# whileEach
> While a test condition is true, invoke a function for each element in a collection.
## Usage
```javascript
var whileEach = require( '@stdlib/utils/while-each' );
```
#### whileEach( collection, predicate, fcn\[, thisArg ] )
Invokes a `function` for each element in a `collection` until either a `predicate` function returns `false` or the function has iterated over all `collection` elements.
```javascript
function predicate( value ) {
return ( value === value );
}
function log( value, index ) {
console.log( '%s: %d', index, value );
}
var arr = [ 1, 2, 3, NaN, 4 ];
whileEach( arr, predicate, log );
/* =>
0: 1
1: 2
2: 3
*/
```
Both the `predicate` function and the `function` to apply are 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 predicate( value ) {
return ( value === value );
}
function log1( value, index, collection ) {
console.log( '%s: %d', index, value );
if ( index === collection.length-1 && collection.length < 10 ) {
collection.push( index+2 );
}
}
var arr = [ 1, 2, 3, 4 ];
whileEach( arr, predicate, log1 );
/* =>
0: 1
1: 2
2: 3
3: 4
4: 5
5: 6
6: 7
7: 8
8: 9
9: 10
*/
function log2( value, index, collection ) {
console.log( '%s: %d', index, value );
collection.shift();
}
arr = [ 1, 2, 3, 4 ];
whileEach( arr, predicate, log2 );
/* =>
0: 1
1: 3
*/
```
To set the function execution context for the applied function, provide a `thisArg`.
```javascript
function predicate( value ) {
return ( value === value );
}
function sum( value ) {
this.sum += value;
this.count += 1;
}
var arr = [ 1, 2, 3, NaN, 4 ];
var context = {
'sum': 0,
'count': 0
};
whileEach( arr, predicate, sum, context );
var mean = context.sum / context.count;
// returns 2.0
```
## 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 ];
whileEach( arr, predicate, 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 randu = require( '@stdlib/random/base/randu' );
var floor = require( '@stdlib/math/base/special/floor' );
var whileEach = require( '@stdlib/utils/while-each' );
function predicate( value ) {
return ( value === value );
}
function log( value, index, collection ) {
console.log( '%s: %d', index, value );
if ( isEven( index ) ) {
collection.shift();
} else {
collection.push( index+1 );
}
}
var arr;
var j;
var i;
arr = new Array( 100 );
j = floor( randu()*arr.length );
for ( i = 0; i < arr.length; i++ ) {
if ( i === j ) {
arr[ i ] = NaN;
} else {
arr[ i ] = i;
}
}
whileEach( arr, predicate, 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