rest parameter in javascript

The rest parameter is introduced in ES6. It allows a function to accept an indefinite number of arguments as an array that is we can call a function with as many arguments as we want and the rest parameter will gather all these arguments into an array.

See the below example to understand how:

Example 1:

function testing(...numbers){
       console.log(numbers)
       console.log(numbers[0])
       console.log(numbers[2])
}

testing(2, 16, 7, 4)

//Result 1: [2, 16, 7, 4]
//Result 2: 2
//Result 3: 7

The syntax of the rest parameter is just 3 dots followed by the array name. In the above ☝️ example, we do not have to know how many arguments are there in order to define parameters. Rather, we can simply use the rest parameter. Using the rest parameter we can tackle an infinite number of arguments.

Let's see some more examples:
Example 2:

function testing(a, ...numbers){
       console.log(a)
       console.log(numbers)
}

testing(2, 16, 7, 4)

//Result 1: 2 
//Result 2: [16, 7, 4]

In the above example, we can see that the parameter "a" is assigned with 2, and the rest of the arguments are stored in the numbers array.

NOTE: The rest parameter must always be the last parameter. The below code will result in an error. Example 3:

function testing(a, ...numbers, b){
       console.log(a)
       console.log(numbers)
       console.log(b)
}

testing(2, 16, 7, 4)

//Error: Uncaught SyntaxError: Rest parameter must be last formal parameter

Example 4: Write a function to sum all the arguments provided? Number of arguments are not known.

function sumAll(...args) {
  let sum = 0;

  for (let arg of args){
        sum += arg;
    }

  return sum;
}

console.log( sumAll(1) ); 
console.log( sumAll(1, 2) ); 
console.log( sumAll(1, 2, 3) ); 

//Result 1: 1
//Result 2: 3
//Result 3: 6

That's all folks. I'll teach spread syntax and destructuring tomorrow.

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rest parameter in javascript