error in v on handler typeerror assignment to constant variable

TypeError: Assignment to Constant Variable in JavaScript

Last updated: Mar 2, 2024 Reading time · 3 min

# TypeError: Assignment to Constant Variable in JavaScript

The "Assignment to constant variable" error occurs when trying to reassign or redeclare a variable declared using the const keyword.

When a variable is declared using const , it cannot be reassigned or redeclared.

Here is an example of how the error occurs.

# Declare the variable using let instead of const

To solve the "TypeError: Assignment to constant variable" error, declare the variable using the let keyword instead of using const .

Variables declared using the let keyword can be reassigned.

We used the let keyword to declare the variable in the example.

Variables declared using let can be reassigned, as opposed to variables declared using const .

You can also use the var keyword in a similar way. However, using var in newer projects is discouraged.

# Pick a different name for the variable

Alternatively, you can declare a new variable using the const keyword and use a different name.

We declared a variable with a different name to resolve the issue.

The two variables no longer clash, so the "assignment to constant" variable error is no longer raised.

# Declaring a const variable with the same name in a different scope

You can also declare a const variable with the same name in a different scope, e.g. in a function or an if block.

The if statement and the function have different scopes, so we can declare a variable with the same name in all 3 scopes.

However, this prevents us from accessing the variable from the outer scope.

# The const keyword doesn't make objects immutable

Note that the const keyword prevents us from reassigning or redeclaring a variable, but it doesn't make objects or arrays immutable.

We declared an obj variable using the const keyword. The variable stores an object.

Notice that we are able to directly change the value of the name property even though the variable was declared using const .

The behavior is the same when working with arrays.

Even though we declared the arr variable using the const keyword, we are able to directly change the values of the array elements.

The const keyword prevents us from reassigning the variable, but it doesn't make objects and arrays immutable.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• SyntaxError: Unterminated string constant in JavaScript
• TypeError (intermediate value)(...) is not a function in JS

Borislav Hadzhiev

Web Developer

Copyright © 2024 Borislav Hadzhiev

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TypeError: invalid assignment to const "x"

Const and immutability, what went wrong.

A constant is a value that cannot be altered by the program during normal execution. It cannot change through re-assignment, and it can't be redeclared. In JavaScript, constants are declared using the const keyword.

Invalid redeclaration

Assigning a value to the same constant name in the same block-scope will throw.

Fixing the error

There are multiple options to fix this error. Check what was intended to be achieved with the constant in question.

If you meant to declare another constant, pick another name and re-name. This constant name is already taken in this scope.

const , let or var ?

Do not use const if you weren't meaning to declare a constant. Maybe you meant to declare a block-scoped variable with let or global variable with var .

Check if you are in the correct scope. Should this constant appear in this scope or was is meant to appear in a function, for example?

The const declaration creates a read-only reference to a value. It does not  mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in case the content is an object, this means the object itself can still be altered. This means that you can't mutate the value stored in a variable:

But you can mutate the properties in a variable:

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Typeerror assignment to constant variable

Doesn’t know how to solve the “Typeerror assignment to constant variable” error in Javascript?

Don’t worry because this article will help you to solve that problem

In this article, we will discuss the Typeerror assignment to constant variable , provide the possible causes of this error, and give solutions to resolve the error.

First, let us know what this error means.

What is Typeerror assignment to constant variable?

“Typeerror assignment to constant variable” is an error message that can occur in JavaScript code.

It means that you have tried to modify the value of a variable that has been declared as a constant.

In JavaScript, when you declare a variable using the const keyword, its value cannot be changed or reassigned.

Attempting to modify a constant variable, you will receive an error stating:

Here is an example code snippet that triggers the error:

In this example, we have declared a constant variable greeting and assigned it the value “Hello” .

When we try to reassign greeting to a different value (“Hi”) , we will get the error:

because we are trying to change the value of a constant variable.

Let us explore more about how this error occurs.

How does Typeerror assignment to constant variable occurs ?

This “ TypeError: Assignment to constant variable ” error occurs when you attempt to modify a variable that has been declared as a constant.

In JavaScript, constants are variables whose values cannot be changed once they have been assigned.

When you declare a variable using the const keyword, you are telling JavaScript that the value of the variable will remain constant throughout the program.

If you try to modify the value of a constant variable, you will get the error:

This error can occur in various situations, such as:

• Attempting to reassign a constant variable:

When you declare a variable using the const keyword, its value cannot be changed.

If you try to reassign the value of a constant variable, you will get this error.

Here is an example :

In this example, we declared a constant variable age and assigned it the value 30 .

