Strict Mode

strict mode is a way to opt in to a restricted variant of JavaScript, this was introduced in ECMAscript 5. To use strict mode just place 'use strict'; before any statements or with in a function.

Some Key Changes

1. Makes debugging easier by highlighting/throwing errors for common mistakes/typos.

2. Prevents accidental globals.

                            // Assuming a global variable mistypedVariable exists
   mistypedVaraible = 17;   // this line throws a ReferenceError due to the
                            // mispelling of variable
   

3. Throws error deletion of undeletable properties.

   "use strict";
   delete Object.prototype; // throws a TypeError
   

4. Disallows duplicate property names or parameter values.

   "use strict";
   //property name
   var o = { p: 1, p: 2 }; // !!! syntax error
   //Parameter name
   function sum(a, a, c){ // !!! syntax error
    "use strict";
     return a + b + c; // wrong if this code ran
   }
   

5. Makes eval() safer. Variables and functions declared inside of an eval() statement are not created in the containing scope, strict mode also forbids deleting plain names.

   "use strict";
   eval("var x; delete x;"); // !!! syntax error
   

For more reading see:

MDN Strict Mode

MDN Strict Mode Transition