# Scenario

We wish to have a function where we can pass in any number of arguments and add them together. So for example:

`sum(1,2)`

or

`sum(1,2,3)`

Also we want another function where we can subtract the first argument from the sum of the rest. Like this:

addThenSubtract(1,2,3) // Result would be 4addThenSubtract(1,2,3,4) // Result would be 8

The SPREAD operator is also expressed as `...`

. We will see this used when we implement a`ddThenSubtract`

.

# Approach

## Sum

We `implement`

sum like this.

`function sum(...arr: number[]): number {`

let sum = 0;

arr.forEach((v) => {

sum += v;

});

return sum;

}

The REST `…`

operator collects all the arguments into an an array `arr`

for us, and we then calculate the sum.

## addThenSubtract

Here we can subtract the first argument from the sum of the rest.

`/**`

* @param subtract Subract this number from the numbers added

* @param add Add tese numbers together

*/

function addThenSubtract(subtract: number, ...add: number[]) {

console.log(`subtract ${subtract}`);

console.log(`add ${add}`);

const total = sum(...add);

return total - subtract;

}

Note that when we call `add`

we use an operator that looks like the REST operator `...`

. In this context it’s a SPREAD operator. It takes the values in the array and spreads them out. So if we pass in `[1,2,3]`

it will become `1,2,3`

.