### `Rx.Observable.prototype.findIndex(predicate, [thisArg])`
[&#x24C8;](https://github.com/Reactive-Extensions/RxJS/blob/master/src/core/linq/observable/findindex.js "View in source")

Searches for an element that matches the conditions defined by the specified predicate, and returns the first occurrence within the entire Observable sequence.

#### Arguments
1. `predicate` *(`Function`)*: A function to test each source element for a condition;  The callback is called with the following information:
    1. the value of the element
    2. the index of the element
    3. the Observable object being subscribed
2. `[thisArg]` *(`Any`)*: Object to use as `this` when executing the predicate.

#### Returns
*(`Observable`)*: An Observable sequence with the first element that matches the conditions defined by the specified predicate, if found; otherwise, undefined.

#### Example
```js
/* Found an element */
var array = [1,2,3,4];

var source = Rx.Observable.from(array)
  .findIndex(function (x, i, obs) { return x === 1; });

var subscription = source.subscribe(
  function (x) {
    console.log('Next: %s', x);
  },
  function (err) {
    console.log('Error: %s', err);
  },
  function () {
    console.log('Completed');
  });

// => Next: 0
// => Completed

/* Not found */
var array = [1,2,3,4];

var source = Rx.Observable.from(array)
  .findIndex(function (x, i, obs) { return x === 5; });

var subscription = source.subscribe(
  function (x) {
    console.log('Next: %s', x);
  },
  function (err) {
    console.log('Error: %s', err);
  },
  function () {
    console.log('Completed');
  });

// => Next: -1
// => Completed
```

### Location

File:
- [`/src/core/linq/observable/findindex.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/src/core/linq/observable/findindex.js)

Dist:
- [`rx.all.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.all.js)
- [`rx.all.compat.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.all.js)
- [`rx.aggregates.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.aggregates.js)

Prerequisites:
- [`rx.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js) | [`rx.compat.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.compat.js) | [`rx.lite.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.lite.js) | [`rx.lite.compat.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.lite.compat.js)

NPM Packages:
- [`rx`](https://www.npmjs.org/package/rx)

NuGet Packages:
- [`RxJS-All`](http://www.nuget.org/packages/RxJS-All/)
- [`RxJS-Aggregates`](http://www.nuget.org/packages/RxJS-Aggregates/)

Unit Tests:
- [`/tests/observable/findindex.js`](https://github.com/Reactive-Extensions/RxJS/blob/master/tests/observable/findindex.js)
