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I have read a lot of articles explaining the difference between @, = and &. I've seen a lot of people using =? in their code. What does this mean?
Unfortunately, I can't seem to search on Google or SO for an answer because search engines ignore special characters.
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These prefixes are used to bind the parent scope's methods and properties to the directive scope. There are 3 types of prefixes in AngularJS: '@' – Text binding / one-way binding. '=' – Direct model binding / two-way binding.
Property binding is used to bind the data from property of a component to DOM elements. It is denoted by []. Let's understand with a simple example.
All three bindings are ways of passing data from your parent scope to your directive's isolated scope through the element's attributes: @ binding is for passing strings. These strings support {{}} expressions for interpolated values.
= or =attr - set up bi-directional binding between a local scope property and the parent scope property of name defined via the value of the attr attribute.
The ? makes the property optional. Otherwise, you'll get NON_ASSIGNABLE_MODEL_EXPRESSION where Angular is looking for a property that doesn't exist.
From the docs:
If the parent scope property doesn't exist, it will throw a NON_ASSIGNABLE_MODEL_EXPRESSION exception. You can avoid this behavior using =? or =?attr in order to flag the property as optional.
https://docs.angularjs.org/api/ng/service/$compile#-scope-
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