# forEachRight
> Invoke a function for each element in a collection, iterating from the right to left.
## Usage
```javascript
var forEachRight = require( '@stdlib/utils/for-each-right' );
```
#### forEachRight( collection, fcn\[, thisArg ] )
Invokes a `function` for each element in a `collection`, iterating from right to left.
```javascript
function log( value, index ) {
console.log( '%s: %d', index, value );
}
var arr = [ 1, 2, 3, 4 ];
forEachRight( arr, log );
/* =>
3: 4
2: 3
1: 2
0: 1
*/
```
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 **not** guaranteed to be monotonically **decreasing**.
```javascript
var arr = [ 1, 2, 3, 4 ];
var i = 0;
function log1( value, index, collection ) {
console.log( '%s: %d', index, value );
i += 1;
if ( index === 0 && collection.length < 10 ) {
collection.unshift( i+1 );
}
}
forEachRight( arr, log1 );
/* =>
3: 4
2: 3
1: 2
0: 1
0: 5
0: 6
0: 7
0: 8
0: 9
0: 10
*/
function log2( value, index, collection ) {
console.log( '%s: %d', index, value );
collection.pop();
}
arr = [ 1, 2, 3, 4 ];
forEachRight( arr, log2 );
/* =>
3: 4
1: 2
*/
```
To set the function execution context, provide a `thisArg`.
```javascript
function sum( value ) {
this.sum += value;
this.count += 1;
}
var arr = [ 1, 2, 3, 4 ];
var context = {
'sum': 0,
'count': 0
};
forEachRight( arr, sum, context );
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.forEach`][mdn-array-foreach] in the following ways:
- The function returns the input `collection`.
- The function does **not** skip `undefined` elements.
```javascript
function log( value, index ) {
console.log( '%s: %s', index, value );
}
var arr = [ 1, , , 4 ];
forEachRight( arr, log );
/* =>
3: 4
2: undefined
1: undefined
0: 1
*/
```
- 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 forEachRight = require( '@stdlib/utils/for-each-right' );
var arr;
var i;
function log( value, index, collection ) {
console.log( '%s: %d', index, value );
i += 1;
if ( isEven( i ) ) {
collection.pop();
} else {
collection.unshift( i+1 );
}
}
arr = new Array( 100 );
for ( i = arr.length-1; i >= 0; i-- ) {
arr[ i ] = i;
}
i = 0;
forEachRight( arr, log );
```
[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-foreach]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach