Best Practice in JavaScript

Avoid Global Variables

Global variables are those variables that are accessible throughout the program. These global variables should be avoided, or their usage must be minimized. This is because other scripts can overwrite them. You can replace them and use local variables. 

Declare Local Variables

In a function, all the variables present must be declared as local variables. The keywords used to declare the local variables are const, var, and let. If such declarations are not made or missed, these variables turn into global variables.

Example

/*
	Declaring variables using 
	var, let, and const.
*/

var h = 10
let i = 20
const j =30

// Printing the values of variables.


console.log(h)
console.log(i)
console.log(j)

You can also try this code with Online Javascript Compiler

Run Code

Output

Position of the Declaration

While declaring a variable, the best practice in JavaScript is to declare it at the top of the function or script. Doing this would make the code appear neater. It is easier for the programmer to understand the variable type when they know the declaration statement's position. It makes it easier to check if a variable is already declared, preventing unwanted redeclarations. It also prevents the clashing of local variable declarations with global variables.

Example

/*
	The declaration statement 
	must be at the top.
*/

let name, age

name = "lia"
age = 18
console.log(name+" "+age)

You can also try this code with Online Javascript Compiler

Run Code

Output

Initialize the Variables

While declaring, always initialize the variables. That is one of the best practices in JavaScript. It prevents storing unnecessary values and possible errors. Initializing variables while declaring them makes the code much cleaner and prevents undefined values.

Example

/*
	Declare and initialize the variables
	at the top of a function or script.
*/

let name = ""
let age = 0
console.log(name+" "+age)

You can also try this code with Online Javascript Compiler

Run Code

Output

Declaration of Objects as Constant

Declaring an object as const is considered the best practice in JavaScript. It prevents unwanted changes in types.

Example

/*
	Not declaring an object as constant
	may cause accidental changes.
*/

let candy = { type:"chocolate",color:"white"}
candy = "caramel"
console.log(candy)

You can also try this code with Online Javascript Compiler

Run Code

Output

Example

/*
	Declaring candy as const
	avoids accidental type changes.
*/

const candy = { type:"chocolate",color:"white"}
candy = "caramel"

You can also try this code with Online Javascript Compiler

Run Code

Output

Declaration of Arrays as Constants

Declaring an array as const is considered the best practice in JavaScript to prevent unwanted changes in types.

Example

/*
	Not declaring the Array as constant
	causes accidental type changes.
*/

let candy = ["caramel", "dark chocolate", "milk"];
console.log(candy)
candy = 6; 
console.log(candy)

You can also try this code with Online Javascript Compiler

Run Code

Output

Example

/*
	Declaring the array as constant
	avoids accidental type changes.
*/

const candy = ["caramel", "dark chocolate", "white chocolate"];
console.log(candy)
candy = 6; 
console.log(candy)

You can also try this code with Online Javascript Compiler

Run Code

Output

Avoid the Use of new 

Creating objects using "new" is a bad practice. It is not technically harmful, but it's lengthy and unusual. Another disadvantage of using "new" is that it may slow down the compiler. Hence, avoiding the "new" keyword is recommended for anything other than classes and constructor function instancing. Here are a few alternatives to new.

Those alternatives are: 

• "" in place of new String()
   
• 0 in place of new Number()
   
• false in place of new Boolean()
   
• {} in place of new Object()
   
• [ ] in place of new Array()
   
• /()/ in place of new RegExp()
   

Example

let primitive_string = ""

let primitive_number = 0

let boolean = false

const object = {}

const array_object = []

const regular_expression = /()/

Also Read, Javascript hasOwnProperty

Be careful about the Automatic Type Conversions

JavaScript is a loosely typed language, meaning all data types can be stored in a variable. It also means that a variable's data type can change without warning. You need to keep this in mind and be careful about accidental type conversions.

Example

// The variable x is of type string.

let x = "Hello";
console.log('Initial value of x: '+ x)
console.log('type of x: '+typeof x)

/*
	The data type of x can get
	changed without warning.
*/

x = 5;
console.log('Changed value of x: '+ x)
console.log('type of x: '+typeof x)

You can also try this code with Online Javascript Compiler

Run Code

Output

Naming variables

A variable name should convey its purpose in the code. When we choose good variable names, it becomes easy for us to debug and for someone else to use the code.

Some points to note while naming variables in JavaScript are:

• The names of variables should not have spaces in them.
   
• Always begin with a letter, underscore, or the dollar sign($)
   
• Note that variables are case-sensitive in JavaScript
   
• Do not use reserved words.
   
• Use descriptive words
   
• Follow a constant case like camel case, upper case, etc.

Use === for Comparison

The best practice in JavaScript is to use === for comparisons. When using == to compare, the values are implicitly converted to matching data types. The === doesn't change the data type, giving more accurate results.

Simply put, the "===" results true only if the operands have the same values as well as the same data types. The operator "==" returns true if the value of the operands is equal. 

