How to Declare Constants in JavaScript?

ES6 provides a new way of declaring a constant by using the const keyword which creates a read-only reference to a value:

const CONSTANT_VALUE = "someValue";

This works in almost all browsers.

Capitalization indicates that some values should be constant.

Local variables with block scope can be declared with const. Assuming, we have a constant variable CONSTANT_VALUE. After declaring a value like a constant variable, you can’t assign a new value to it:

Javascript const keyword
// declared ‘CONSTANT_VALUE’ as a constant variable.
const CONSTANT_VALUE = 200;
// Assigning a new value to `CONSTANT_VALUE` will fail since ‘CONSTANT_VALUE’ is a constant variable
CONSTANT_VALUE = 0;
console.log(CONSTANT_VALUE);
 
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Javascript const keyword
// declared ‘CONSTANT_VALUE’ as a constant variable.
const CONSTANT_VALUE = 200;
CONSTANT_VALUE++;
console.log(CONSTANT_VALUE);
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Javascript const keyword
// declared ‘CONSTANT_VALUE’ as a constant variable.
const CONSTANT_VALUE = 200;
// The below declarations won’t work
var CONSTANT_VALUE = 0;
console.log(CONSTANT_VALUE);
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The constant cannot change through re-assignment constant cannot be re-declared. Const can be changed if it is an object because when you're adding to an array or object you're not re-assigning or re-declaring the constant, it's already declared and assigned, you're just adding to the "list" that the constant points to. So this works fine:

Javascript const keyword for objects
const obj = {};
obj.name= 'John';
console.log(obj); // {name : 'John'}
obj.name = 'Jack';
console.log(obj); // {name : 'Jack'}
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Javascript const keyword for array
const arr = [];
arr.push('elem1');
console.log(arr); // ['elem1']
arr.unshift("elem2");
console.log(arr); // ['elem2', 'elem']
arr.pop();
console.log(arr); // ['elem2']
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The const Variable

ES2015 introduced two new JavaScript keywords: let and const. Variables defined with const behave like let variables, except they cannot be re-assigned. In the programming world, a constant is something that does not change. The const creates a constant that is a read-only reference to a value, which does not mean the value it holds is immutable. It only indicates that the variable identifier cannot be re-assigned.