JavaScript Best Practices

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

Avoid Global Variables

Minimize the use of 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.

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.

Note Strict mode does not allow undeclared variables.

Declarations on Top

It is good coding practice, to put all declarations at the top of each script or function. This makes it easier to avoid unwanted (implied) global variables. It also gives cleaner code, and reduces the possibility of unwanted re-declarations.

var firstName, lastName;
var price, discount, fullPrice;

firstName = "John";
lastName = "Doe";

price = 19.90;
discount = 0.10;

fullPrice = price * 100 / discount;

Variable declarations should always be the first statements in scripts and functions.

This also goes for variables in loops:

var i;
for (i = 0; i < 5; i++)

Note Since JavaScript moves the declarations to the top anyway (JavaScript hoisting), it is always a good rule.

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:


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

Try it yourself »

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()


var x1 = {};           // new object
var x2 = "";           // new primitive string
var x3 = 0;            // new primitive number
var x4 = false;        // new primitive boolean
var x5 = [];           // new array object
var x6 = /()/;         // new regexp object
var x7 = function(){}; // new function object

Try it Yourself »

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:


var x = "Hello";     // typeof x is a string
x = 5;               // changes typeof x to a number

Try it yourself »

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


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

Try it yourself »

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


"Hello" - "Dolly"    // returns NaN

Try it yourself »

Use === Comparison

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

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


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

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

Try it yourself »

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.


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

Or, even simpler:

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

Try it Yourself »

Read more about function parameters and arguments at Function Parameters

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.

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