# inmap
> Invoke a function for each element in a collection and update the collection in-place.
## Usage
```javascript
var inmap = require( '@stdlib/utils/inmap' );
```
#### inmap( collection, fcn\[, thisArg ] )
Invokes a `function` for each element in a `collection` and updates the `collection` in-place.
```javascript
function scale( value, index ) {
return value * index;
}
var arr = [ 1, 2, 3, 4 ];
var out = inmap( arr, scale );
// returns [ 0, 2, 6, 12 ]
var bool = ( out === arr );
// returns true
```
The invoked `function` is provided three arguments:
- `value`: collection element
- `index`: collection index
- `collection`: input collection
Basic support for dynamic collections is provided. Note, however, that index incrementation is monotonically increasing.
```javascript
function scale1( value, index, collection ) {
if ( index === collection.length-1 && collection.length < 10 ) {
collection.push( index+2 );
}
return value * index;
}
var arr = [ 1, 2, 3, 4 ];
var out = inmap( arr, scale1 );
// returns [ 0, 2, 6, 12, 20, 30, 42, 56, 72, 90 ]
function scale2( value, index, collection ) {
collection.shift();
return value * index;
}
arr = [ 1, 2, 3, 4 ];
out = inmap( arr, scale2 );
// returns [ 3, 3 ]
```
To set the function execution context, provide a `thisArg`.
```javascript
function sum( value ) {
this.sum += value;
this.count += 1;
return value;
}
var arr = [ 1, 2, 3, 4 ];
var context = {
'sum': 0,
'count': 0
};
var out = inmap( arr, sum, context );
// returns [ 1, 2, 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`).
- The function differs from [`Array.prototype.map`][mdn-array-map] in the following ways:
- The function returns the input `collection`.
- The function modifies `collection` elements in-place.
- The function does **not** skip `undefined` elements.
```javascript
function log( value, index ) {
console.log( '%s: %s', index, value );
return value;
}
var arr = [ 1, , , 4 ];
var out = inmap( arr, log );
/* =>
0: 1
1: undefined
2: undefined
3: 4
*/
```
- The function provides limited support for dynamic collections (i.e., collections whose `length` changes during execution).
## Examples
```javascript
var isEven = require( '@stdlib/assert/is-even' ).isPrimitive;
var inmap = require( '@stdlib/utils/inmap' );
var bool;
var arr;
var out;
var i;
function scale( value, index, collection ) {
if ( isEven( index ) ) {
collection.shift();
} else {
collection.push( index+1 );
}
return value * index;
}
arr = new Array( 100 );
for ( i = 0; i < arr.length; i++ ) {
arr[ i ] = i;
}
out = inmap( arr, scale );
bool = ( out === arr );
console.log( bool );
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
[mdn-array-map]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map