When we try to reassign age to a different value ( 35 ), we will get the error:

• Attempting to modify a constant object:

If you declare an object using the const keyword, you can still modify the properties of the object.

However, you cannot reassign the entire object to a new value.

If you try to reassign a constant object, you will get the error.

For example:

In this example, we declared a constant object person with two properties ( name and age ).

We are able to modify the age property of the object without triggering an error.

However, when we try to reassign person to a new object, we will get the Typeerror.

• Using strict mode:

In strict mode, JavaScript throws more errors to help you write better code.

If you try to modify a constant variable in strict mode, you will get the error.

In this example, we declared a constant variable name and assigned it the value John .

However, because we are using strict mode, any attempt to modify the value of name will trigger the error.

Now let’s fix this error.

Typeerror assignment to constant variable – Solutions

Here are the alternative solutions that you can use to fix “Typeerror assignment to constant variable” :

Solution 1: Declare the variable using the let or var keyword:

If you need to modify the value of a variable, you should declare it using the let or var keyword instead of const .

Just like the example below:

Solution 2: Use an object or array instead of a constant variable:

If you need to modify the properties of a variable, you can use an object or array instead of a constant variable.

Solution 3: Declare the variable outside of strict mode:

If you are using strict mode and need to modify a variable, you can declare the variable outside of strict mode:

Solution 4: Use the const keyword and use a different name :

This allows you to keep the original constant variable intact and create a new variable with a different value.

Solution 5: Declare a const variable with the same name in a different scope :

This allows you to create a new constant variable with the same name as the original constant variable.

But with a different value, without modifying the original constant variable.

For Example:

You can create a new constant variable with the same name, without modifying the original constant variable.

By declaring a constant variable with the same name in a different scope.

This can be useful when you need to use the same variable name in multiple scopes without causing conflicts or errors.

So those are the alternative solutions that you can use to fix the TypeError.

By following those solutions, you can fix the “Typeerror assignment to constant variable” error in your JavaScript code.

Here are the other fixed errors that you can visit, you might encounter them in the future.

• typeerror unsupported operand type s for str and int
• typeerror: object of type int64 is not json serializable
• typeerror: bad operand type for unary -: str

In conclusion, in this article, we discussed   “Typeerror assignment to constant variable” , provided its causes and give solutions that resolve the error.

By following the given solution, surely you can fix the error quickly and proceed to your coding project again.

I hope this article helps you to solve your problem regarding a  Typeerror   stating  “assignment to constant variable” .

We’re happy to help you.

Happy coding! Have a Good day and God bless.

关于“TypeError: Assignment to constant variable”的问题解决方案

在项目开发过程中，在使用变量声明时，如果不注意，可能会造成类型错误 比如：

Uncaught (in promise) TypeError: Assignment to constant variable. 未捕获的类型错误：赋值给常量变量。

我们使用 const 定义了变量且存在初始值。 后面又给这个变量赋值，所以报错了。

ES6 标准引入了新的关键字 const 来定义常量， const 与 let 都具有块级作用域：

• 使用 const 定义的常量，不能修改它的值，且定义的常量必须赋初值；
• let 定义的是变量，可以进行变量赋值操作，且不需要赋初值。

这个错误就是因为我们修改了常量而引起的错误，虽然某些浏览器不报错，但是无效果！

将 const 改为 let 进行声明。

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JavaScript’s var, let, and const variables explained with a story

by Prarthana S. Sannamani

In this article, we will explore the history of var in JavaScript, the need for let and const , and the differences between them.

This post consists of two sections: Fictional piece and Technical explanation.

The fictional piece is intended to ease beginners into the concepts, but several parts are simplified and do not always present an accurate 1:1 analogy.

Let’s start!

A tale of three variables

JavaScript town was a bustling town beside the sea with a commercial district filled with high rise buildings.

Since time immemorial, the residents of JavaScript town used Vary boxes to store their valuables, especially their prized gold marbles. To do so, the residents had two options:

• They could place the gold marbles directly in the box (pass by value)
• If they had a large number of gold marbles so that they would not fit in the box, they could place a special piece of paper in the box, which indicated where they had stored them. For example, the piece of paper could say “second drawer in the storage cabinet” (pass by reference)

Since the town prided itself on law and order, they set up several rules and procedures.

Rules for shops

• To maintain the serenity of the town, shops could be built only on hills (functions create their own local scope)
• The only exception to Rule 1 was the special shop at sea level (global scope).
• A shop could have inner shops to help cover the rent (nested functions). However, each inner shop was required to be on a higher hill than the landlord shop’s hill (local function scope).
• A shop could have “special offer” counters, such as “ If you are over 20 years old, buy a special box here.” And “ For (every) child of your family, buy a kid’s box here” (other blocks such as if and loops).
• Each shop was required to have a “declaration-initialization” counter with a guard at the entrance, who maintained a registration log book (hoisting at the top of corresponding scope).
• Each shop could have unlimited “assignment” counters with a shop assistant, who would place a resident’s gold marbles in the box.

