Iterator.prototype.reduce()
===========================

**Experimental:** **This is an experimental technology**  
Check the Browser compatibility table carefully before using this in production.

The **`reduce()`** method of `Iterator` instances is similar to `Array.prototype.reduce`: it executes a user-supplied "reducer" callback function on each element produced by the iterator, passing in the return value from the calculation on the preceding element. The final result of running the reducer across all elements is a single value.

Syntax
------


```js
reduce(callbackFn)
reduce(callbackFn, initialValue)
```

### Parameters

`callbackFn`

A function to execute for each element produced by the iterator. Its return value becomes the value of the `accumulator` parameter on the next invocation of `callbackFn`. For the last invocation, the return value becomes the return value of `reduce()`. The function is called with the following arguments:

`accumulator`

The value resulting from the previous call to `callbackFn`. On the first call, its value is `initialValue` if the latter is specified; otherwise its value is the first element of the iterator.

`currentValue`

The value of the current element. On the first call, its value is the first element of the iterator if `initialValue` is specified; otherwise its value is the second element.

`currentIndex`

The index position of `currentValue`. On the first call, its value is `0` if `initialValue` is specified, otherwise `1`.


`initialValue` Optional

A value to which `accumulator` is initialized the first time the callback is called. If `initialValue` is specified, `callbackFn` starts executing with the first element as `currentValue`. If `initialValue` is *not* specified, `accumulator` is initialized to the first element, and `callbackFn` starts executing with the second element as `currentValue`. In this case, if the iterator is empty (so that there's no first value to return as `accumulator`), an error is thrown.

### Return value

The value that results from running the "reducer" callback function to completion over the entire iterator.

### Exceptions

`TypeError`

Thrown if the iterator contains no elements and `initialValue` is not provided.

Description
-----------

See `Array.prototype.reduce()` for details about how `reduce()` works. Unlike most other iterator helper methods, it does not work well with infinite iterators, because it is not lazy.

Examples
--------

### Using reduce()

The following example creates an iterator that yields terms in the Fibonacci sequence, and then sums the first ten terms:


```js
function\* fibonacci() {
  let current = 1;
  let next = 1;
  while (true) {
    yield current;
    [current, next] = [next, current + next];
  }
}

console.log(
  fibonacci()
    .take(10)
    .reduce((a, b) => a + b),
); // 143
```

Specifications
--------------

| Specification |
| --- |
| Iterator Helpers # sec-iteratorprototype.reduce |

Browser compatibility
---------------------

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

See also
--------

* Polyfill of `Iterator.prototype.reduce` in `core-js`
* `Iterator`
* `Iterator.prototype.map()`
* `Iterator.prototype.flatMap()`
* `Array.prototype.reduce()`