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
let firstName, lastName, price, discount, fullPrice;
// Use later
firstName = "John";
lastName = "Doe";
price = 19.90;
discount = 0.10;
fullPrice = price - discount;
This also goes for loop variables:
for (let i = 0; i < 5; i++)
{
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
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).
Declare Objects with const
Declaring objects with const will prevent any accidential change of type:
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
Declaring arrays with const will prevent any accidential change of type:
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
""instead ofnew String() - Use
0instead ofnew Number() - Use
falseinstead ofnew Boolean() - Use
{}instead ofnew Object() - Use
[]instead ofnew Array() - Use
/()/instead ofnew RegExp() - Use
function (){}instead ofnew Function()
Example
let x1 = ""; // new primitive string
let x2 = 0; // new primitive number
let x3 = false; // new primitive boolean
const x4 = {}; // new object
const x5 = []; // new array object
const x6 = /()/; // new regexp object
const x7 = function(){}; // new function object
Try it Yourself »
Beware of Automatic Type Conversions
JavaScript is loosely typed.
A variables can contain all data types.
A variable can change its data type:
Example
let x = "Hello"; // typeof x is a string
x = 5; // changes typeof x to a number
Try it Yourself »
Beware that numbers can accidentally be converted to strings or NaN (Not a
Number).
When doing mathematical operations, JavaScript can convert numbers to strings:
Example
let x = 5 + 7; // x.valueOf() is 12, typeof x is a number
let x = 5 + "7"; // x.valueOf() is 57, typeof x is a string
let x = "5" + 7; // x.valueOf() is 57, typeof x is a string
let x = 5 - 7; // x.valueOf() is -2, typeof x is a number
let x = 5 - "7"; // x.valueOf() is -2, typeof x is a number
let x = "5" - 7; // x.valueOf() is -2, typeof x is a number
let 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):
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
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.
ECMAScript 2015 allows default parameters in the function definition:
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";
}
Try it Yourself »
Avoid Number, String, and Boolean as 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
let x = "John";
let y = new String("John");
(x === y) // is false because x is a string and y is an object.
Try it Yourself »
Or even worse:
Example
let x = new String("John");
let y = new String("John");
(x == y) // is false because you cannot compare objects.
Try it Yourself »
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.
