What does “use strict” do in JavaScript, and what is the reasoning behind it?

Question

Recently, I ran some of my JavaScript code through Crockford\'s JSLint, and it gave the following error:

Problem at line 1 character 1: Missing \"use

Answer 1

When adding "use strict";, the following cases will throw a SyntaxError before the script is executing:

• Paving the way for future ECMAScript versions, using one of the newly reserved keywords (in prevision for ECMAScript 6): implements, interface, let, package, private, protected, public, static, and yield.

• Declaring function in blocks

  if(a

• Octal syntax

  var n = 023;
  

• this point to the global object.

   function f() {
        "use strict";
        this.a = 1;
   };
   f(); 
  

• Declaring twice the same name for a property name in an object literal

   {a: 1, b: 3, a: 7} 
  

  This is no longer the case in ECMAScript 6 (bug 1041128).

• Declaring two function arguments with the same name function

  f(a, b, b){}
  

• Setting a value to an undeclared variable

  function f(x){
     "use strict";
     var a = 12;
     b = a + x*35; // error!
  }
  f();
  

• Using delete on a variable name delete myVariable;

• Using eval or arguments as variable or function argument name

  "use strict";
  arguments++;
  var obj = { set p(arguments) { } };
  try { } catch (arguments) { }
  function arguments() { } 
  

Sources:

• Transitioning to strict mode on MDN

• Strict mode on MDN

• JavaScript’s Strict Mode and Why You Should Use It on Colin J. Ihrig's blog (archived version)