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# Error: $compile:nonassign
Non-Assignable Expression

    Expression '{0}' in attribute '{1}' used with directive '{2}' is non-assignable!

## Description

This error occurs when a directive defines an isolate scope property (using the `=` mode in the `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
        }
        ...
      }
    });