JavaScript Program to Set a Default Parameter Value For a Function

The syntax to set the default parameter value for a function is:

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

Example 1: Set Default Parameter Value For a Function

// program to set default parameter value

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

    // return sum
    return x + y;
}

console.log(sum(5, 15));
console.log(sum(7));
console.log(sum());

Output

20
12
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 the default value 5.
• sum() - When no argument is passed to the sum() function, x takes the default value 3 and y takes the default value 5.

Example 2: Using Previous Parameter in Another Parameter

// using previous parameter in default value expression

let calculate = function(x = 15, y = x + 2) {
    return x + y;
}

const result1 = calculate(10);
console.log(result1);

const result2 = calculate();
console.log(result2);

Output

22
32

You can also pass a parameter as the default value for another parameter.

In the above program,

• When 10 is passed to the calculate() function, x becomes 10, and y becomes 12 (the sum function returns 22).
• When no value is passed to the calculate() function, x becomes 15, and y becomes 17 (the sum function returns 32).