How to declare the optional function parameters in JavaScript ?

Declaring optional function parameters in JavaScript means defining function parameters that aren’t required when the function is called. You can assign default values to these parameters using the = syntax, so if no argument is provided, the default value is used instead.

These are the following approaches to doing so:

Using the Logical OR operator (‘||’)

The Logical OR operator (||) is used to assign a default value to an optional parameter. If the parameter is null, undefined, or falsy, the default value is used instead, ensuring the function works even with missing arguments.

Note: The optional parameters should always come at the end of the parameter list. 

Syntax:

function myFunc(a,b) {
  b = b || 0;
  // b will be set either to b or to 0.
}

Example: In this example the check function assigns b to 0 if b is not provided. The function logs the values of a and b to the console.

function check(a, b) {
    b = b || 0;
    console.log("Value of a is: " + a +
        " Value of b is: " + b);
}
check(5, 3);
check(10);

Value of a is: 5 Value of b is: 3
Value of a is: 10 Value of b is: 0

Using the Assignment operator (“=”)

The assignment operator (`=`) approach allows you to set default values directly in the function’s parameter list. If an argument is not provided, the parameter automatically takes the default value, simplifying the handling of optional parameters in functions.

Syntax

function myFunc(a, b = 0) {
   // function body
}

Example: In this example the check function sets b to 0 by default if no value is provided. It logs the values of a and b to the console.

function check(a, b = 0) {
    console.log("Value of a is: " + a +
        " Value of b is: " + b);
}
check(9, 10);
check(1);

Value of a is: 9 Value of b is: 10
Value of a is: 1 Value of b is: 0

Using argument variable

The arguments variable is an array-like object holding all arguments passed to a function. This approach checks the number of passed arguments and assigns default values or handles missing ones based on the argument count, providing flexibility in parameter handling.

Example: This exanpmle shows the implementation of the above-explained appraoch.

function gfg(a, b) {
    // No parameters are passed
    if (arguments.length == 0) {
        a = "hello";
        b = "geeks"
    }
    // Only one parameter is passed
    if (arguments.length == 1) {
        b = "geeks";
    }
    return `${a + b}`;
}

console.log(gfg("hey"));

heygeeks

Using Rest Parameters

The rest parameter syntax (…rest) allows you to collect all remaining arguments into an array. This can be particularly useful for functions that can accept a variable number of arguments.

Syntax:

function myFunc(a, ...rest) {
    // function body
}

Example: Using rest parameters to handle optional parameters

// Function that uses rest parameters to handle optional parameters
function check(a, ...rest) {
    let b = rest.length > 0 ? rest[0] : 0;
    console.log(`Value of a is: ${a} Value of b is: ${b}`);
}

// Calling the function with both parameters
check(5, 3); 

// Calling the function with only the first parameter
check(10); 

// Calling the function with more than two parameters
check(1, 2, 3); 

Value of a is: 5 Value of b is: 3
Value of a is: 10 Value of b is: 0
Value of a is: 1 Value of b is: 2