Hence, it is recommended that you use "===" or "!==" instead of "==" or "!=" for Comparisons.

Example

/*
	The "==" operator implicitly changes 
	the data type of operands to compare.
*/

if(0 == "") { 
	console.log('they are equal')
}
else {
	console.log('they are not equal')
}

/*
	The "===" operator does not change
	the data type of operands to compare.
*/

if(0 === "") {
	console.log('they are equal')
}
else {
	console.log('they are not equal')
}

You can also try this code with Online Javascript Compiler

Run Code

Output

Using Parameter Defaults

When there is a missing argument when calling a function, the missing argument is assigned as undefined. This possesses many disadvantages. Automatic conversion like this may result in errors and code breakage. To rectify this problem, set a default value for the arguments.

Example

/*
	Not setting default values for 
	arguments can cause code breakage.
*/

function function1(a,b) {
	c = a+b
	console.log(c)
}

/*
	Setting default values for arguments 
	avoids code breakage.
*/

function function2(a, b) {
	if (b === undefined) {
		b = 0
	}
	c = a + b
	console.log(c)
}

/*
	Calling the function without
	parameter defaults.
*/

function1(50)

/*
	Calling the function with
	parameter defaults.
*/

function2(30)

You can also try this code with Online Javascript Compiler

Run Code

Output

Ending Switch Statements With Default

It is considered a best practice in JavaScript to end each switch statement with a default case. This would give information on what to do if any irrelevant value is entered in the switch statement.

Example

/*
	Using a default case in Switch statements
	to handle unexpected cases.
*/

let x = 4;
switch (x) {
	case 0:
		num = 0;
		break;
	case 1:
		num = 1;
		break;
	case 2:
		num = 2;
		break;
	case 3:
		num = 3;
		break;
	default:
		num = "greater than 3 or less than 0";
}
console.log(num);

You can also try this code with Online Javascript Compiler

Run Code

Output

Do not treat number, string, or boolean as an object

Number, String, and Boolean values should not be treated as objects. This is because of the following reasons:

• Treating them as objects can slow down the speed of execution.
   
• Objects can not be compared. So if you use them as objects, you won't be able to compare them.

Hence, they should be treated as primitive values for better performance of the code.

Avoid using the eval function

The eval() function in JavaScript allows us to use the JavaScript compiler in our code. It can be used to evaluate or run a string of codes. It seems useful, but its use is not recommended for three main reasons. These are:

1. It degrades the performance of your program.

2. The eval function introduces several security flaws. If the end user gives malicious input to this function, they might even crash your system or steal your data.
For example - Suppose there's a website that takes input from the user, and in the backend, the developer has coded it using the eval() function. Now, imagine if a user enters the code for an infinite loop as the input. The eval() function will evaluate it, which might end up crashing the system.

3. Using eval() function can unnecessarily complicate your otherwise simple code. That is why the code written using eval() becomes tough to debug.

var x = 1;

console.log(eval('var y = 2;\
\
function add() {\
	return x + y;\
}\
\
add();\
'));

You can also try this code with Online Javascript Compiler

Run Code

Due to reasons mentioned above, you should avoid using the eval() function.

Example

/*
	This example shows how the
	use of the eval function degrades 
	the performance of code.
*/
   
str = "for(;;){}" 
console.log(eval(str)) 

You can also try this code with Online Javascript Compiler

Run Code

Output

Let us now answer some frequently asked questions.

Frequently Asked Questions

What is the primary use of JavaScript?

Javascript is a programming or scripting language. It is used to build dynamic and interactive web content, such as browsers and applications.

Can I name a variable as "final"?

No, you cannot use reserved words to name variables in Javascript. The "final" is a reserved keyword. Other reserved keywords in Javascript include if, else, const, null, etc. 

What is the use of "==" and "===" in JavaScript?

The "==" operator compares two variables' values. The "===" operator also compares values, but it also checks the data type of the values.

Is it a good practice to use a library compared to raw JavaScript?

Using libraries can help you complete work faster. They are easier to implement than writing raw code. But always remember that libraries are slower than raw Javascript in terms of performance. As a good practice, always use raw Javascript over libraries.

Why is the use of eval() not recommended in JavaScript?

Use of the eval() function is not recommended because using it might pave the way to security problems as it may allow arbitrary code to run.

Conclusion

In this article, we have talked about the best practices in JavaScript. The use of these techniques makes the code easier to understand and gives assurance that the code will run without any problems in the future.

You can refer to these articles to learn more about JavaScript.

• Loops in Javascript
• Functions in Javascript
• Operators in Javascript
• Hello World Program in JavaScript
• String() Functions in JavaScript
• Difference Between "var", "let" and "const" in JS

You may refer to our Guided Path on Code Studios for enhancing your skill set on DSA, Competitive Programming, System Design, etc. Check out essential interview questions, practice our available mock tests, look at the interview bundle for interview preparations, and so much more!

Happy Learning, Ninjas!