Rules for box market regulation

• The boxes could be purchased only from the sea level special shop or from a shop on the hills (variable can have global or local scope).
• At the sea level or on any hill, residents could own ONLY a single colored Vary box (duplicate identifiers not allowed).
• The Vary box could never be empty from the moment it was created. It had to contain cotton ( undefined ) or gold marbles at all times (effect of hoisting).
• Once a resident exited a shop (and hence, descended the hill), all boxes they purchased in it disappeared (end of variable’s scope).

Procedure for residents to buy the `Vary` boxes

We will follow the journey of a resident, John, in this article.

• John enters the shop and declares what color of Vary box he desires to buy at the “declaration-initialization” counter. The guard notes this in his registration book.
• The guard conjures the colored Vary box, fills it with cotton and hands it to John.
• John gets a ticket for his turn and when it arrives, he heads to the “assignment” counter. Until then, he can hold his box but cannot place his gold marbles in it.
• At the counter, John hands over his box and gold marbles to the shop assistant, who removes the cotton, places the gold marbles inside and hands it back to him.

Naturally, these rules brought along peculiar problems.

With long waiting times for the “assignment” counter, John would forget that he had not placed his gold marbles in his box yet. He would open it to brag to his friends and find only cotton. Bummer!

• Often, John would forget that he had already bought a certain colored box in a shop and newly register for the same colored box again. This would instantly result in the disappearance of his existing box (and gold marbles!!), followed by the guard conjuring a new box filled with cotton. No warning! This was especially prevalent at the “special offers” counters.

You can imagine how frustrating this situation was. With the residents of JavaScript town losing their marbles, the Town Council decided to take action.

In a grand Town Meeting in 2015, they proudly introduced two new boxes: Lety and Consty .

They also introduced the other major change: the removal of “special offer” counters from Lety and Consty shops. Instead, these counters were upgraded to inner shops, which were built on a hill inside the shop.

Rules for purchasing `Lety` and `Consty` boxes

• John enters the shop and declares what type and color of box he desires to buy at the “declaration” counter. The guard notes this in his registration book. This information hazily appears on the huge wall clock, which can be seen, but not used, and is referred to as the “temporal dead zone”.
• John gets a ticket for his turn. Since the box is not created at declaration, it is not available for use.

This is where Lety and Consty purchase rules diverge.

`Lety` rules:

• Once John’s turn arrives, he heads to the “initialization” counter.
• At the counter, John has the choice to buy an empty Lety box, or buy a Lety box and have his gold marbles placed inside it immediately.
• Depending on his choice, the shop assistant conjures the Lety box, and fills it with cotton or hands it over to the “assignment” counter, where John’s gold marbles are placed inside it.

`Consty` rules

Consty boxes are extremely special. Lined with a layer of gold inside and sealed with a lock, these boxes are so dear to the shop assistants that they refuse to sell them without knowing what exactly will be placed in them.

• Once John’s turn arrives, he heads to the “initialization-assignment” counter.
• John is required to hand over his gold marbles to the shop assistant, who conjures the colored Consty box, places the gold marbles inside, and locks the box forever .

If you remember, John could directly place his gold marbles in the box or place a special piece of paper which indicated the location of his gold marbles.

• If he places his gold marbles inside the Consty box, he cannot add or remove them anymore. They are locked forever.
• However, if he places the special piece of paper, it is a little different. While he cannot replace the paper, he can add or remove his gold marbles at the location he has specified on the paper.

Let’s go back to the peculiar problems that prompted the invention of Lety and Consty boxes, and decide if they are resolved.

Since Lety and Consty boxes are not created until John heads over to the “initialization” or ”initialization-assignment” counter, respectively, he knows he does not have the box, and thus, does not try to use it. Even if he does, loud alarms installed in the shops start ringing to alert him of the fact.

Often, John would forget that he had already bought a certain colored box in a shop and newly register for same colored box again. This would instantly result in the disappearance of his existing box (and gold marbles!!), followed by the guard conjuring a new box filled with cotton. No warning! This was especially prevalent at the “special offers” counters.

This is handled by the removal of the “special offers” counters and the introduction of the below rule:

Once a resident registers for a certain colored box at the “declaration”desk in the Lety or Consty shops, he cannot re-register for the same colored box anymore in that shop! If he does, loud alarms will start blaring.

