Optional arguments, Default parameters and REST parameters in JavaScript

Optional Arguments

Take a look at this JavaScript functions post first before reading further: Different Types of Functions in JavaScript

Now that we have a better understanding of JavaScript Functions, we can now provide optional arguments to those functions.

JavaScript doesn’t raise an error if the function has multiple arguments, but we don’t specify them when we call the function. For example,

function printTime(times, message) {
  for(var a = 0; a < times; a++) {
    console.log(times);
  }
}
printTime(3);

//output
3
3
3 
 

JavaScript would raise an error and return undefined if we were to use message in the function but we are not using it.

Just like this example.

function multiply(a, b) {
  return a * b
}
multiply(5, 2)  // output 10
multiply(5)     // output NaN !

There are few ways to handle optional arguments if we are using those arguments in the function. One of those ways is default parameters.

Default parameters

Default function parameters allow named parameters to be initialized with default values if no value or undefined is passed.

Let’s modify the example above by adding default parameters.

function multiply(a, b = 1) {
  return a * b 
}
                
multiply(5, 2)          // output 10
multiply(5)             // output 5
multiply(5, undefined)  // output 5

REST parameters

The rest parameter syntax allows to represent an indefinite number of arguments as an array.

A function’s last parameter can be prefixed with … (3 dots) which will cause all remaining arguments to be placed within a JavaScript array.

Syntax and example:

function myFunc(...args) {
  console.log(args);
}

//example

myFunc( 1, 2, 3, 4, 5);       // output [1,2,3,4,5]

One more example:

function myFun(a, b, ...manyMoreArgs) {
  console.log("a", a)
  console.log("b", b)
  console.log("manyMoreArgs", manyMoreArgs)
}
                
myFun("one", "two", "three", "four", "five", "six")

// a, one
// b, two
// manyMoreArgs, [three, four, five, six]

In the example above, the first argument is mapped to “a” and the second to “b” and the third argument will be an array that contains the 3rd, 4th, … till nth — all the arguments we add.

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