165 lines
3.9 KiB
Markdown
165 lines
3.9 KiB
Markdown
<!--
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@license Apache-2.0
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Copyright (c) 2018 The Stdlib Authors.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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-->
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# keyByRight
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> Convert a collection to an object whose keys are determined by a provided function and whose values are the collection values, iterating from right to left.
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<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
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<section class="intro">
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</section>
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<!-- /.intro -->
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<!-- Package usage documentation. -->
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<section class="usage">
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## Usage
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```javascript
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var keyByRight = require( '@stdlib/utils/key-by-right' );
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```
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#### keyByRight( collection, fcn\[, thisArg ] )
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Converts a `collection` to an object whose keys are determined by a provided `function` and whose values are the `collection` values, iterating from right to left.
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<!-- eslint-disable object-curly-newline -->
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```javascript
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function toKey( value ) {
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return value.a;
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}
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var arr = [
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{ 'a': 1 },
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{ 'a': 2 }
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];
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var out = keyByRight( arr, toKey );
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// returns { '2': { 'a': 2 }, '1': { 'a': 1 } }
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```
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The invoked `function` is provided two arguments:
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- `value`: collection element
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- `index`: collection index
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To set the function execution context, provide a `thisArg`.
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```javascript
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function toKey( value, index ) {
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this.sum += value;
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this.count += 1;
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return index;
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}
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var arr = [ 1, 2, 3, 4 ];
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var context = {
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'sum': 0,
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'count': 0
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};
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var out = keyByRight( arr, toKey, context );
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// returns { '3': 4, '2': 3, '1': 2, '0': 1 }
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var mean = context.sum / context.count;
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// returns 2.5
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```
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</section>
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<!-- /.usage -->
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<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
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<section class="notes">
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## Notes
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- A `collection` may be either an [`Array`][mdn-array], [`Typed Array`][mdn-typed-array], or an array-like [`Object`][mdn-object] (excluding `strings` and `functions`).
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- If more than one element in a `collection` resolves to the same key, the key value is the `collection` element which last resolved to the key.
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- Object values are shallow copies.
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</section>
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<!-- /.notes -->
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<!-- Package usage examples. -->
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<section class="examples">
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## Examples
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<!-- eslint no-undef: "error" -->
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```javascript
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var keyByRight = require( '@stdlib/utils/key-by-right' );
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var arr;
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var obj;
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var i;
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function toKey( value ) {
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return value.name;
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}
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arr = new Array( 100 );
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for ( i = 0; i < arr.length; i++ ) {
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arr[ i ] = {
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'name': 'v'+i,
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'value': i
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};
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}
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obj = keyByRight( arr, toKey );
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console.log( obj );
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```
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</section>
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<!-- /.examples -->
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<!-- 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. -->
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<section class="references">
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</section>
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<!-- /.references -->
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<!-- Section for all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
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<section class="links">
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[mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array
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[mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
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[mdn-object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object
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</section>
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<!-- /.links -->
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