# keyBy
> Convert a collection to an object whose keys are determined by a provided function and whose values are the collection values.
## Usage
```javascript
var keyBy = require( '@stdlib/utils/key-by' );
```
#### keyBy( collection, fcn\[, thisArg ] )
Converts a `collection` to an object whose keys are determined by a provided `function` and whose values are the `collection` values.
```javascript
function toKey( value ) {
return value.a;
}
var arr = [
{ 'a': 1 },
{ 'a': 2 }
];
var out = keyBy( arr, toKey );
// returns { '1': { 'a': 1 }, '2': { 'a': 2 } }
```
The invoked `function` is provided two arguments:
- `value`: collection element
- `index`: collection index
To set the function execution context, provide a `thisArg`.
```javascript
function toKey( value, index ) {
this.sum += value;
this.count += 1;
return index;
}
var arr = [ 1, 2, 3, 4 ];
var context = {
'sum': 0,
'count': 0
};
var out = keyBy( arr, toKey, context );
// returns { '0': 1, '1': 2, '2': 3, '3': 4 }
var mean = context.sum / context.count;
// returns 2.5
```
## 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 more than one element in a `collection` resolves to the same key, the key value is the `collection` element which last resolved to the key.
- Object values are shallow copies.
## Examples
```javascript
var keyBy = require( '@stdlib/utils/key-by' );
var arr;
var obj;
var i;
function toKey( value ) {
return value.name;
}
arr = new Array( 100 );
for ( i = 0; i < arr.length; i++ ) {
arr[ i ] = {
'name': 'v'+i,
'value': i
};
}
obj = keyBy( arr, toKey );
console.log( obj );
```
[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