Examples

Basic const usage

Constants can be declared with uppercase or lowercase, but a common convention is to use all-uppercase letters. 

// define MY_FAV as a constant and give it the value 7
const MY_FAV = 7;

// this will throw an error - Uncaught TypeError: Assignment to constant variable.
MY_FAV = 20;

// MY_FAV is 7
console.log('my favorite number is: ' + MY_FAV);

// trying to redeclare a constant throws an error
// Uncaught SyntaxError: Identifier 'MY_FAV' has already been declared
const MY_FAV = 20;

// the name MY_FAV is reserved for constant above, so this will fail too
var MY_FAV = 20;

// this throws an error too
let MY_FAV = 20;


Block scoping

It's important to note the nature of block scoping.

if (MY_FAV === 7) {
  // this is fine and creates a block scoped MY_FAV variable
  // (works equally well with let to declare a block scoped non const variable)
  let MY_FAV = 20;

  // MY_FAV is now 20
  console.log('my favorite number is ' + MY_FAV);

  // this gets hoisted into the global context and throws an error
  var MY_FAV = 20;
}

// MY_FAV is still 7
console.log('my favorite number is ' + MY_FAV);


const needs to be initialized

// throws an error
// Uncaught SyntaxError: Missing initializer in const declaration

const FOO;


const in objects and arrays

const also works on objects and arrays. Attempting to overwrite the object throws an error "Assignment to constant variable".

const MY_OBJECT = { key: 'value' };
MY_OBJECT = { OTHER_KEY: 'value' };

However, object keys are not protected, so the following statement is executed without problem.

MY_OBJECT.key = 'otherValue';

You would need to use Object.freeze() to make an object immutable.

The same applies to arrays. Assigning a new array to the variable throws an error "Assignment to constant variable".

const MY_ARRAY = [];
MY_ARRAY = ['B'];

Still, it's possible to push items into the array and thus mutate it.

MY_ARRAY.push('A'); // ["A"]