These wonderful new boxes and rules bought peace and serenity to JavaScript Town once again, and everyone lived happily ever after.

Diving into the technical details

Let’s go over the technical aspects of var , let and const to understand the story.

If you are unfamiliar with hoisting and scope (function-level and block-level), I recommend that you read my previous article here .

Here is an extract to understand the hills analogy I have used above:

To increase our understanding of block level and function level scope, let us consider the analogy of hills. Assume that global scope is the land at sea level and local scopes are hills. If you stand on top of a hill, you can see (access) variables below your altitude. However, if you are sea level, you cannot see (access) variables at a higher altitude.

In C++, every block {} results in the formation of a new hill (local scope), at an altitude one level higher than the one it is enclosed in. Nested blocks result in multi-level hills.

In JavaScript, only a function results in the formation of a new hill (local scope). Other blocks such as if blocks are present on the same altitude.

Therefore, if a variable is declared on a certain hill (block), it can be accessed from that hill (block) and all hills (blocks) above it.

Life cycle of a variable

Declaration Phase : Registration of a variable in its scope, which can be global/function/block scope. In this phase, no memory is allocated yet.

Initialization Phase : Allocation of memory for the variable, where a binding is created, and the variable is initialized with undefined .

Assignment Phase : Assignment of a value to the variable.

It is important to note that variable declaration and declaration phase are not the same!

A variable declaration is a statement such as var a .

The declaration phase is a step carried out by the JavaScript compiler. In this step, when the compiler encounters a variable declaration, it declares/registers it in its corresponding scope (if the declaration does not already exist). Later on, the code generated by the compiler is executed by the JavaScript engine.

• global scope or function scope
• value can be updated
• can be re-declared
• hoisted: registered in the scope, and initialized with undefined

Below is a simple example where we initialize a variable, update its value, and re-declare it.

At the top of the scope, all variables are declared in their corresponding scope and initialized with a value of undefined . Registration and initialization are coupled. Thus, variable a is available for use from the top of the scope. So when we try to access the value of a before it is declared, it does not throw an error. Rather, undefined is printed. This is known as variable hoisting.

Below is an example that shows the function scope of var .

Variable a is initially declared in the scope of outerFunc . Since the if block does not create a new scope, when we re-declare variable a , the earlier variable a gets wiped away and a new variable a gets created with a value of 20 .

Accidental re-declaration of var variables is a common mistake developers make due to silent re-declaration and confusion in understanding function scope.

• block scoped
• cannot be re-declared
• hoisted but not initialized

Below is a simple example where we initialize a variable, update its value, and try to re-declare it.

Updating a let variable is allowed. However, if you try to re-declare it, you encounter a SyntaxError . This protects developers from silent and accidental re-declaration of variables.

Are let variables hoisted?

This is a tricky question. The internet is divided on this: there are arguments for both sides. Some developers believe that let (and const ) variables are not hoisted, because they cannot be accessed before their declaration statement is reached, unlike var . However, this answer really depends on your definition of hoisting. If hoisting is the coupling of the declaration and initialization phases of a variable at the top of its corresponding scope, then let and const variables are not hoisted.

However, after reading several opinions and not being any closer to the truth, I decided to go with MDN’s definition of hoisting.

let bindings are created at the top of the (block) scope containing the declaration, commonly referred to as "hoisting". (MDN)

According to this definition, the answer to our question is yes. let variables are hoisted, but they are not initialized with undefined . Thus, they exist in a time period called the “Temporal Dead Zone” from the start of the block until their definition is evaluated. Trying to access them in TDZ throws a ReferenceError , as seen in the example.

Below is an example that shows block scope of let .

The first declaration of variable a is in the scope of outerFunc . The if block creates a new scope, and when we make the second declaration of variable a , it gets registered in the new scope. This is independent from the outerFunc scope. Hence, a separate variable a is created, and we can observe that changes to the inner variable a do not affect the outer variable a .

This allows developers to easily create temporary variables inside condition and looping blocks, without having to search if the variable already exists in the function.

• binding is immutable (but value may or may not be changed)

Below is a simple example where we initialize a variable, try to update its value, and try to re-declare it.

Similar to let variables, const variables are hoisted, but not initialized with undefined . Trying to access them in the Temporal Dead Zone throws a ReferenceError .

If we try to initialize a const variable without an assignment, as in the example above for const b; , we encounter a SyntaxError: Missing initializer in const declaration . Similarly, we cannot re-declare const variables. It leads to a SyntaxError .

Let’s temporarily hold off our discussion of updating const variables.

