Variable Scope in JavaScript

Variables belong to a scope, and that particular scope determines where a variable can be used. Scope is delineated with { and }

A function can use a variable declared outside of it, since it is available globally:

var world = "world";

function sayHello() {
  var hello = "Hello ";
  console.log(hello + world);

// Hello world

But trying to invoke a function’s variable from outside will result in an error: hello is not defined.

Be careful when defining global variables - you don’t want to mistakenly overwrite a value you are relying on. There are techniques to try and counter the possibility of this happening: concepts such as closures, or design patterns like the module pattern.