time-to-botec

README.md (9813B)

---

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 @license Apache-2.0
 
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 # countByAsync
 
 > Group values according to an indicator function and return group counts.
 
 <!-- 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="usage">
 
 ## Usage
 
 ```javascript
 var countByAsync = require( '@stdlib/utils/async/count-by' );
 ```
 
 #### countByAsync( collection, \[options,] indicator, done )
 
 Groups values according to an `indicator` function, which specifies which group an element in the input `collection` belongs to, and returns group counts.
 
 ```javascript
 function indicator( value, next ) {
     setTimeout( onTimeout, value );
     function onTimeout() {
         console.log( value );
         next( null, (value > 2000) );
     }
 }
 
 function done( error, result ) {
     if ( error ) {
         throw error;
     }
     console.log( result );
 }
 
 var arr = [ 3000, 2500, 1000 ];
 
 countByAsync( arr, indicator, done );
 /* =>
     1000
     2500
     3000
     { 'true': 2, 'false': 1 }
 */
 ```
 
 The function accepts the following `options`:
 
 -   `limit`: the maximum number of pending invocations at any one time. Default: `infinity`.
 -   `series`: `boolean` indicating whether to sequentially invoke the `indicator` function for each `collection` element. If `true`, the function sets `options.limit=1`. Default: `false`.
 -   `thisArg`: the execution context for `indicator`.
 
 By default, all elements are processed concurrently, which means that the function does **not** guarantee completion order. To process each `collection` element sequentially, set the `series` option to `true`.
 
 ```javascript
 function indicator( value, next ) {
     setTimeout( onTimeout, value );
     function onTimeout() {
         console.log( value );
         next( null, (value > 2000) );
     }
 }
 
 function done( error, result ) {
     if ( error ) {
         throw error;
     }
     console.log( result );
 }
 
 var arr = [ 3000, 2500, 1000 ];
 
 var opts = {
     'series': true
 };
 
 countByAsync( arr, opts, indicator, done );
 /* =>
     3000
     2500
     1000
     { 'true': 2, 'false': 1 }
 */
 ```
 
 To limit the maximum number of pending function invocations, set the `limit` option.
 
 ```javascript
 function indicator( value, next ) {
     setTimeout( onTimeout, value );
     function onTimeout() {
         console.log( value );
         next( null, (value > 2000) );
     }
 }
 
 function done( error, result ) {
     if ( error ) {
         throw error;
     }
     console.log( result );
 }
 
 var arr = [ 3000, 2500, 1000 ];
 
 var opts = {
     'limit': 2
 };
 
 countByAsync( arr, opts, indicator, done );
 /* =>
     2500
     3000
     1000
     { 'true': 2, 'false': 1 }
 */
 ```
 
 To set the execution context of the `indicator` function, set the `thisArg` option.
 
 ```javascript
 function indicator( value, next ) {
     this.count += 1;
     setTimeout( onTimeout, value );
     function onTimeout() {
         next( null, (value > 2000) );
     }
 }
 
 var arr = [ 3000, 2500, 1000 ];
 
 var context = {
     'count': 0
 };
 
 var opts = {
     'thisArg': context
 };
 
 countByAsync( arr, opts, indicator, done );
 
 function done( error, result ) {
     if ( error ) {
         throw error;
     }
     console.log( result );
     // => { 'true': 2, 'false': 1 }
 
     console.log( context.count );
     // => 3
 }
 ```
 
 When invoked, the `indicator` function is provided a maximum of four arguments:
 
 -   `value`: collection value.
 -   `index`: collection index.
 -   `collection`: the input `collection`.
 -   `next`: a callback which should be called once the `indicator` function has finished processing a collection `value`.
 
