This block could be conveniently rewritten as 

```js
field.options[i].selected = value.indexOf(field.options[i].value) !== -1;
```

Or a bit more clearly:

```js
var option = field.options[i];
option.selected = value.indexOf(option.value) !== -1;
```

Also, maybe using `forEach` would be a bit more clean:

```js
[].forEach.call(field.options, function(option) {
    option.selected = value.indexOf(option.value) !== -1;
});
```