Below is an example of block level scope of const variables:

The above behavior is similar to let variables, where a new scope is created for the if block, and hence, changes to the inner variable a do not affect the outer variable a .

Let’s return to the discussion of updating const variables.

There is a common misunderstanding that const variables hold constant values, and cannot ever be updated. However, const works differently.

After the initial assignment, the binding of const variables is immutable ., and therefore, the reference to what is stored inside the const variable cannot be modified. In the simplest terms, this means you cannot have a statement with just the const variable on the left hand side, followed by an equal sign = , and a new value on the right hand side.

However, whether the value can be updated depends on what is stored in it. Let’s consider the two cases:

• Primitive data type: Boolean, Null, Undefined, Number, String, Symbol

If a variable is assigned a primitive data type, the data type gets passed by value . Hence, if we have a statement let x = 10 , we can visualize x containing the Number 10 .

If a variable is assigned an object, the object is passed by reference . Hence, if we have a statement let x = [1,2,3] , x does not contain the array [1,2,3] . Instead, it contains a reference (address) of where the array [1,2,3] is stored in memory after its creation. Hence, we can visualize x containing an address such as 5274621 .

Let’s see examples from primitive and object data types:

As we can see above, trying to update the value of any primitive data type results in a TypeError .

As we can see above, we can push and pop items from the array since this only modifies the contents of what the const variable is pointing to, but does not try to overwrite the contents of the const variable itself. However, if we try to update the binding of the const variable by re-assigning it a completely new array c = [4,5,6] , it throws a TypeError .

As we can see above, we can modify and add properties to the object since this only modifies the contents of what the const variable is pointing to, but does not try to overwrite the contents of the const variable itself. However, if we try to update the binding of the const variable by re-assigning it a completely new object d = { name: 'Mary Jane', age: 25 }; , it throws a TypeError .

What went wrong?

A constant is a value that cannot be altered by the program during normal execution. It cannot change through re-assignment, and it can't be redeclared. In JavaScript, constants are declared using the const keyword.

Invalid redeclaration

Assigning a value to the same constant name in the same block-scope will throw.

Fixing the error

There are multiple options to fix this error. Check what was intended to be achieved with the constant in question.

If you meant to declare another constant, pick another name and re-name. This constant name is already taken in this scope.

const , let or var ?

Do not use const if you weren't meaning to declare a constant. Maybe you meant to declare a block-scoped variable with let or global variable with var .

Check if you are in the correct scope. Should this constant appear in this scope or was it meant to appear in a function, for example?

const and immutability

The const declaration creates a read-only reference to a value. It does not  mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in case the content is an object, this means the object itself can still be altered. This means that you can't mutate the value stored in a variable:

But you can mutate the properties in a variable:

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Mobile app development, nodejs typeerror: assignment to constant variable.

Published By: Divya Mahi

Published On: November 17, 2023

Published In: Development

Grasping and Fixing the 'NodeJS TypeError: Assignment to Constant Variable' Issue

Introduction.

Node.js, a powerful platform for building server-side applications, is not immune to errors and exceptions. Among the common issues developers encounter is the “NodeJS TypeError: Assignment to Constant Variable.” This error can be a source of frustration, especially for those new to JavaScript’s nuances in Node.js. In this comprehensive guide, we’ll explore what this error means, its typical causes, and how to effectively resolve it.

Understanding the Error

In Node.js, the “TypeError: Assignment to Constant Variable” occurs when there’s an attempt to reassign a value to a variable declared with the const keyword. In JavaScript, const is used to declare a variable that cannot be reassigned after its initial assignment. This error is a safeguard in the language to ensure the immutability of variables declared as constants.

Diving Deeper

This TypeError is part of JavaScript’s efforts to help developers write more predictable code. Immutable variables can prevent bugs that are hard to trace, as they ensure that once a value is set, it cannot be inadvertently changed. However, it’s important to distinguish between reassigning a variable and modifying an object’s properties. The latter is allowed even with variables declared with const.

Common Scenarios and Fixes

Example 1: reassigning a constant variable.

Javascript:

Fix: Use let if you need to reassign the variable.

Example 2: Modifying an Object's Properties

Fix: Modify the property instead of reassigning the object.

Example 3: Array Reassignment

Fix: Modify the array’s contents without reassigning it.

Example 4: Within a Function Scope

Fix: Declare a new variable or use let if reassignment is needed.

Example 5: In Loops

Fix: Use let for variables that change within loops.

Example 6: Constant Function Parameters

Fix: Avoid reassigning function parameters directly; use another variable.

Example 7: Constants in Conditional Blocks

Fix: Use let if the variable needs to change.

