---
id: 5ddb965c65d27e1512d44dad
title: Part 20
challengeType: 0
isHidden: true
---
## Description
Let's says we have an array `[1, 3, 5]` named `arr` and we want to sum up all the numbers.
We can use the reduce function as follows:
```js
arr.reduce((accumulator, currentValue) => {
return accumulator + currentValue;
}, 0);
```
At `arr[0]`, the function is `(0, 1) => { return 0 + 1 }`,
since `arr[0] = 1 = currentValue`.
At `arr[1]`, the function is `(1, 3) => 1 + 3`,
Finally at `arr[2]`, the function is `(4, 5) => 4 + 5`. Now the accumulator is `9` and since we have gone through all of the items in `arr`, the `reduce()` method will return `9`.
In the body of the callback function, replace `/* code to run */` with `return accumulator + currentValue`.
## Instructions
## Tests
```yml
tests:
- text: See description above for instructions.
testString: assert( code.replace(/\s/g, '').match(/reduce\(\(accumulator\,currentValue\)\=\>{returnaccumulator\+currentValue\;?},0\)/) );
```
## Challenge Seed
```html
```
### Before Test
### After Test
```html