JavaScript Best Practices

Avoid Global Variables

Minimize the use of global variables.

Always Declare Local Variables

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

Declarations on Top

This will:

Give cleaner code

Reduce the possibility of unwanted re-declarations

// Declare at the beginning let firstName, lastName, price, discount, fullPrice;
// Use later firstName = "John";
lastName = "Doe";
price = 19.90;
discount = 0.10;
fullPrice = price - discount;

for (let i = 0; i < 5; i++)
{

Initialize Variables

This will:

Give cleaner code

Provide a single place to initialize variables

Avoid undefined values

// Declare and initiate at the beginning let firstName = "";
let lastName = "";
let price = 0;
let discount = 0;
let fullPrice = 0, const myArray = [];
const myObject = {};
Initializing variables provides an idea of the intended use (and intended data type).

Re-Declaring JavaScript Variables

If you re-declare a JavaScript variable declared with var, it will not lose its value.
The variable carName will still have the value "Volvo" after the execution of these statements:

Not

Recommended)

var carName = "Volvo";
var carName;
You cannot re-declare a variable declared with let or const.

Examples

This will not work:

let carName = "Volvo";
let carName;

This will not work:

const carName = "Volvo";
const carName;

Declare Objects with const

Example

let car = {type:"Fiat", model:"500", color:"white"};
car = "Fiat";      // Changes object to string const car = {type:"Fiat", model:"500", color:"white"};
car = "Fiat";      // Not possible

Declare Arrays with const

Example

let cars = ["Saab", "Volvo", "BMW"];
cars = 3;    // Changes array to number const cars = ["Saab", "Volvo", "BMW"];
cars = 3;    // Not possible

Don't Use new Object()

Use