Example 8: Reassigning Properties of a Constant Object

Fix: Modify only the properties of the object.

Strategies to Prevent Errors

Understand const vs let: Familiarize yourself with the differences between const and let. Use const for variables that should not be reassigned and let for those that might change.

Code Reviews: Regular code reviews can catch these issues before they make it into production. Peer reviews encourage adherence to best practices.

Linter Usage: Tools like ESLint can automatically detect attempts to reassign constants. Incorporating a linter into your development process can prevent such errors.

Best Practices

Immutability where Possible: Favor immutability in your code to reduce side effects and bugs. Normally use const to declare variables, and use let only if you need to change their values later .

Descriptive Variable Names: Use clear and descriptive names for your variables. This practice makes it easier to understand when a variable should be immutable.

Keep Functions Pure: Avoid reassigning or modifying function arguments. Keeping functions pure (not causing side effects) leads to more predictable and testable code.

The “NodeJS TypeError: Assignment to Constant Variable” error, while common, is easily avoidable. By understanding JavaScript’s variable declaration nuances and adopting coding practices that embrace immutability, developers can write more robust and maintainable Node.js applications. Remember, consistent coding standards and thorough code reviews are your best defense against common errors like these.

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Variable Declaration

let and const are two relatively new concepts for variable declarations in JavaScript. As we mentioned earlier , let is similar to var in some respects, but allows users to avoid some of the common “gotchas” that users run into in JavaScript.

const is an augmentation of let in that it prevents re-assignment to a variable.

With TypeScript being an extension of JavaScript, the language naturally supports let and const . Here we’ll elaborate more on these new declarations and why they’re preferable to var .

If you’ve used JavaScript offhandedly, the next section might be a good way to refresh your memory. If you’re intimately familiar with all the quirks of var declarations in JavaScript, you might find it easier to skip ahead.

var declarations

Declaring a variable in JavaScript has always traditionally been done with the var keyword.

As you might’ve figured out, we just declared a variable named a with the value 10 .

We can also declare a variable inside of a function:

and we can also access those same variables within other functions:

In this above example, g captured the variable a declared in f . At any point that g gets called, the value of a will be tied to the value of a in f . Even if g is called once f is done running, it will be able to access and modify a .

Scoping rules

var declarations have some odd scoping rules for those used to other languages. Take the following example:

Some readers might do a double-take at this example. The variable x was declared within the if block , and yet we were able to access it from outside that block. That’s because var declarations are accessible anywhere within their containing function, module, namespace, or global scope - all which we’ll go over later on - regardless of the containing block. Some people call this var -scoping or function-scoping . Parameters are also function scoped.

These scoping rules can cause several types of mistakes. One problem they exacerbate is the fact that it is not an error to declare the same variable multiple times:

Maybe it was easy to spot out for some experienced JavaScript developers, but the inner for -loop will accidentally overwrite the variable i because i refers to the same function-scoped variable. As experienced developers know by now, similar sorts of bugs slip through code reviews and can be an endless source of frustration.

Variable capturing quirks

Take a quick second to guess what the output of the following snippet is:

For those unfamiliar, setTimeout will try to execute a function after a certain number of milliseconds (though waiting for anything else to stop running).

Ready? Take a look:

Many JavaScript developers are intimately familiar with this behavior, but if you’re surprised, you’re certainly not alone. Most people expect the output to be

Remember what we mentioned earlier about variable capturing? Every function expression we pass to setTimeout actually refers to the same i from the same scope.

Let’s take a minute to consider what that means. setTimeout will run a function after some number of milliseconds, but only after the for loop has stopped executing; By the time the for loop has stopped executing, the value of i is 10 . So each time the given function gets called, it will print out 10 !

A common work around is to use an IIFE - an Immediately Invoked Function Expression - to capture i at each iteration:

This odd-looking pattern is actually pretty common. The i in the parameter list actually shadows the i declared in the for loop, but since we named them the same, we didn’t have to modify the loop body too much.

let declarations

By now you’ve figured out that var has some problems, which is precisely why let statements were introduced. Apart from the keyword used, let statements are written the same way var statements are.

The key difference is not in the syntax, but in the semantics, which we’ll now dive into.

Block-scoping

When a variable is declared using let , it uses what some call lexical-scoping or block-scoping . Unlike variables declared with var whose scopes leak out to their containing function, block-scoped variables are not visible outside of their nearest containing block or for -loop.

Here, we have two local variables a and b . a ’s scope is limited to the body of f while b ’s scope is limited to the containing if statement’s block.

Variables declared in a catch clause also have similar scoping rules.

