Could you please explain the significance of scope: [variable-name]='@' in Angular?

Question

Could you please explain the significance of scope: [variable-name]='@' in Angular?

I absolutely despise asking what may seem like foolish questions, however, it seems that the AngularJS documentation authors have left out some crucial clarifications.

When constructing a directive, you have the ability to connect variables within your directive to those in your HTML using the following code snippet:

angular.module('app', []).directive('someModule', ['$window', function($window) {
  return {
    restrict: 'E',
    transclude: true,
    template: '...',
    scope: {
      x:          '=',
      front:      '@',
      markers:    '=?'
    },
    controller: ...
  }]);

I grasp that the '=' following x signifies x=x, however, could you please shed some light on the meanings of @ and ?=?

javascript angularjs scope

Answer 1

Answer №1

In my opinion, it's important to understand that the @ symbol provides the direct string representation of the DOM attribute. Don't get caught up in labeling it as one-way binding or anything like that - it will only add unnecessary confusion.

When the value is interpolated, if you check it before the interpolation, you'll see "{{something}}", but once the interpolation occurs, you'll see the actual interpolated value.

On the other hand, the = sign signifies an expression that is evaluated within the parent scope - a method to grant an isolated scope access to the parent scope.

Answer 2

In my opinion, it's important to understand that the @ symbol provides the direct string representation of the DOM attribute. Don't get caught up in labeling it as one-way binding or anything like that - it will only add unnecessary confusion.

When the value is interpolated, if you check it before the interpolation, you'll see "{{something}}", but once the interpolation occurs, you'll see the actual interpolated value.

On the other hand, the = sign signifies an expression that is evaluated within the parent scope - a method to grant an isolated scope access to the parent scope.

Answer 3

Answer №2

@ refers to one-way binding with directive isolated scope variable.

When the bound scope variable changes within the controller where the element is placed, the directive isolated scope will be notified of that change. However, changes in the directive isolated scope will not be observed by the controller where the directive is placed.

An expression is required to assign a value to the isolated scope variable.

For Example:

<my-directive foo-id="{{fooId}}"></my-directive>

Directive:

scope: {
   'fooId': '@'
}

When the fooId in the directive changes, the expression will be evaluated and will update the fooId in the isolated scope variable. You can access this isolated scope using scope.fooId. Changing scope.fooId from the directive will also update the binding of the fooId variable in the parent controller where the directive is placed. This is why it is known as one-way binding as it does not update a binding in the reverse direction.

'=?'

This indicates that you can use = for two-way binding, but the value of the attribute is optional. The ? after = signifies that it is an optional field.

If you only use = without ?, then you must provide a value for the variable reference within that attribute. If this does not happen, Angular will throw a noassign error.

Answer 4

@ refers to one-way binding with directive isolated scope variable.

When the bound scope variable changes within the controller where the element is placed, the directive isolated scope will be notified of that change. However, changes in the directive isolated scope will not be observed by the controller where the directive is placed.

An expression is required to assign a value to the isolated scope variable.

For Example:

<my-directive foo-id="{{fooId}}"></my-directive>

Directive:

scope: {
   'fooId': '@'
}

When the fooId in the directive changes, the expression will be evaluated and will update the fooId in the isolated scope variable. You can access this isolated scope using scope.fooId. Changing scope.fooId from the directive will also update the binding of the fooId variable in the parent controller where the directive is placed. This is why it is known as one-way binding as it does not update a binding in the reverse direction.

'=?'

This indicates that you can use = for two-way binding, but the value of the attribute is optional. The ? after = signifies that it is an optional field.

If you only use = without ?, then you must provide a value for the variable reference within that attribute. If this does not happen, Angular will throw a noassign error.

Could you please explain the significance of scope: [variable-name]='@' in Angular?

Answer №1

Answer №2

'=?'

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