Function hoisting and variable hoisting

Let's look at two similar codes first

a = 2;var a ;console.log(a);

console.log(a);var a = 2;

The first output is 2

The second output value is undefined

You may be wondering why?

Let's look into it

This is because all declarations including variables and functions are processed first before any code is executed

So the first code actually handles this

var a;a = 2;console.log(a);

The second code is handled like this

var a ;console.log(a);a = 2;

This process is as if variable and function declarations are "moved" to the top from where they appear in the code.This process is called ascension

Note: Only the declaration itself will be hoisted, and assignments or other run logic will be left in place.If hoisting changes the order in which code is executed, it can cause very serious damage.Function declarations can be hoisted, but function expressions cannot be hoisted.

Function first

Both function declarations and variable declarations are hoisted, but in duplicated code, functions are hoisted first, then variables

foo();//1var foo;function foo() {console.log(1);}foo = function() {console.log(2);}

This will output 1 instead of 2, this code will be understood by the engine as the following format

function foo(){console.log(1);}foo(); // 1foo = function(){console.log(2);}

Here var foo although appears before the declaration of function foo()......, but because it is a duplicate declaration (and thus ignored) because when there is a duplicate declaration, the function declarationwill be hoisted before variable declarations.