# doWhileEachRight

> While a test condition is true, invoke a function for each element in a collection, iterating from right to left.


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## 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
```


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## 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.

  <!-- eslint-disable no-sparse-arrays -->

  ``` 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).

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## Examples

``` javascript
var isEven = require( '@stdlib/assert/is-even' ).isPrimitive;
var randu = require( '@stdlib/math/base/random/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 );
```

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[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

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