Hoistingis the concept in which Javascript, by default, moves all declarations to the top of the current scope. As such, a variable can be used before it has been declared.

Note that Javascript only hoists declarations and not initializations.

Source

• https://github.com/kennymkchan/interview-questions-in-javascript

There are two types of hoisting:

• variable hoisting - rare
• function hoisting - more common

Wherever a var (or function declaration) appears inside a scope, that declaration is taken to belong to the entire scope and accessible everywhere throughout.

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var a = 2;

foo();  // works because `foo()`

        // declaration is "hoisted"

​

function foo() {

  a = 3;

  console.log( a );   // 3

  var a;              // declaration is "hoisted"

                            // to the top of `foo()`

}

​

console.log( a ); // 2