Another property of block-scoped variables is that they can’t be read or written to before they’re actually declared. While these variables are “present” throughout their scope, all points up until their declaration are part of their temporal dead zone . This is just a sophisticated way of saying you can’t access them before the let statement, and luckily TypeScript will let you know that.

Something to note is that you can still capture a block-scoped variable before it’s declared. The only catch is that it’s illegal to call that function before the declaration. If targeting ES2015, a modern runtime will throw an error; however, right now TypeScript is permissive and won’t report this as an error.

For more information on temporal dead zones, see relevant content on the Mozilla Developer Network .

Re-declarations and Shadowing

With var declarations, we mentioned that it didn’t matter how many times you declared your variables; you just got one.

In the above example, all declarations of x actually refer to the same x , and this is perfectly valid. This often ends up being a source of bugs. Thankfully, let declarations are not as forgiving.

The variables don’t necessarily need to both be block-scoped for TypeScript to tell us that there’s a problem.

That’s not to say that a block-scoped variable can never be declared with a function-scoped variable. The block-scoped variable just needs to be declared within a distinctly different block.

The act of introducing a new name in a more nested scope is called shadowing . It is a bit of a double-edged sword in that it can introduce certain bugs on its own in the event of accidental shadowing, while also preventing certain bugs. For instance, imagine we had written our earlier sumMatrix function using let variables.

This version of the loop will actually perform the summation correctly because the inner loop’s i shadows i from the outer loop.

Shadowing should usually be avoided in the interest of writing clearer code. While there are some scenarios where it may be fitting to take advantage of it, you should use your best judgement.

Block-scoped variable capturing

When we first touched on the idea of variable capturing with var declaration, we briefly went into how variables act once captured. To give a better intuition of this, each time a scope is run, it creates an “environment” of variables. That environment and its captured variables can exist even after everything within its scope has finished executing.

Because we’ve captured city from within its environment, we’re still able to access it despite the fact that the if block finished executing.

Recall that with our earlier setTimeout example, we ended up needing to use an IIFE to capture the state of a variable for every iteration of the for loop. In effect, what we were doing was creating a new variable environment for our captured variables. That was a bit of a pain, but luckily, you’ll never have to do that again in TypeScript.

let declarations have drastically different behavior when declared as part of a loop. Rather than just introducing a new environment to the loop itself, these declarations sort of create a new scope per iteration . Since this is what we were doing anyway with our IIFE, we can change our old setTimeout example to just use a let declaration.

and as expected, this will print out

const declarations

const declarations are another way of declaring variables.

They are like let declarations but, as their name implies, their value cannot be changed once they are bound. In other words, they have the same scoping rules as let , but you can’t re-assign to them.

This should not be confused with the idea that the values they refer to are immutable .

Unless you take specific measures to avoid it, the internal state of a const variable is still modifiable. Fortunately, TypeScript allows you to specify that members of an object are readonly . The chapter on Interfaces has the details.

let vs. const

Given that we have two types of declarations with similar scoping semantics, it’s natural to find ourselves asking which one to use. Like most broad questions, the answer is: it depends.

Applying the principle of least privilege , all declarations other than those you plan to modify should use const . The rationale is that if a variable didn’t need to get written to, others working on the same codebase shouldn’t automatically be able to write to the object, and will need to consider whether they really need to reassign to the variable. Using const also makes code more predictable when reasoning about flow of data.

Use your best judgement, and if applicable, consult the matter with the rest of your team.

The majority of this handbook uses let declarations.

Destructuring

Another ECMAScript 2015 feature that TypeScript has is destructuring. For a complete reference, see the article on the Mozilla Developer Network . In this section, we’ll give a short overview.

Array destructuring

The simplest form of destructuring is array destructuring assignment:

This creates two new variables named first and second . This is equivalent to using indexing, but is much more convenient:

Destructuring works with already-declared variables as well:

And with parameters to a function:

You can create a variable for the remaining items in a list using the syntax ... :

Of course, since this is JavaScript, you can just ignore trailing elements you don’t care about:

Or other elements:

Tuple destructuring

Tuples may be destructured like arrays; the destructuring variables get the types of the corresponding tuple elements:

It’s an error to destructure a tuple beyond the range of its elements:

As with arrays, you can destructure the rest of the tuple with ... , to get a shorter tuple:

Or ignore trailing elements, or other elements:

Object destructuring

You can also destructure objects:

This creates new variables a and b from o.a and o.b . Notice that you can skip c if you don’t need it.

Like array destructuring, you can have assignment without declaration:

Notice that we had to surround this statement with parentheses. JavaScript normally parses a { as the start of block.

