Case Sensitivity

JavaScript is case-sensitive. This means that Hello() is not the same as HELLO() or hello().

Whitespace

Whitespace can be: extra indents, line breaks, and spaces. JavaScript ignores it, but it makes the code easier for people to read.

The following is JavaScript with very little whitespace.

function filterEmailKeys(event){
event=event||window.event;
var charCode=event.charCode||event.keyCode;
var char=String.fromCharCode(charCode);
if(/[a-zA-Z0-9_\-\.@]/.exec(char))
return true;
return false;
}

The following is the same JavaScript with a typical amount of whitespace.

function filterEmailKeys(event) {
  event = event || window.event;
  var charCode = event.charCode || event.keyCode;
  var char = String.fromCharCode(charCode);
  if (/[a-zA-Z0-9_\-\.@]/.exec(char)) {
    return true;
  }
  return false;
}

The following is the same JavaScript with a lot of whitespace.

function filterEmailKeys( evt )
  {
    evt = evt || window.event;

    var charCode = evt.charCode || evt.keyCode;
    var char = String.fromCharCode ( charCode );

    if ( /[a-zA-Z0-9_\-\.@]/.exec ( char ) )
      {
        return true;
      }

    return false;
  }

Comments

Comments allow you to leave notes in your code to help other people understand it. They also allow you to comment out code that you want to hide from the parser, but you don't want to delete.

Single-line comments

A double slash, //, turns all of the following text on the same line into a comment that will not be processed by the JavaScript interpreter.

// Shows a welcome message
alert("Hello, World!")

Multi-line comments

Multi-line comments are begun with slash asterisk, /*, and end with the reverse asterisk slash, */.

Here is an example of how to use the different types of commenting techniques.

/* This is a multi-line comment
that contains multiple lines
of commented text. */
var a = 1;
/* commented out to perform further testing
a = a + 2;
a = a / (a - 3); // is something wrong here?
*/
alert('a: ' + a);

Semicolons

In many computer languages, semicolons are required at the end of each code statement. In JavaScript the use of semicolons is optional, as a new line indicates the end of the statement. This is called automatic semicolon insertion, and the rules for it are quite complex.^[1] Leaving out semicolons and allowing the parser to automatically insert them can create complex problems.

a = b + c
(d + e).print()

The above code is not interpreted as two statements. Because of the parentheses on the second line, JavaScript interprets the above as if it were

a = b + c(d + e).print();

when instead, you may have meant it to be interpreted as

a = b + c;
(d + e).print();

Even though semicolons are optional, it's preferable to end statements with a semicolon to prevent any misunderstandings from taking place.

Literals

A literal is a hard coded value. Literals provide a means of expressing specific values in your script. For example, to the right of the equals sign:

var myLiteral = "a fixed value";

There are several types of literals available. The most common are the string literals, but there are also integer and floating-point literals, array and boolean literals, and object literals.

Example of an object literal:

var myObject = { name:"value", anotherName:"anotherValue"};

Details of these different types are covered in Variables and Types.

Identifiers

An identifier is a name for a piece of data such as a variable, array, or function. There are rules:

• Letters, dollar signs, underscores, and numbers are allowed in identifiers.
• The first character cannot be a number.

Examples of valid identifiers:

• u
• $hello
• _Hello
• hello90

1A2B3C is an invalid identifier, as it starts with a number.

References

1. ↑ Standard ECMA-262 ECMAScript Language Specification, Chapter 7.9 - Automatic Semicolon Insertion