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 or the let keyword, otherwise they will become global variables.

Strict mode does not allow undeclared variables.

Declarations on Top
It is a good coding practice to put all declarations at the top of each script or function.

This will:

Give cleaner code
Provide a single place to look for local variables
Make it easier to avoid unwanted (implied) global variables
Reduce the possibility of unwanted re-declarations
// Declare at the beginning
var firstName, lastName, price, discount, fullPrice;

// Use later
firstName = "John";
lastName = "Doe";

price = 19.90;
discount = 0.10;

fullPrice = price * 100 / discount;
This also goes for loop variables:

// Declare at the beginning
var i;

// Use later
for (i = 0; i < 5; i++) {
By default, JavaScript moves all declarations to the top (JavaScript Hoisting).

Initialize Variables
It is a good coding practice to initialize variables when you declare them.

This will:

Give cleaner code
Provide a single place to initialize variables
Avoid undefined values
// Declare and initiate at the beginning
var firstName = "",
lastName = "",
price = 0,
discount = 0,
fullPrice = 0,
myArray = [],
myObject = {};
Initializing variables provides an idea of the intended use (and intended data type).

Never Declare Number, String, or Boolean Objects
Always treat numbers, strings, or booleans as primitive values. Not as objects.

Declaring these types 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.
Or even worse:

Example
var x = new String("John");           
var y = new String("John");
(x == y) // is false because you cannot compare objects.
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 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
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
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
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;
  }
}
ECMAScript 2015 allows default parameters in the function call:

function (a=1, b=1) { /*function code*/ }
Read more about function parameters and arguments at Function Parameters

End Your Switches with Defaults
Always end your switch statements with a default. Even if you think there is no need for it.

Example
switch (new Date().getDay()) {
  case 0:
    day = "Sunday";
    break;
  case 1:
    day = "Monday";
    break;
  case 2:
    day = "Tuesday";
    break;
  case 3:
    day = "Wednesday";
    break;
  case 4:
    day = "Thursday";
    break;
  case 5:
    day = "Friday";
    break;
  case 6:
    day = "Saturday";
    break;
  default:
    day = "Unknown";
}
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.