Default Parameters

A JavaScript function can have default parameter values. Using default function parameters, you can initialize formal parameters with default values. If you do not initialize a parameter with some value, then the default value of the parameter is undefined.

Example 1 Without passing argument value

function Hello(name) {
    console.log('Value Of Name parameter = ' + name )
}

Hello();

Output
Value Of Name parameter = undefined

Example 2 passing argument value

function Hello(name) {
    console.log('Value Of Name parameter = ' + name )
}

Hello("Hello");

Output
Value Of Name parameter = Hello

Example 1 With Set Parameters Default vlaue

// Add Name parameter default value is 'Hello World' check below
    
function Hello( name='Hello World' ) { 
   console.log('Value Of Name parameter = ' + name )
}

// Calling without passing arguments show the default value

Hello(); 

Output
Value Of Name parameter = Hello World

Example With Pass Default Parameters 2

// Add Name parameter default value is 'Hello World' check below
    
function Hello( name='Hello World' ) { 
   console.log('Value Of Name parameter = ' + name )
}

// Calling Passing arguments value Overwrite the default value

Hello('Overwrite the default parameter Value'); 

Output
Value Of Name parameter = Overwrite the default parameter Value

Example With Pass 'undefined' in function argument

function Hello( name='Hello World' ) { 
    console.log('Value Of Name parameter = ' + name )
}


Hello(undefined);

Output
Value Of Name parameter = Hello World

Even if you explicitly pass undefined as the parameter value when calling the function, the parameter value will be set to the default value.

Reusing Default Parameters

function foo(num1 = 9, num2 = num1 + 8){
    console.log('Value Of Num2 = ' + num2);
}

foo();

Output

Value Of Num2 = 17

Example With Passing 3 Arguments

function Sum(a, b=70 ,c) { 
    console.log(`A = ${a}, B = ${b}, C = ${c}`)
}

Sum(10, 20 , 30);

Output
A = 10, B = 20, C = 30

Example With Passing 2 Arguments

function Sum(a, b=70 ,c) { 
    console.log(`A = ${a}, B = ${b}, C = ${c}`)
}

Sum(10, 20);

Output
A = 10, B = 20, C = undefined