JavaScript - constants

In this article, we would like to show you how to declare and use constants using the const keyword in JavaScript.

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const myConstant = value;

Constants are declared using the const keyword and they must be initialized. They are block-scoped such as variables declared using the let keyword. The constant values can't be changed through reassignment, and they can't be redeclared. However, if you create a constant object or array its properties or items can be updated or removed.

In this example, we create a constant and try to reassign a value to it.

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const myConstant = 1;

​

try {

  myConstant = 2;

} catch (error) {

  console.log(error); // TypeError: Assignment to constant variable.

}

​

console.log('myConstant = ' + myConstant); // 1

Output:

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TypeError: Assignment to constant variable.

number = 1

In this example, we create an uninitialized constant.

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const myConstant;

Output:

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Uncaught SyntaxError: Missing initializer in const declaration

This example shows, that constants are block-scoped.

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const myConstant = 3;

​

if (true) {

  const myConstant = 2;

  console.log(myConstant); // Output: 2

}

​

console.log(myConstant); // Output: 3

Output:

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2

3

1. const - JavaScript | MDN