Copied from the official AngularJS 1 documentation:
To make a binding optional, simply add
?:<?or<?attr.
What are the differences between the optional and non-optional one-way bindings?
I can't seem to find the dissimilarities for the optional versions of two-way (=) and delegate (&) bindings on my fiddle: https://jsfiddle.net/glenn/ze2wo0s1/, except for the one-way binding.
Wishing you all a joyful and Merry Christmas! 🎅🏻🎄❤️
If you take a look at how it's handled in the source code, you can find more information here: https://github.com/angular/angular.js/blob/master/src/ng/compile.js#L3523.
From my perspective, it seems that by using <? to make the binding optional, it will break early without setting up a watch. On the other hand, if you use < to make it required, it sets the binding to undefined and sets up a watch. However, since it is just watching undefined, there may not be any practical difference except for the call to recordChanges. When a required binding is omitted, it will be a key in the changes object passed to the $onChanges hook on the first call. But when an optional binding is omitted, it will not be a key in the changes object.
To see an example, you can check out this JSFiddle. Both requiredBinding and optionalBinding are omitted, so they are initialized to undefined. However, requiredBinding is a key in the changes object while optionalBinding is not.
Utilizing the <? symbol allows the controller to alter the value of the variable intended to be bound, only if that variable does not exist.
When optional bindings are absent, they can be adjusted in the controller. However, once a value is supplied to the component, it becomes immutable.
In contrast, non-optional bindings remain unchangeable. If these bindings are missing, they default to being undefined and cannot be altered in any way.
For instance, consider the following scenario:
bindings: {
nameOptional: '<?',
nameRequired: '<'
}
Attempting to modify
$ctrl.nameRequired = 'something else'nameOptional is possible under one condition: the absence of name-optional being passed to the component grants control over the variable to the controller.
To gain a better grasp of this concept, you can review this fiddle.
It's important to note that for simplicity, we are using a string which is passed by value. When dealing with objects, their properties remain modifiable under normal circumstances.
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