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What's the difference between using “let” and “var”?

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1

What are the differences? When should let be used over var?

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2 Answers

1

Scoping rules
Main difference is scoping rules. Variables declared by var keyword are scoped to the immediate function body (hence the function scope) while let variables are scoped to the immediate enclosing block denoted by { } (hence the block scope).

function run() {
  var foo = "Foo";
  let bar = "Bar";

  console.log(foo, bar);

  {
    let baz = "Bazz";
    console.log(baz);
  }

  console.log(baz); // ReferenceError
}

run();

The reason why let keyword was introduced to the language was function scope is confusing and was one of the main source of bug in javascript.

Hoisting
While variables declared with var keyword are “hoisted” to the top of the block which means they are accessible in their enclosing scope even before they are declared:

function run() {
  console.log(foo); // undefined
  var foo = "Foo";
  console.log(foo); // Foo
}

run();

let variables are not initialized until their definition is evaluated. Accessing them before the initialization results in a ReferenceError. Variable said to be in “temporal dead zone” from the start of the block until the initialization is processed.

function checkHoisting() {
  console.log(foo); // ReferenceError
  let foo = "Foo";
  console.log(foo); // Foo
}

checkHoisting();

Creating global object property
At the top level, let, unlike var, does not create a property on the global object:

var foo = "Foo";  // globally scoped
let bar = "Bar"; // globally scoped

console.log(window.foo); // Foo
console.log(window.bar); // undefined

Redeclaration
In strict mode, var will let you re-declare the same variable in the same scope while let raises a SyntaxError.

'use strict';
var foo = "foo1";
var foo = "foo2"; // No problem, 'foo' is replaced.

let bar = "bar1";
let bar = "bar2"; // SyntaxError: Identifier 'bar' has already been declared
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0

var and let are both used for variable declaration in javascript but the difference between them is that var is function scoped and let is block scoped.
It can be said that a variable declared with var is defined throughout the program as compared to let.

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