What sets apart the definition of isolated scope variables for an Angular directive when done within {} as opposed to in

When working with Angular directives, I am aware that I can assign an isolated scope by adding '=' or '@' or '&' in the {} to define variables. However, it seems like this is not required within the link function. For example:

 scope: {
   foo:'=foo',
   bar:'@bar'
 }

This setup works as expected.

link: function($scope, $element){
  $scope.foo = 'foo';
  $scope.bar = 'bar';
}

Even though the link function is inside the directive, the above code functions perfectly fine.

 scope: {
   foo:'foo',
   bar:'bar'
 }

However, without adding '=' or '@' or '&', defining variables does not work as intended.

My question is, why is it possible to define variables within the link function without the need for '=' or '@' or '&' when both $scope and $scope:{} represent the same isolated scope?

Thank you!!!

Answer №1

When it comes to the link property, you are specifically dealing with the isolated scope of a directive. On the other hand, the scope property defines how attributes from the parent element are brought into your isolated scope. These two serve distinct purposes.

The =, @, and & prefixes determine how attributes are understood when they are imported into your isolated scope:

  1. modelParent: '=modelIsolate' - This imports a model into your isolated scope by creating a two-way binding between the model defined in the parent scope (modelParent) and the model defined in the isolated scope (modelIsolate). Here, the modelParent attribute within the directive is viewed as a model.

  2. attrib1: '@attrib1' - This brings a string into your isolated scope by interpreting the attribute value as a string data type. The attribute may contain expressions that need evaluation. For example, if 'hello {{ name }}' is passed as an attribute to your directive where name is linked to 'James' in the parent scope, attrib1 in your isolated scope will be $scope.attrib1 = 'hello James'. As the 'name' binding changes (e.g. from James to Mike), attrib1 in your isolated scope will also automatically update. This is why @ binding is sometimes known as one-way binding. I personally see it as just strings with interpolation capabilities.

  3. express: '&express' - This involves importing an expression into your isolated scope which can be evaluated within the parent scope. It is often used to execute a function that exists in the parent scope, from within the isolated scope of your directive. For instance, if you have a click handler defined in your parent scope: $scope.onclick = function() {}; and you pass this into your directive:

    <directive attrib1="onclick" />
    , you can call the function from your directive's template:
    '<div ng-click="attrib1()" />'
    .

All three methods bring scope variables from the parent scope into your isolated scope. However, depending on how you intend to interpret the directive attributes, you may find yourself favoring one method over the others.

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