Usestrict(厳格モード)

なぜUse Strictを使うのか

UseStrictの適用

1.アプリケーションコードの前

"use strict"

let v = "use strict example";

2.関数への適用

function strict(){
 'use strict';
  function online() {return "hi my name is bohun"}
  }

3.strictモードで許可されていない構文

'use strict'
x = 3 //index.js:3 Uncaught ReferenceError: x is not defined

'use strict'
x = 3 
delete x //Delete of an unqualified identifier in strict mode

'use strict'
function code(x1 ,x1){}; //Duplicate parameter name not allowed in this context 

'use strict'
let x = 010; //Octal literals are not allowed in strict mode.

"use strict"; 
var obj = {};

Object.defineProperty(obj, "x", {value:0, writable:false}); obj.x = 3.14;    

//Cannot assign to read only property 'x' of object '#<Object>'

"use strict"; 
var obj = {get x() {return 0} }; obj.x = 3.14;  

// TypeError: Cannot set property x of #<Object> which has only a getter
    at index.js:2

"use strict"; 
let eval = 3.14;         
let arguments = 3.14;

// Unexpected eval or arguments in strict mode

"use strict"; 
with (Math){x = cos(2)};

//Strict mode code may not include a with statement

"use strict"; 
eval ("var x = 2"); 
alert (x);

// x is not defined