 The actual number of provided arguments depends on function `length`. If the `indicator` function accepts two arguments, the `indicator` function is provided `value` and `next`. If the `indicator` function accepts three arguments, the `indicator` function is provided `value`, `index`, and `next`. For every other `indicator` function signature, the `indicator` function is provided all four arguments.
 
 ```javascript
 function indicator( value, i, collection, next ) {
     console.log( 'collection: %s. %d: %d', collection.join( ',' ), i, value );
     setTimeout( onTimeout, value );
     function onTimeout() {
         console.log( value );
         next( null, (value > 2000) );
     }
 }
 
 function done( error, result ) {
     if ( error ) {
         throw error;
     }
     console.log( result );
 }
 
 var arr = [ 3000, 2500, 1000 ];
 
 countByAsync( arr, indicator, done );
 /* =>
     collection: 3000,2500,1000. 0: 3000
     collection: 3000,2500,1000. 1: 2500
     collection: 3000,2500,1000. 2: 1000
     1000
     2500
     3000
     { 'true': 2, 'false': 1 }
 */
 ```
 
 #### countByAsync.factory( \[options,] indicator )
 
 Returns a `function` which invokes an `indicator` function once for each element in a `collection` and returns group counts.
 
 ```javascript
 function indicator( value, next ) {
     setTimeout( onTimeout, value );
     function onTimeout() {
         console.log( value );
         next( null, (value > 2000) );
     }
 }
 
 function done( error, result ) {
     if ( error ) {
         throw error;
     }
     console.log( result );
 }
 
 var f = countByAsync.factory( indicator );
 
 var arr1 = [ 3000, 2500, 1000 ];
 
 f( arr1, done );
 /* =>
     1000
     2500
     3000
     { 'true': 2, 'false': 1 }
 */
 
 var arr2 = [ 300, 250, 100 ];
 
 f( arr2, done );
 /* =>
     100
     250
     300
     { 'false': 3 }
 */
 ```
 
 The function accepts the same `options` as `countByAsync()`.
 
 </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
 
 -   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`).
 
 -   If a provided function calls the `next` callback with a truthy `error` argument, the function suspends execution and immediately calls the `done` callback for subsequent `error` handling.
 
 -   The function does **not** support dynamic `collection` resizing.
 
 -   The function does **not** skip `undefined` elements.
 
 -   If provided an empty `collection`, the function calls the `done` callback with an empty `object` for the group results.
 
 -   **Neither** `countByAsync` nor the function returned by the `factory` method **guarantee** asynchronous execution. To guarantee asynchrony, wrap the `done` callback in a function which either executes at the end of the current stack (e.g., `nextTick`) or during a subsequent turn of the event loop (e.g., `setImmediate`, `setTimeout`).
 
 -   The `group` returned by an `indicator` function should be a value which can be serialized as an `object` key. As a counterexample,
 
     ```javascript
     function indicator( value, next ) {
         setTimeout( onTimeout, value );
         function onTimeout() {
             console.log( value );
             next( null, {} );
         }
     }
 
     function done( error, result ) {
         if ( error ) {
             throw error;
         }
         console.log( result );
     }
 
     var arr = [ 3000, 2500, 1000 ];
 
     countByAsync( arr, indicator, done );
     /* =>
         1000
         2500
         3000
         { '[object Object]': 3 }
     */
     ```
 
     while each group identifier is unique, all collection elements resolve to the same group because each group identifier serializes to the same `string`.
 
 </section>
 
 <!-- /.notes -->
 
 <!-- Package usage examples. -->
 
 <section class="examples">
 
 ## Examples
 
 <!-- eslint no-undef: "error" -->
 
 ```javascript
 var resolve = require( 'path' ).resolve;
 var readFile = require( '@stdlib/fs/read-file' );
 var countByAsync = require( '@stdlib/utils/async/count-by' );
 
 var files = [
     resolve( __dirname, 'package.json' ),
     resolve( __dirname, 'README.md' ),
     resolve( __dirname, 'beep.boop.md' )
 ];
 
 function done( error, result ) {
     if ( error ) {
         throw error;
     }
     console.log( result );
 }
 
 function indicator( file, next ) {
     var opts = {
         'encoding': 'utf8'
     };
     readFile( file, opts, onFile );
 
     function onFile( error ) {
         if ( error ) {
             return next( null, 'nonreadable' );
         }
         next( null, 'readable' );
     }
 }
 
 countByAsync( files, indicator, done );
 ```
 
 </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 all links. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
 
 <section class="links">
 
 [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
 
 </section>
 
 <!-- /.links -->