# doWhileEachRight
> While a test condition is true, invoke a function for each element in a collection, iterating from right to left.
## Usage
```javascript
var doWhileEachRight = require( '@stdlib/utils/do-while-each-right' );
```
#### doWhileEachRight( collection, fcn, predicate\[, thisArg ] )
Invokes a `function` for each element in a `collection`, iterating from right to left, until either a `predicate` function returns `false` or the function has iterated over all `collection` elements. Note that the condition is evaluated **after** executing `fcn`; thus, `fcn` **always** executes at least once.
```javascript
function predicate( value ) {
return ( value === value );
}
function log( value, index ) {
console.log( '%s: %d', index, value );
}
var arr = [ 1, NaN, 2, 3, 4 ];
doWhileEachRight( arr, log, predicate );
/* =>
4: 4
3: 3
2: 2
1: NaN
*/
```
Both the `predicate` function and the `function` to apply are provided three arguments:
- `value`: collection element
- `index`: collection index
- `collection`: input collection
If provided an empty `collection`, both `value` and `index` are `undefined`.
```javascript
function predicate( value ) {
return ( value === value );
}
function log( value, index ) {
console.log( '%s: %s', index, value );
}
var arr = [];
doWhileEachRight( arr, log, predicate );
/* =>
undefined: undefined
*/
```
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 predicate( value ) {
return ( value === value );
}
function log1( value, index, collection ) {
console.log( '%s: %d', index, value );
i += 1;
if ( index === 0 && collection.length < 10 ) {
collection.unshift( i+1 );
}
}
doWhileEachRight( arr, log1, predicate );
/* =>
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 ];
doWhileEachRight( arr, log2, predicate );
/* =>
3: 4
1: 2
*/
```
To set the function execution context for the applied function, provide a `thisArg`.
```javascript
function predicate( value ) {
return ( value > 2 );
}
function sum( value ) {
this.sum += value;
this.count += 1;
}
var arr = [ 1, NaN, 2, 3, 4 ];
var context = {
'sum': 0,
'count': 0
};
doWhileEachRight( arr, sum, predicate, context );
var mean = context.sum / context.count;
// returns 3.0
```
## 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 returns the input `collection`.
- The function does **not** skip `undefined` elements.
```javascript
function predicate( value ) {
return ( value === value );
}
function log( value, index ) {
console.log( '%s: %s', index, value );
}
var arr = [ 1, , , 4 ];
doWhileEachRight( arr, log, predicate );
/* =>
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 randu = require( '@stdlib/random/base/randu' );
var floor = require( '@stdlib/math/base/special/floor' );
var doWhileEachRight = require( '@stdlib/utils/do-while-each-right' );
var arr;
var i;
var j;
function predicate( value ) {
return ( value === value );
}
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 );
j = floor( randu()*arr.length );
for ( i = arr.length-1; i >= 0; i-- ) {
if ( i === j ) {
arr[ i ] = NaN;
} else {
arr[ i ] = i;
}
}
i = 0;
doWhileEachRight( arr, log, predicate );
```
[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