Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope (ie: the top of the current script or the current function).

In JavaScript, a variable can be declared after it has been used. In other words; a variable can be used before it has been declared.

x = 5; // Assign 5 to x
elem = document.getElementById("demo"); // Find an element
elem.innerHTML = x;                     // Display x in the element
var x; // Declare x

Variables and constants declared with let or const are not hoisted!

JavaScript only hoists declarations, not initializations.

var x = 5; // Initialize x
var y = 7; // Initialize y
elem = document.getElementById("demo"); // Find an element
elem.innerHTML = x + " " + y;           // Display x and y

Hoisting is (to many developers) an unknown or overlooked behavior of JavaScript. If a developer doesn't understand hoisting, programs may contain bugs (errors). To avoid bugs, always declare all variables at the beginning of every scope. Since this is how JavaScript interprets the code, it is always a good rule. JavaScript in strict mode does not allow variables to be used if they are not declared.

Hoisting applies to variable declarations and to function declarations. Because of this, JavaScript functions can be called before they are declared:

myFunction(5);

function myFunction(y) {
  return y * y;
}

Functions defined using an expression are not hoisted.