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Group values according to an indicator function and return group counts.
npm install @stdlib/utils-async-count-byAlternatively,
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var countByAsync = require( '@stdlib/utils-async-count-by' );Groups values according to an indicator function, which specifies which group an element in the input collection belongs to, and returns group counts.
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:booleanindicating whether to sequentially invoke theindicatorfunction for eachcollectionelement. Iftrue, the function setsoptions.limit=1. Default:false.thisArg: the execution context forindicator.
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.
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.
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.
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 inputcollection.next: a callback which should be called once theindicatorfunction has finished processing a collectionvalue.
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.
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 }
*/Returns a function which invokes an indicator function once for each element in a collection and returns group counts.
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().
-
A
collectionmay be either anArray,Typed Array, or an array-likeObject(excludingstringsandfunctions). -
If a provided function calls the
nextcallback with a truthyerrorargument, the function suspends execution and immediately calls thedonecallback for subsequenterrorhandling. -
The function does not support dynamic
collectionresizing. -
The function does not skip
undefinedelements. -
If provided an empty
collection, the function calls thedonecallback with an emptyobjectfor the group results. -
Neither
countByAsyncnor the function returned by thefactorymethod guarantee asynchronous execution. To guarantee asynchrony, wrap thedonecallback 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
groupreturned by anindicatorfunction should be a value which can be serialized as anobjectkey. As a counterexample,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.
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 );@stdlib/utils-count-by: group values according to an indicator function and return group counts.@stdlib/utils-async/group-by: group values according to an indicator function.@stdlib/utils-async/tabulate-by: generate a frequency table according to an indicator function.
This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.
For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.
See LICENSE.
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