JavaScript Best Practices

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Avoid global variables,  avoid new,  avoid  ==,  avoid eval()

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Avoid Global Variables

Avoid using global variables.

This includes all data types, objects, and functions.

Global variables and functions can be overwritten by other scripts.

Use local variables instead, and learn how to use closures.

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Always Declare Local Variables

All variables used in a function should be declared as local variables.

Local variables must be declared with the var keyword, otherwise they will become global variables.

	Strict mode does not allow undeclared variables.

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Never Declare Numbers, Strings, or Booleans as Objects

Always treat numbers, strings, or booleans as primitive values. Not as objects.

Declaring numbers, strings, or booleans as objects, slows down execution speed, and produces nasty side effects:

Example

var x = "John";            
var y = new String("John");
(x === y) // is false because x is a string and y is an object.

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Don't Use new Object()

• Use {} instead of new Object()
• Use "" instead of new String()
• Use 0 instead of new Number()
• Use false instead of new Boolean()
• Use [] instead of new Array()
• Use /(:)/ instead of new RegExp()
• Use function (){} instead of new function()

Example

var x1 = {};           // new objectvar x2 = "";           // new primitive stringvar x3 = 0;            // new primitive numbervar x4 = false;        // new primitive booleanvar x5 = [];           // new array objectvar x6 = /()/          // new regexp objectvar x7 = function(){}; // new function object

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Beware of Automatic Type Conversions

Beware that numbers can accidentally be converted to strings or NaN (Not a Number).

JavaScript is loosely typed. A variable can contain different data types, and a variable can change its data type:

Example

var x = "Hello";     // typeof x is a string 

x = 5;               // changes typeof x to a number

When doing mathematical operations, JavaScript can convert numbers to strings:

Example

var x = 5 + 7;       // x.valueOf() is 12,  typeof x is a number

var x = 5 + "7";     // x.valueOf() is 57,  typeof x is a string

var x = "5" + 7;     // x.valueOf() is 57,  typeof x is a string

var x = 5 - 7;       // x.valueOf() is -2,  typeof x is a number

var x = 5 - "7";     // x.valueOf() is -2,  typeof x is a number

var x = "5" - 7;     // x.valueOf() is -2,  typeof x is a number

var x = 5 - "x";     // x.valueOf() is NaN, typeof x is a number

Subtracting a string from a string, does not generate an error but returns NaN (Not a Number):

Example

"Hello" - "Dolly"    // returns NaN

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Use === Comparison

The == comparison operator always converts (to matching types) before comparison.

The === operator forces comparison of values and type:

Example

0 == "";        // true

1 == "1";       // true

1 == true;      // true
0 === "";       // false

1 === "1";      // false

1 === true;     // false

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Never End a Definition with a Comma

Bad Examples

points = [40, 100, 1, 5, 25, 10, ];

person = {firstName:"John", lastName:"Doe", age:46, }

Some JSON and JavaScript engines will fail, or behave unexpectedly.

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Use Parameter Defaults

If a function is called with a missing argument, the value of the missing argument is set to undefined.

Undefined values can break your code. It is a good habit to assign default values to arguments.

Example

function myFunction(x, y) {
    if (y === undefined) {
        y = 0;
    }
}

Or, even simpler:

function myFunction(x, y) {
    y = y || 0;
}

Read more about function parameters and arguments at Function Parameters

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Avoid Using eval()

The eval() function is used to run text as code. In almost all cases, it should not be necessary to use it.

Because it allows arbitrary code to be run, it also represents a security problem.