# tabulateByAsync
> Generate a frequency table according to an indicator function.
## Usage
```javascript
var tabulateByAsync = require( '@stdlib/utils/async/tabulate-by' );
```
#### tabulateByAsync( collection, \[options,] indicator, done )
Generates a frequency table according to an `indicator` function, i.e., a function which specifies how to categorize an element in the input `collection`.
```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, 750 ];
tabulateByAsync( arr, indicator, done );
/* =>
750
1000
2500
3000
[ [ false, 2, 0.5 ], [ true, 2, 0.5 ] ]
*/
```
The returned frequency table is an `array` of `arrays`. Each sub-array corresponds to a unique value in the input `collection` and is structured as follows:
- `0`: unique value
- `1`: value count
- `2`: frequency percentage
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, 750 ];
var opts = {
'series': true
};
tabulateByAsync( arr, opts, indicator, done );
/* =>
3000
2500
1000
750
[ [ true, 2, 0.5 ], [ false, 2, 0.5 ] ]
*/
```
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, 750 ];
var opts = {
'limit': 2
};
tabulateByAsync( arr, opts, indicator, done );
/* =>
2500
3000
1000
750
[ [ true, 2, 0.5 ], [ false, 2, 0.5 ] ]
*/
```
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, 750 ];
var context = {
'count': 0
};
var opts = {
'thisArg': context
};
tabulateByAsync( arr, opts, indicator, done );
function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
// => [ [ false, 2, 0.5 ], [ true, 2, 0.5 ] ]
console.log( context.count );
// => 4
}
```
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, 750 ];
tabulateByAsync( arr, indicator, done );
/* =>
collection: 3000,2500,1000,750. 0: 3000
collection: 3000,2500,1000,750. 1: 2500
collection: 3000,2500,1000,750. 2: 1000
collection: 3000,2500,1000,750. 3: 750
750
1000
2500
3000
[ [ false, 2, 0.5 ], [ true, 2, 0.5 ] ]
*/
```
#### tabulateByAsync.factory( \[options,] indicator )
Returns a `function` which invokes an `indicator` function once for each element in a `collection` and generates a frequency table.
```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 = tabulateByAsync.factory( indicator );
var arr1 = [ 3000, 2500, 1000, 750 ];
f( arr1, done );
/* =>
750
1000
2500
3000
[ [ false, 2, 0.5 ], [ true, 2, 0.5 ] ]
*/
var arr2 = [ 300, 250, 100 ];
f( arr2, done );
/* =>
100
250
300
[ [ false, 3, 1.0 ] ]
*/
```
The function accepts the same `options` as `tabulateByAsync()`.
## 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 `array` for the tabulated results.
- **Neither** `tabulateByAsync` 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`).
## Examples
```javascript
var resolve = require( 'path' ).resolve;
var readFile = require( '@stdlib/fs/read-file' );
var tabulateByAsync = require( '@stdlib/utils/async/tabulate-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' );
}
}
tabulateByAsync( files, indicator, done );
```
[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