# Non-Index Keys
> Return an array of an object's own enumerable property names which are not integer indices.
## Usage
```javascript
var nonIndexKeys = require( '@stdlib/utils/nonindex-keys' );
```
#### nonIndexKeys( obj )
Returns an `array` of an object's own enumerable property names which are not integer indices.
```javascript
var arr = [ 1, 2, 3 ];
arr.a = 'foo';
arr.b = 'bar';
var keys = nonIndexKeys( arr );
// e.g., returns [ 'a', 'b' ]
```
## Notes
- Name order is not guaranteed, as `object` key enumeration is not specified according to the [ECMAScript specification][ecma-262-for-in]. In practice, however, most engines use insertion order to sort an `object`'s keys, thus allowing for deterministic extraction.
- In contrast to the built-in `Object.keys()`, if provided `null` or `undefined`, the function returns an empty `array`, rather than throwing an error.
## Examples
```javascript
var nonIndexKeys = require( '@stdlib/utils/nonindex-keys' );
function Foo() {
this[ 0 ] = 3.14;
this.beep = 'boop';
this.a = {
'b': 'c'
};
return this;
}
Foo.prototype.foo = [ 'bar' ];
var obj = new Foo();
var keys = nonIndexKeys( obj );
console.log( keys );
// e.g., => [ 'beep', 'a' ]
```
[ecma-262-for-in]: http://www.ecma-international.org/ecma-262/5.1/#sec-12.6.4