#use strict

Basically, 'use strict'; makes code more consistent and less error prone. You can read about the exact details here.

So should it be used everywhere? Contrary to most sources on the internet, the answer is no. For new projects and completely new code it should be used by default, but applying it to old code can in some rare cases cause that code to work differently. For example, here is some code made in an old JS style without 'use strict':

function addMessage(message) { var self = this || {}; self.messages = self.messages || []; self.messages.push(message); } addMessage('message1'); console.log(window.messages); // ["A message"]

The above occurs because the this value refers to the global scope in non-strict code. With adding 'use strict';:

'use strict'; function addMessage(message) { var self = this || {}; self.messages = self.messages || []; self.messages.push(message); } addMessage('message2'); console.log(window.messages); // undefined

Now this does not automatically set to the global scope in that function. Also, notice how no errors occured. In this case, unit testing that old code first before adding 'use strict'; would have caught this error. Though in some cases even unit testing may not be enough, so it is also necessary to manually analyze if old code will act differently under strict mode. Here is a good source on how to do that.

The good news, is in the future most code will automatically be in strict mode either via tools or by using new language features. For example, the upcoming language feature ES6 Modules will be supported natively in browsers and those modules are automatically in strict mode. Note these modules are different from the Node.js modules (there was no universal module format when Node.js was created). So Node.js code is not automatically in strict mode unless a compiler flag is used.

This statement indicates that the code should be executed in “strict mode”. It is only declared at the beginning of a function/script.Strict mode will help save developers from errors they didn’t even know that existed in their code.

Read the complete post at: http://www.pro-tekconsulting.com/blog/what-is-use-strict-what-are-the-advantages-and-disadvantages-to-using-it/

This post explains JavaScript's "Use Strict" directive. The notes are paraphrased from here. Some Javascript functions include the line:

"use strict";

at the top. This places the script in question into a "strict" operating mode. While in "strict" operating more, the program in question is more restricted, does stricter checking, and throws more exceptions. This can be useful because: 

by throwing more exceptions, it helps to catch common coding bloopers

It prevents or throws errors when a user accesses something global.

javascript 에서 파일 또는 함수의 시작 부분에  'use strict' 를 추가하면 

좀더 엄격한 모드 선언으로 오류 검사를 해줍니다. 

ex) 

(function() { 'use strict';

function test(){

  aa = 10; //ReferenceError : aa is not defined

})();