@ngdoc error
@name $compile:nonassign
@fullName Non-Assignable Expression
@description

This error occurs when a directive defines an isolate scope property
(using the `=` mode in the {@link ng.$compile#directive-definition-object
`scope` option} of a directive definition) but the directive is used with an expression that is not-assignable.

In order for the two-way data-binding to work, it must be possible to write new values back into the path defined with the expression.

For example, given a directive:

```
myModule.directive('myDirective', function factory() {
  return {
    ...
    scope: {
      localValue: '=bind'
    }
    ...
  }
});
```

Following are invalid uses of this directive:
```
<!-- ERROR because `1+2=localValue` is an invalid statement -->
<my-directive bind="1+2">

<!-- ERROR because `myFn()=localValue` is an invalid statement -->
<my-directive bind="myFn()">

<!-- ERROR because attribute bind wasn't provided -->
<my-directive>
```


To resolve this error, do one of the following options:

- use path expressions with scope properties that are two-way data-bound like so:

```
<my-directive bind="some.property">
<my-directive bind="some[3]['property']">
```

- Make the binding optional

```
myModule.directive('myDirective', function factory() {
  return {
    ...
    scope: {
      localValue: '=?bind' // <-- the '?' makes it optional
    }
    ...
  }
});
```
