• Hoisting is when during the compilation phase, JS moves the declaration to the top of the code. For example

// Code 1
a = 2;
var a;
console.log(a); // 2

// This will be processed as
var a;  // declaration moved to top
a = 2;
console.log(a);

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

// This will be processed as
var a; // declaration moved to the top
console.log(a); // undefined
a = 2;

• In the second example above, var a = 2 is first broken into var a (the declaration) and a = 2 (assignment). The declaration part only moves to the top, not the assignment because of this tokenization during compilation.
• Hoisting happens per-scope.
• The functions are always hoisted first and then the variables.

foo(); // 1

var foo;

function foo() {
	console.log(1);
}

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

// This will be interpreted as
function foo() {
	console.log(1);
}

foo(); // 1

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

// Here var foo; was just a redundant line