time-to-botec

README.md (9268B)

---

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 @license Apache-2.0
 
 Copyright (c) 2018 The Stdlib Authors.
 
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 # mapKeysAsync
 
 > Map keys from one object to a new object having the same values.
 
 <!-- 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 mapKeysAsync = require( '@stdlib/utils/async/map-keys' );
 ```
 
 #### mapKeysAsync( obj, \[options,] transform, done )
 
 Map keys from one `object` to a new `object` having the same values.
 
 ```javascript
 function transform( key, next ) {
     setTimeout( onTimeout, 0 );
     function onTimeout() {
         next( null, key+':beep' );
     }
 }
 
 function done( error, out ) {
     if ( error ) {
         throw error;
     }
     console.log( out );
     // => { 'a:beep': 1, 'b:beep': 2 }
 }
 
 var obj = {
     'a': 1,
     'b': 2
 };
 
 mapKeysAsync( obj, transform, done );
 ```
 
 The `next` callback accepts two arguments: `error` and `key`. The second argument to the `next` callback is the transformed property name. If a `transform` function calls the `next` callback with a truthy error argument, the function stops processing any additional own properties and calls the `done` callback for error processing.
 
 ```javascript
 function transform( key, value, next ) {
     setTimeout( onTimeout, value );
     function onTimeout() {
         if ( value === 1 ) {
             return next( new Error( 'boop' ) );
         }
         next( null, key+':beep' );
     }
 }
 
 function done( error ) {
     if ( error ) {
         console.error( error.message );
         // => 'boop'
     }
 }
 
 var obj = {
     'a': 1,
     'b': 2
 };
 
 mapKeysAsync( obj, transform, done );
 ```
 
 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 `transform` function for each own property. If `true`, the function sets `options.limit=1`. Default: `false`.
 -   `thisArg`: the execution context for `fcn`.
 
 By default, all properties are processed concurrently, which means that the function does **not** guarantee completion order. To process each property sequentially, set the `series` option to `true`.
 
 ```javascript
 function transform( key, value, next ) {
     setTimeout( onTimeout, value );
     function onTimeout() {
         next( null, key+':beep' );
     }
 }
 
 function done( error, out ) {
     if ( error ) {
         throw error;
     }
     console.log( out );
     // => { 'a:beep': 1, 'b:beep': 2 }
 }
 
 var obj = {
     'a': 1,
     'b': 2
 };
 
 var opts = {
     'series': true
 };
 
 mapKeysAsync( obj, opts, transform, done );
 ```
 
 To limit the maximum number of pending function invocations, set the `limit` option.
 
 ```javascript
 function transform( key, value, next ) {
     setTimeout( onTimeout, value );
     function onTimeout() {
         next( null, key+':beep' );
     }
 }
 
 function done( error, out ) {
     if ( error ) {
         throw error;
     }
     console.log( out );
     // => { 'a:beep': 1, 'b:beep': 2, 'c:beep': 3 }
 }
 
 var obj = {
     'a': 1,
     'b': 2,
     'c': 3
 };
 
 var opts = {
     'limit': 2
 };
 
 mapKeysAsync( obj, opts, transform, done );
 ```
 
 To set the execution context of the `transform` function, set the `thisArg` option.
 
 ```javascript
 function transform( key, value, next ) {
     this.count += 1;
     setTimeout( onTimeout, value );
     function onTimeout() {
         next( null, key+':beep' );
     }
 }
 
 var obj = {
     'a': 1,
     'b': 2,
     'c': 3
 };
 
 var context = {
     'count': 0
 };
 
 var opts = {
     'thisArg': context
 };
 
 mapKeysAsync( obj, opts, transform, done );
 
 function done( error, out ) {
     if ( error ) {
         throw error;
     }
     console.log( out );
     // => { 'a:beep': 1, 'b:beep': 2, 'c:beep': 3 }
 
     console.log( context.count );
     // => 3
 }
 ```
 
 When invoked, the `transform` function is provided a maximum of four arguments:
 
 -   `key`: object key.
 -   `value`: object value corresponding to `key`.
 -   `obj`: source object.
 -   `next`: a callback which should be called once the `transform` function has finished processing a property.
 
 The actual number of provided arguments depends on function `length`. If the `transform` function accepts two arguments, the `transform` function is provided `key` and `next`. If the `transform` function accepts three arguments, the `transform` function is provided `key`, `value`, and `next`. For every other `transform` function signature, the `transform` function is provided all four arguments.
 
 ```javascript
 function transform( key, value, obj, next ) {
     console.log( 'obj: %s. %s: %d', JSON.stringify( obj ), key, value );
     setTimeout( onTimeout, value );
     function onTimeout() {
         next( null, key+':'+value );
     }
 }
 
 function done( error, out ) {
     if ( error ) {
         throw error;
     }
     console.log( out );
 }
 
 var obj = {
     'a': 1,
     'b': 2
 };
 
 mapKeysAsync( obj, transform, done );
 /* => e.g.,
     obj: {"a": 1, "b": 2}. a: 1
     obj: {"a": 1, "b": 2}. b: 2
     { 'a:1': 1, 'b:2': 2 }
 */
 ```
 
 #### mapKeysAsync.factory( \[options,] transform )
 
 Returns a `function` which invokes a `transform` function once for each own property.
 
 ```javascript
 function transform( key, value, next ) {
     setTimeout( onTimeout, value );
     function onTimeout() {
         next( null, key+':beep' );
     }
 }
 
 function done( error, out ) {
     if ( error ) {
         throw error;
     }
     console.log( out );
 }
 
 var f = mapKeysAsync.factory( transform );
 
 var obj1 = {
     'a': 1,
     'b': 2
 };
 
 f( obj1, done );
 // => { 'a:beep': 1, 'b:beep': 2 }
 
 var obj2 = {
     'c': 3,
     'd': 4
 };
 
 f( obj2, done );
 // => { 'c:beep': 3, 'd:beep': 4 }
 ```
 
 The function accepts the same `options` as `mapKeysAsync()`.
 
 </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 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.
 -   If provided an empty `object`, the function calls the `done` callback with an empty `object`.
 -   Key iteration order is **not** guaranteed, as `object` key enumeration is not specified according to the [ECMAScript specification][ecma-262]. In practice, however, most engines use insertion order to sort an `object`'s keys, thus allowing for iteration order.
 -   Key insertion order is **not** guaranteed.
 -   The value returned by a `transform` function should be a value which can be serialized as an `object` key.
 -   The function only maps **own** properties. Hence, the function does **not** map inherited properties.
 -   The function **shallow** copies key values.
 -   **Neither** `mapKeysAsync` 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`).
 
 </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 mapKeysAsync = require( '@stdlib/utils/async/map-keys' );
 
 var files = {
     'file1': resolve( __dirname, 'package.json' ),
     'file2': resolve( __dirname, 'README.md' )
 };
 
 function read( key, value, next ) {
     var opts = {
         'encoding': 'utf8'
     };
     readFile( value, opts, onFile );
 
     function onFile( error ) {
         if ( error ) {
             return next( error, key+':unreadable' );
         }
         next( null, key+':readable' );
     }
 }
 
 function done( error, out ) {
     if ( error ) {
         throw error;
     }
     console.log( out );
 }
 
 mapKeysAsync( files, read, done );
 ```
 
 </section>
 
 <!-- /.examples -->
 
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 <section class="references">
 
 </section>
 
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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. -->
 
 <section class="links">
 
 [ecma-262]: http://www.ecma-international.org/ecma-262/5.1/#sec-12.6.4
 
 </section>
 
 <!-- /.links -->