npm package: @stdlib/iter-while-each
Description: Create an iterator which, while a test condition is true, invokes a function for each iterated value before returning the iterated value.
Author: The Stdlib Authors
Latest version: 0.2.3

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</details>

# iterWhileEach

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] <!-- [![dependencies][dependencies-image]][dependencies-url] -->

> Create an iterator which, while a test condition is true, invokes a function for each iterated value before returning the iterated value.

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/iter-while-each
```

</section>

<section class="usage">

## Usage

```javascript
var iterWhileEach = require( '@stdlib/iter-while-each' );
```

#### iterWhileEach( iterator, predicate, fcn\[, thisArg] )

Returns an iterator which invokes a function for each iterated value **before** returning the iterated value until either a `predicate` function returns `false` or the iterator has iterated over all values.

```javascript
var array2iterator = require( '@stdlib/array-to-iterator' );

function predicate( v ) {
    return v < 3;
}

function assert( v ) {
    if ( v !== v ) {
        throw new Error( 'should not be NaN' );
    }
}

var it = iterWhileEach( array2iterator( [ 1, 2, 3, 4 ] ), predicate, assert );
// returns {}

var r = it.next().value;
// returns 1

r = it.next().value;
// returns 2

r = it.next().value;
// undefined

// ...
```

The returned iterator protocol-compliant object has the following properties:

-   **next**: function which returns an iterator protocol-compliant object containing the next iterated value (if one exists) assigned to a `value` property and a `done` property having a boolean value indicating whether the iterator is finished.
-   **return**: function which closes an iterator and returns a single (optional) argument in an iterator protocol-compliant object.

Both the `predicate` function and the function to invoke for each iterated value are provided two arguments:

-   **value**: iterated value.
-   **index**: iteration index (zero-based).

```javascript
var array2iterator = require( '@stdlib/array-to-iterator' );

function predicate( v ) {
    return v < 3;
}

function assert( v, i ) {
    if ( i > 1 ) {
        throw new Error( 'unexpected error' );
    }
}

var it = iterWhileEach( array2iterator( [ 1, 2, 3, 4 ] ), predicate, assert );
// returns <Object>

var r = it.next().value;
// returns 1

r = it.next().value;
// returns 2

r = it.next().value;
// undefined

// ...
```

To set the execution context for `fcn`, provide a `thisArg`.

<!-- eslint-disable no-invalid-this -->

```javascript
var array2iterator = require( '@stdlib/array-to-iterator' );

function assert( v ) {
    this.count += 1;
    if ( v !== v ) {
        throw new Error( 'should not be NaN' );
    }
}

function predicate( v ) {
    return v < 3;
}

var ctx = {
    'count': 0
};

var it = iterWhileEach( array2iterator( [ 1, 2, 3 ] ), predicate, assert, ctx );
// returns <Object>

var r = it.next().value;
// returns 1

r = it.next().value;
// returns 2

var count = ctx.count;
// returns 2
```

</section>

<!-- /.usage -->

<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

## Notes

-   If an environment supports `Symbol.iterator` **and** a provided iterator is iterable, the returned iterator is iterable.

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var randu = require( '@stdlib/random-iter-randu' );
var isnan = require( '@stdlib/math-base-assert-is-nan' );
var iterWhileEach = require( '@stdlib/iter-while-each' );

function assert( v ) {
    if ( isnan( v ) ) {
        throw new Error( 'should not be NaN' );
    }
}

function predicate( v ) {
    return v <= 0.75;
}

// Create a seeded iterator for generating pseudorandom numbers:
var rand = randu({
    'seed': 1234,
    'iter': 10
});

// Create an iterator which validates generated numbers:
var it = iterWhileEach( rand, predicate, assert );

// Perform manual iteration...
var r;
while ( true ) {
    r = it.next();
    if ( r.done ) {
        break;
    }
    console.log( r.value );
}
```

</section>

<!-- /.examples -->

<!-- Section to include cited references. If references are included, add a horizontal rule *before* the section. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="references">

</section>

<!-- /.references -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/iter-until-each`][@stdlib/iter/until-each]</span><span class="delimiter">: </span><span class="description">create an iterator which, while a test condition is false, invokes a function for each iterated value before returning the iterated value.</span>

</section>

<!-- /.related -->

<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->


<section class="main-repo" >

* * *

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

#### Community

[![Chat][chat-image]][chat-url]

---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

</section>

<!-- /.stdlib -->

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[deno-url]: https://github.com/stdlib-js/iter-while-each/tree/deno
[deno-readme]: https://github.com/stdlib-js/iter-while-each/blob/deno/README.md
[umd-url]: https://github.com/stdlib-js/iter-while-each/tree/umd
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[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/iter-while-each/main/LICENSE

<!-- <related-links> -->

[@stdlib/iter/until-each]: https://www.npmjs.com/package/@stdlib/iter-until-each

<!-- </related-links> -->

</section>

<!-- /.links -->