HTML SELECT element with angular data-binding.

ngOptions

The ngOptions attribute can be used to dynamically generate a list of <option> elements for the <select> element using the array or object obtained by evaluating the ngOptions comprehension expression.

In many cases, ngRepeat can be used on <option> elements instead of ngOptions to achieve a similar result. However, ngOptions provides some benefits such as reducing memory and increasing speed by not creating a new scope for each repeated instance, as well as providing more flexibility in how the <select>'s model is assigned via the select as part of the comprehension expression. ngOptions should be used when the <select> model needs to be bound to a non-string value. This is because an option element can only be bound to string values at present.

When an item in the <select> menu is selected, the array element or object property represented by the selected option will be bound to the model identified by the ngModel directive.

Optionally, a single hard-coded <option> element, with the value set to an empty string, can be nested into the <select> element. This element will then represent the null or "not selected" option. See example below for demonstration.

select as

Using select as will bind the result of the select expression to the model, but the value of the <select> and <option> html elements will be either the index (for array data sources) or property name (for object data sources) of the value within the collection. If a track by expression is used, the result of that expression will be set as the value of the option and select elements.

select as and track by

Consider the following example:

<select ng-options="item.subItem as item.label for item in values track by item.id" ng-model="selected">

$scope.values = [{
  id: 1,
  label: 'aLabel',
  subItem: { name: 'aSubItem' }
}, {
  id: 2,
  label: 'bLabel',
  subItem: { name: 'bSubItem' }
}];

$scope.selected = { name: 'aSubItem' };

With the purpose of preserving the selection, the track by expression is always applied to the element of the data source (to item in this example). To calculate whether an element is selected, we do the following:

1. Apply track by to the elements in the array. In the example: [1, 2]
2. Apply track by to the already selected value in ngModel. In the example: this is not possible as track by refers to item.id, but the selected value from ngModel is {name: 'aSubItem'}, so the track by expression is applied to a wrong object, the selected element can't be found, <select> is always reset to the "not selected" option.