@stdlib/iter/unique-by/README.md

Standard iterator utilities.

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# iterUniqueBy

> Create an [iterator][mdn-iterator-protocol] which returns unique values according to a predicate function.

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## Usage

```javascript
var iterUniqueBy = require( '@stdlib/iter/unique-by' );
```

#### iterUniqueBy( iterator, predicate\[, thisArg] )

Returns an [iterator][mdn-iterator-protocol] which returns unique values according to a `predicate` function.

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

function predicate( a, b ) {
    return ( a !== b );
}

var src = array2iterator( [ 2, 1, 1, 2, 4, 3, 4, 3 ] );

var it = iterUniqueBy( src, predicate );
// returns <Object>

var v = it.next().value;
// returns 2

v = it.next().value;
// returns 1

v = it.next().value;
// returns 4

v = it.next().value;
// returns 3

var bool = it.next().done;
// returns true
```

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

-   **next**: function which returns an [iterator][mdn-iterator-protocol] 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][mdn-iterator-protocol] is finished.
-   **return**: function which closes an [iterator][mdn-iterator-protocol] and returns a single (optional) argument in an [iterator][mdn-iterator-protocol] protocol-compliant object.

A `predicate` function is provided two arguments:

-   **a**: a previously identified unique value
-   **b**: the value whose uniqueness is being determined

To set the execution context of the `predicate` function, provide a `thisArg`.

<!-- eslint-disable object-curly-newline -->

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

function predicate( a, b ) {
    this.count += 1;
    return ( a.v !== b.v );
}

var values = [
    { 'v': 2 },
    { 'v': 1 },
    { 'v': 1 },
    { 'v': 2 },
    { 'v': 4 },
    { 'v': 3 },
    { 'v': 4 },
    { 'v': 3 }
];

var src = array2iterator( values );

var ctx = {
    'count': 0
};

var it = iterUniqueBy( src, predicate, ctx );
// returns <Object>

var v = it.next().value;
// returns { 'v': 2 }

v = it.next().value;
// returns { 'v': 1 }

v = it.next().value;
// returns { 'v': 4 }

v = it.next().value;
// returns { 'v': 3 }

var bool = it.next().done;
// returns true

bool = ( ctx.count > 0 );
// returns true
```

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## Notes

-   A returned [iterator][mdn-iterator-protocol] internally buffers unique values and, thus, has `O(N)` memory requirements.
-   A `predicate` function is invoked for each iterated value against each value in an internal buffer consisting of previously identified unique values. Thus, as the number of unique values grows, so, too, does the number of `predicate` function invocations per iterated value.
-   An iterated value is considered "unique" if the `predicate` function returns truthy values for **all** comparisons of the iterated value with each value in the internal buffer.
-   If an environment supports `Symbol.iterator` **and** a provided [iterator][mdn-iterator-protocol] is iterable, the returned [iterator][mdn-iterator-protocol] is iterable.

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## Examples

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

```javascript
var discreteUniform = require( '@stdlib/random/iter/discrete-uniform' );
var iterUniqueBy = require( '@stdlib/iter/unique-by' );

function predicate( a, b ) {
    return ( a !== b );
}

// Create a seeded iterator which can generate 1000 pseudorandom numbers:
var rand = discreteUniform( 1, 10, {
    'seed': 1234,
    'iter': 1000
});

// Create an iterator which returns unique values:
var it = iterUniqueBy( rand, predicate );

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

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* * *

## See Also

-   <span class="package-name">[`@stdlib/iter/unique`][@stdlib/iter/unique]</span><span class="delimiter">: </span><span class="description">create an iterator which returns unique values.</span>
-   <span class="package-name">[`@stdlib/iter/unique-by-hash`][@stdlib/iter/unique-by-hash]</span><span class="delimiter">: </span><span class="description">create an iterator which returns unique values according to a hash function.</span>

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[mdn-iterator-protocol]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#The_iterator_protocol

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[@stdlib/iter/unique]: https://github.com/stdlib-js/iter/tree/main/unique

[@stdlib/iter/unique-by-hash]: https://github.com/stdlib-js/iter/tree/main/unique-by-hash

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