Hoisting

It is a term used to explain the behaviour of variable declarations in our code.

when var is used to declare a variable, it is hoisted (moved up) to the top of its scope, hovever only the declaration is hoisting, assignment stays.

this is because the Javascript engine parses the declarations during compilation, and the assignments at runtime.

console.log(foo); // undefined
var foo = 1;
console.log(foo); // 1

Function declarations are also hoisted but their function expressions only has their variable declaration hoisted

// Function Declaration
console.log(foo); // [Function: foo]
foo(); // 'FOOOOO'
function foo() {
  console.log("FOOOOO");
}
console.log(foo); // [Function: foo]
 
// Function Expression
console.log(bar); // undefined
bar(); // Uncaught TypeError: bar is not a function
var bar = function () {
  console.log("BARRRR");
};
console.log(bar); // [Function: bar]

Variables declared with let and const are hoisted but unlike var they are not initialized to undefined. They are in a state called "temporal dead zone" where they are not accessible.

foo; // undefined
bar; // ReferenceError: bar is not defined
 
var foo = "hello";
let bar = "world";

References

Generators Promises