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Set a default parameter value for a JavaScript function

The default parameters in the function allow named parameters to be initialized with default values if no value or undefined is passed. Here, we will learn how we can set default parameters for a function?
Submitted by Pratishtha Saxena, on May 25, 2022

First, let's understand what is parameter in a function. What is the difference between a parameter and an argument?

When a function is declared in JavaScript, the values that are specified are the parameters. Whereas, the values given when a function is called are called arguments.

function sum(a, b) {
	return a + b;
}

sum(5, 4);

Here, a and b are parameters while 5 and 4 are arguments for the function.

Now, to set a default parameter value for a JavaScript function we use the following way.

Syntax:

function functionName(param1 = default1, param2 = default2, ...) {
	// function body
}

A default value is set for the parameters in the function. While calling the function, if the arguments are specified then the function takes those values, but if no arguments are passed then the function will return the result by considering the default parameters.

Let's understand this with the help of an example.

function mul(x = 4, y = 50) {
	// return mul
	return x * y;
}

console.log(mul(2, 17));
console.log(mul(3));
console.log(mul());

Output:

34
150
200

Here, the default parameters are 4 and 50. So, if the arguments are not passed then these values will be taken into consideration.

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