strict-mode.md

Strict mode

Strict mode makes several changes to normal JavaScript semantics:

• Eliminates some JavaScript silent errors by changing them to throw errors. It prevents, or throws errors, when relatively "unsafe" actions are taken (such as gaining access to the global object).
• Fixes mistakes that make it difficult for JavaScript engines to perform optimizations: strict mode code 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. Words like: implements, interface, let, package, private, protected, public, static, yield can't be used as name of variables or functions

Example

Silent failing assignments will throw error in strict mode

NaN = 5; // error

Example

Using an object, without declaring it, is not allowed:

"use strict";
x = 3.14;

Example

Deleting a variable (or object) is not allowed.

"use strict";
var x = 3.14;
delete x;

Example

Writing to a get-only property is not allowed:

"use strict";
var obj = {
  get x() {
    return 0;
  }
};
obj.x = 3.14; // This will cause an error

Example

Deleting an undeletable property is not allowed:

"use strict";
delete Object.prototype; // This will cause an error