ES6 : Strict Mode - wesleyduff/Blog-and-Tutorials GitHub Wiki

#Strict Mode

Strict mode was introduced in ECMAScript 5 to clean up the language. Make sure to feature-test for strict mode if you want to use it. It is switched on by adding by putting the following line first in a file or in a function

'use strict';

##Facts

• Intentionally changes semantics for normal code.
• Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do support strict mode.
• Eliminates some JavaScript silent errors by changing them to throw errors.
• Fixes mistakes that make it difficult for JavaScript engines to perform optimizations.
• Can sometimes be made to run faster than identical code that's not strict mode.
• Prohibits some syntax likely to be defined in future versions of ECMAScript.

###Breaking changes:

• with statement is forbidden. The "with" statement lets users add arbitrary objects to the chain of variable scopes, which slows down execution and makes it tricky to figure out what a variable refers to.
• Deleting a variable is forbidden.
• Functions can only be declared at the top level of a scope.
• More keywords are reserved :
  • implements
  • interface
  • let
  • package
  • private
  • protected
  • public
  • static
  • yield

###More Errors

• Assigning to an undeclared variable causes a ReferenceError.
  • In non-strict mode the variable becomes a global variable.

'use strict';
mystring = "test";

//Uncaught ReferenceError: mystring is not defined

codepen

• Changing read-only properties cause a TypeError. In non-strict mode, it has no effects.

'use strict';
var myString = "This is a  length of a string";
myString.length = 3;

//Uncaught TypeError: Cannot assign to read only property 'length' of string 'This is a  length of a string'

codepen