# countBy
> Group values according to an indicator function and return group counts.
## Usage
```javascript
var countBy = require( '@stdlib/utils/count-by' );
```
#### countBy( collection, \[options,] indicator )
Groups values according to an `indicator` function, i.e., a function which specifies which group an element in the input `collection` belongs to, and returns group counts.
```javascript
function indicator( v ) {
return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = countBy( arr, indicator );
// returns { 'b': 3, 'f': 1 }
```
An `indicator` function is provided two arguments:
- `value`: collection element
- `index`: collection index
```javascript
function indicator( v, i ) {
console.log( '%d: %d', i, v );
return v[ 0 ];
}
var arr = [ 'beep', 'boop', 'foo', 'bar' ];
var out = countBy( arr, indicator );
// returns { 'b': 3, 'f': 1 }
```
The function accepts the following `options`:
- `thisArg`: execution context.
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 = countBy( arr, opts, indicator );
// returns { 'b': 3, 'f': 1 }
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 = countBy( arr, indicator );
// returns { '[object Object]': 4 }
```
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 countBy = require( '@stdlib/utils/count-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 group counts:
out = countBy( 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