Variable Identifiers

  1. Naming rules
  • Start with a letter, underscore, or $
  • Contain letters, underscores, $, and digits

Examples of valid and invalid names

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var abc;
var _abc;
var $abc;
var _abc1$;
function $$abc_(){}
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var 1abc;
var *abc;
var abc&;
function #abc(){}

Keywords and Reserved Words

Case Sensitivity

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var Arr = [1,2,3];
var arr = function(){
	For(var i = 0; i < A.length; i++){
		console.log(A[i]);
	}
}
for is a keyword and must be lowercase

Strict Mode

Why strict mode was introduced
Benefits

  • Removes unreasonable, loose, and unsafe parts of JS syntax, reduces quirky behavior, and keeps code running safely
  • Makes the compiler more efficient and code run faster

Usage
Enable strict mode globally

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<script type="text/javascript">
	"user strict";
	(function)(){
		console.log("hello world!");
	})()
</script>

Enable strict mode inside a function

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<script type="text/javascript">
	(function)(){
		"user strict";
		console.log("hello world!");
	})()
</script>

Differences between strict mode and normal mode

  • In strict mode, you can't implicitly declare and define variables
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(function)(){
use "strict"
abc = 0 // ReferenceError
})()
  • In strict mode, objects can't have duplicate property names
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use "strict"
var obj = {a:1,b:2,a:3}
// SyntaxError
  • You can't call arguments.callee (used for recursion in anonymous functions)
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var count = 0;
(function)(){
    user "strict"
    if (count > 10){
        return;
    }
    count++;
    arguments.callee(); //TypeError
})();
  • The with statement
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    var x,y;
    (function)(){
        "use strict";
        with(Math) {    //TypeError
            x = cos(3 * PI) + sin (LN10);
            Y = tan(14 * E);
        }
    })();

Strict mode has other restrictions too

Comments

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/*
This is a JS multi-line comment
Both can span multiple lines
*/
// This is a JS single-line comment