# groupBy
> Group values according to an indicator function.
## Usage
```javascript
var groupBy = require( '@stdlib/utils/group-by' );
```
#### groupBy( collection, \[options,] indicator )
Groups values according to an `indicator` function, which specifies which group an element in the input `collection` belongs to.
```javascript
function indicator( v ) {
return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = groupBy( arr, indicator );
// returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }
```
An `indicator` function is provided two arguments:
- `value`: collection element
- `index`: collection index
```javascript
function indicator( v, i ) {
console.log( '%d: %s', i, v );
return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = groupBy( arr, indicator );
// returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }
```
The function accepts the following `options`:
- `returns`: specifies the output format. If the option equals `'values'`, the function outputs element values. If the option equals `'indices'`, the function outputs element indices. If the option equals `'*'`, the function outputs both element indices and values. Default: `'values'`.
- `thisArg`: execution context.
By default, the function returns element values. To return element indices, set the `returns` option to `'indices'`.
```javascript
function indicator( v ) {
return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var opts = {
'returns': 'indices'
};
var out = groupBy( arr, opts, indicator );
// returns { 'b': [ 0, 1, 3 ], 'f': [ 2 ] }
```
To return index-element pairs, set the `returns` option to `'*'`.
```javascript
function indicator( v ) {
return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var opts = {
'returns': '*'
};
var out = groupBy( arr, opts, indicator );
// returns { 'b': [ [ 0, 'beep' ], [ 1, 'boop' ], [ 3, 'bar' ] ], 'f': [ [ 2, 'foo' ] ] }
```
To set the `indicator` execution context, provide a `thisArg`.
```javascript
function indicator( v ) {
this.count += 1;
return v[ 0 ];
}
var context = {
'count': 0
};
var opts = {
'thisArg': context
};
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = groupBy( arr, opts, indicator );
// returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }
console.log( context.count );
// => 4
```
## 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 value returned by an `indicator` function should be a value which can be serialized as an `object` key. As a counterexample,
```javascript
function indicator( v ) {
return {};
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = groupBy( arr, indicator );
// returns { '[object Object]': [ 'beep', 'boop', 'foo', 'bar' ] }
```
while each group identifier is unique, all collection elements resolve to the same group because each group identifier serializes to the same `string`.
## Examples
```javascript
var randu = require( '@stdlib/random/base/randu' );
var floor = require( '@stdlib/math/base/special/floor' );
var groupBy = require( '@stdlib/utils/group-by' );
var vals;
var arr;
var out;
var i;
var j;
vals = [ 'beep', 'boop', 'foo', 'bar', 'woot', 'woot' ];
// Generate a random collection...
arr = new Array( 100 );
for ( i = 0; i < arr.length; i++ ) {
j = floor( randu()*vals.length );
arr[ i ] = vals[ j ];
}
function indicator( v ) {
// Use the first letter of each element to define groups:
return v[ 0 ];
}
// Compute the groups:
out = groupBy( arr, indicator );
console.log( out );
```
[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