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Start your free trial###### rymatech

4,671 Points# Can someone explain how this sum code works?

```
var sum = [1, 2, 3].reduce((a, b) => a + b, 0);
console.log(sum); // 6
```

I know the above code calculates the sum of an array but how?

"a" and "b" are the parameters, then "a" and "b" are added together but what is the 0 for?

## 1 Answer

###### Moosa Bonomali

6,297 PointsThe reduce function is executed as follows;

```
arr.reduce(callback[, initialValue])
```

The initialValue is optional. In your case 0 is the initial value. In this case it may not be necessary to include the 0, if you had a different initial value, then this value would have been added to sum of your array.

The callback function takes 4 parameters, 2 of which are required. In your case a and b.

```
(a , b)
```

a is the accumulator - it keeps track of the total so far b is the current value been processed. The return value of the callback function

```
=> a + b
```

will be stored in a until all array elements are processed.