You can create a variable for the remaining items in an object using the syntax ... :

Property renaming

You can also give different names to properties:

Here the syntax starts to get confusing. You can read a: newName1 as ” a as newName1 ”. The direction is left-to-right, as if you had written:

Confusingly, the colon here does not indicate the type. The type, if you specify it, still needs to be written after the entire destructuring:

Default values

Default values let you specify a default value in case a property is undefined:

In this example the b? indicates that b is optional, so it may be undefined . keepWholeObject now has a variable for wholeObject as well as the properties a and b , even if b is undefined.

Function declarations

Destructuring also works in function declarations. For simple cases this is straightforward:

But specifying defaults is more common for parameters, and getting defaults right with destructuring can be tricky. First of all, you need to remember to put the pattern before the default value.

The snippet above is an example of type inference, explained earlier in the handbook.

Then, you need to remember to give a default for optional properties on the destructured property instead of the main initializer. Remember that C was defined with b optional:

Use destructuring with care. As the previous example demonstrates, anything but the simplest destructuring expression is confusing. This is especially true with deeply nested destructuring, which gets really hard to understand even without piling on renaming, default values, and type annotations. Try to keep destructuring expressions small and simple. You can always write the assignments that destructuring would generate yourself.

The spread operator is the opposite of destructuring. It allows you to spread an array into another array, or an object into another object. For example:

This gives bothPlus the value [0, 1, 2, 3, 4, 5] . Spreading creates a shallow copy of first and second . They are not changed by the spread.

You can also spread objects:

Now search is { food: "rich", price: "$$", ambiance: "noisy" } . Object spreading is more complex than array spreading. Like array spreading, it proceeds from left-to-right, but the result is still an object. This means that properties that come later in the spread object overwrite properties that come earlier. So if we modify the previous example to spread at the end:

Then the food property in defaults overwrites food: "rich" , which is not what we want in this case.

Object spread also has a couple of other surprising limits. First, it only includes an objects’ own, enumerable properties . Basically, that means you lose methods when you spread instances of an object:

Second, the TypeScript compiler doesn’t allow spreads of type parameters from generic functions. That feature is expected in future versions of the language.

© 2012-2021 Microsoft Licensed under the Apache License, Version 2.0. https://www.typescriptlang.org/docs/handbook/variable-declarations.html

TypeError: Assignment to constant variable when using React useState hook

Abstract: Learn about the common error 'TypeError: Assignment to constant variable' that occurs when using the React useState hook in JavaScript. Understand the cause of the error and how to resolve it effectively.

If you are a React developer, you have probably come across the useState hook, which is a powerful feature that allows you to manage state in functional components. However, there may be times when you encounter a TypeError: Assignment to constant variable error while using the useState hook. In this article, we will explore the possible causes of this error and how to resolve it.

Understanding the Error

The TypeError: Assignment to constant variable error occurs when you attempt to update the value of a constant variable that is declared using the const keyword. In React, when you use the useState hook, it returns an array with two elements: the current state value and a function to update the state value. If you mistakenly try to assign a new value to the state variable directly, you will encounter this error.

Common Causes

There are a few common causes for this error:

• Forgetting to invoke the state update function: When using the useState hook, you need to call the state update function to update the state value. For example, instead of stateVariable = newValue , you should use setStateVariable(newValue) . Forgetting to invoke the function will result in the TypeError: Assignment to constant variable error.
• Using the wrong state update function: If you have multiple state variables in your component, make sure you are using the correct state update function for each variable. Mixing up the state update functions can lead to this error.
• Declaring the state variable inside a loop or conditional statement: If you declare the state variable inside a loop or conditional statement, it will be re-initialized on each iteration or when the condition changes. This can cause the TypeError: Assignment to constant variable error if you try to update the state value.

Resolving the Error

To resolve the TypeError: Assignment to constant variable error, you need to ensure that you are using the state update function correctly and that you are not re-declaring the state variable inside a loop or conditional statement.

If you are forgetting to invoke the state update function, make sure to add parentheses after the function name when updating the state value. For example, change stateVariable = newValue to setStateVariable(newValue) .

If you have multiple state variables, double-check that you are using the correct state update function for each variable. Using the wrong function can result in the error. Make sure to match the state variable name with the corresponding update function.

Lastly, if you have declared the state variable inside a loop or conditional statement, consider moving the declaration outside of the loop or conditional statement. This ensures that the state variable is not re-initialized on each iteration or when the condition changes.

The TypeError: Assignment to constant variable error is a common mistake when using the useState hook in React. By understanding the causes of this error and following the suggested resolutions, you can overcome this issue and effectively manage state in your React applications.

Tags: :  javascript reactjs react-state

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