JavaScript interview question

What are the differences between variables created using let, var or const?


Variables declared using the var keyword are scoped to the function in which they are created, or if created outside of any function, to the global object. let and const are block scoped, meaning they are only accessible within the nearest set of curly braces (function, if-else block, or for-loop).

 * All variables are accessible within functions.
function variableScope() {

  var x = 10;
  let y = 20;
  const z = 30;

  console.log(x); // 10
  console.log(y); // 20
  console.log(z); // 30

console.log(x); // ReferenceError: x is not defined
console.log(y); // ReferenceError: y is not defined
console.log(z); // ReferenceError: z is not defined

 * var declared variables are accessible anywhere in the function scope.
if (true) {
  var a = 10;
  let b = 20;
  const c = 30;

console.log(a); // 10
console.log(b); // ReferenceError: b is not defined
console.log(c); // ReferenceError: c is not defined

var allows variables to be hoisted, meaning they can be referenced in code before they are declared. let and const will not allow this, instead throwing an error.

console.log(a); // undefined
var a = 'foo';

console.log(b); // ReferenceError: can't access lexical declaration 'b' before initialization
let b = 'baz';

console.log(c); // ReferenceError: can't access lexical declaration 'c' before initialization
const c = 'bar';

Redeclaring a variable with var will not throw an error, but 'let' and 'const' will.

var a = 'foo';
var a = 'bar';
console.log(a); // "bar"

let b = 'baz';
let b = 'qux'; // Uncaught SyntaxError: Identifier 'b' has already been declared

let and const differ in that let allows reassigning the variable's value while const does not.

// This is ok.
let a = 'foo';
a = 'bar';
console.log(a); // bar

// This causes an exception.
const b = 'baz';
b = 'qux';
console.log(b) // TypeError: Assignment to constant variable.

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