time-to-botec

README.md (5226B)

---

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 # reduce
 
 > Apply a function against an accumulator and each element in a collection and return the accumulated result.
 
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 </section>
 
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 <!-- Package usage documentation. -->
 
 <section class="usage">
 
 ## Usage
 
 ```javascript
 var reduce = require( '@stdlib/utils/reduce' );
 ```
 
 #### reduce( collection, initial, reducer\[, thisArg ] )
 
 Applies a `function` against an accumulator and each element in a `collection` and returns the accumulated result.
 
 ```javascript
 function sum( accumulator, value ) {
     return accumulator + value;
 }
 
 var arr = [ 1, 2, 3, 4 ];
 
 var out = reduce( arr, 0, sum );
 // returns 10
 ```
 
 The `reducer` function is provided four arguments:
 
 -   `accumulator`: accumulated value
 -   `value`: collection element
 -   `index`: collection index
 -   `collection`: input collection
 
 Basic support for dynamic collections is provided. Note, however, that index incrementation is monotonically increasing.
 
 ```javascript
 function sum1( accumulator, value, index, collection ) {
     if ( index === collection.length-1 && collection.length < 10 ) {
         collection.push( index+2 );
     }
     return accumulator + value;
 }
 
 var arr = [ 1, 2, 3, 4 ];
 
 var out = reduce( arr, 0, sum1 );
 // returns 55
 
 function sum2( accumulator, value, index, collection ) {
     collection.shift();
     return accumulator + value;
 }
 
 arr = [ 1, 2, 3, 4 ];
 
 out = reduce( arr, 0, sum2 );
 // returns 4
 ```
 
 To set the function execution context, provide a `thisArg`.
 
 ```javascript
 function sum( accumulator, value ) {
     this.count += 1;
     return accumulator + value;
 }
 
 var arr = [ 1, 2, 3, 4 ];
 
 var context = {
     'count': 0
 };
 
 var out = reduce( arr, 0, sum, context );
 // returns 10
 
 var mean = out / context.count;
 // returns 2.5
 ```
 
 </section>
 
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 <section class="notes">
 
 ## Notes
 
 -   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`).
 
 -   The function differs from [`Array.prototype.reduce`][mdn-array-reduce] in the following ways:
 
     -   The function **requires** an `initial` value for the `accumulator`. The `initial` value is used during the first invocation of the `reducer` function.
 
     -   The function does **not** skip the first element in the `collection`.
 
     -   The function does **not** skip `undefined` elements.
 
         <!-- eslint-disable no-sparse-arrays, stdlib/doctest-marker -->
 
         ```javascript
         function log( accumulator, value, index ) {
             console.log( '%s: %s', index, value );
             return accumulator;
         }
 
         var arr = [ 1, , , 4 ];
 
         var out = reduce( arr, 0, log );
         /* =>
             0: 1
             1: undefined
             2: undefined
             3: 4
         */
         ```
 
     -   The function provides limited support for dynamic collections (i.e., collections whose `length` changes during execution).
 
 </section>
 
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 ## Examples
 
 <!-- eslint no-undef: "error" -->
 
 ```javascript
 var reduce = require( '@stdlib/utils/reduce' );
 
 var arr;
 var s;
 var i;
 
 function sum( acc, value ) {
     return acc + value;
 }
 
 arr = new Array( 1000 );
 for ( i = 0; i < arr.length; i++ ) {
     arr[ i ] = i;
 }
 
 s = reduce( arr, 0, sum );
 console.log( s );
 ```
 
 </section>
 
 <!-- /.examples -->
 
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 [mdn-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array
 
 [mdn-typed-array]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray
 
 [mdn-object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object
 
 [mdn-array-reduce]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce
 
 </section>
 
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