Hoisting variables in JavaScript

This is how the interpreter sees your code,

do_something() {
 var foo;
 console.log(foo); // undefined
 foo = 2;
}

do_something();

So it is printing undefined. This is a basic of variable hoisting. Your declarations will be moved to the top, and your assignation will remain in the same place. And the case is different when you use let over var.

Javascript only hoists declarations, not initializations.

var x = y, y = 'A';
console.log(x + y); // undefinedA

Here, x and y are declared before any code is executed, the assignments occur later. At the time "x = y" is evaluated, y exists so no ReferenceError is thrown and its value is 'undefined'. So, x is assigned the undefined value. Then, y is assigned a value of 'A'. Consequently, after the first line, x === undefined && y === 'A', hence the result.

Variable Declaration

Only the declaration is hoisted. the assigned variables are not hoisted. So you are