The concept of default parameters is a new feature introduced in the ES6 version of JavaScript. This allows us to give default values to function parameters. Let’s take an example,

function sum(x = 3, y = 5) {

    // return sum
    return x + y;
}

console.log(sum(5, 15));  // 20 
console.log(sum(7));        // 12
console.log(sum());          // 8

In the above example, the default value of x is 3 and the default value of y is 5.

• sum(5, 15) – When both arguments are passed, x takes 5 and y takes 15.
• sum(7) – When 7 is passed to the sum() function, x takes 7 and y takes default value 5.
• sum() – When no argument is passed to the sum() function, x takes default value 3 and y takes default value 5.

Using Expressions as Default Values

It is also possible to provide expressions as default values.

Example 1: Passing Parameter as Default Values

function sum(x = 1, y = x,  z = x + y) {
    console.log( x + y + z );
}

sum(); // 4

In the above program,

• The default value of x is 1
• The default value of y is set to x parameter
• The default value of z is the sum of x and y

If you reference the parameter that has not been initialized yet, you will get an error. For example,

function sum( x = y, y = 1 ) {
    console.log( x + y);
}

sum(); 

Output

ReferenceError: Cannot access 'y' before initialization

Example 2: Passing Function Value as Default Value

// using a function in default value expression

const sum = () => 15;

const calculate = function( x, y = x * sum() ) {
    return x + y;
}

const result = calculate(10);
console.log(result);            // 160

In the above program,

• 10 is passed to the calculate() function.
• x becomes 10, and y becomes 150 (the sum function returns 15).
• The result will be 160.

Passing undefined Value

In JavaScript, when you pass undefined to a default parameter function, the function takes the default value. For example,

function test(x = 1) {
  console.log(x);
}

// passing undefined
// takes default value 1
test(undefined); // 1