Understanding Default Parameters in JavaScript

by

In JavaScript, default parameters are used when a value for a given parameter is not provided or is “undefined”. This feature was introduced in ECMAScript 2015 (ES6) and allows for more concise and readable code.

Syntax

function functionName(parameter1 = defaultValue1, parameter2 = defaultValue2, ...) {
  // function body
}

How to set default parameters in JavaScript

To set default parameters for a function in JavaScript, you can use the “equal sign (=) followed by the default value”. Default parameters are used when a value for a given parameter is not provided or is undefined. This feature was introduced in ECMAScript 2015 (ES6).

Example 1

function greet(name = 'Anonymous', age = 0) {
  console.log(`Hello, ${name}! You are ${age} years old.`);
}

greet();
greet('John');
greet('John', 30);
greet(undefined, 25);

Output

Hello, Anonymous! You are 0 years old.
Hello, John! You are 0 years old.
Hello, John! You are 30 years old.
Hello, Anonymous! You are 25 years old.

In the example above, the “greet()” function has two parameters: name and age, with default values ‘Anonymous’ and 0, respectively.

If you call the function without any arguments or with “undefined” as the value for a parameter, the default value will be used instead.

Example 2

Default parameter values can also be expressions or function calls, which are evaluated only when the default value is needed

function getDefaultAge() {
  return 18;
}

function greet(name = 'Anonymous', age = getDefaultAge()) {
  console.log(`Hello, ${name}! You are ${age} years old.`);
}

greet('Niva');

Output

Hello, Niva! You are 18 years old.

That’s it.

Leave a Comment