AngularJS : What is the usage of '=?' in scope of angular directive

Question

I have seen developers using '=?' in angular scope for a directive. Can someone please explain me its usage.

The =? means optional two way binding. Without the ? if you didn't provide an attribute in the DOM it would throw an exception. — ste2425
– ste2425, Commented Jan 6, 2016 at 8:15
so ? can be given to any other directive scope? ex:- '@?' and '&?' — Shashi
– Shashi, Commented Jan 6, 2016 at 8:20
@ste2425 can you provide the documentation/source for your answer? — devqon
– devqon, Commented Jan 6, 2016 at 8:23

RiesvGeffen · Accepted Answer · 2016-01-06 08:24:27Z

2

= 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. If no attr name is specified then the attribute name is assumed to be the same as the local name. Given <widget my-attr="parentModel"> and widget definition of scope: { localModel:'=myAttr' }, then widget scope property localModel will reflect the value of parentModel on the parent scope. Any changes to parentModel will be reflected in localModel and any changes in localModel will reflect in parentModel. 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. If you want to shallow watch for changes (i.e. $watchCollection instead of $watch) you can use =* or =*attr (=*? or =*?attr if the property is optional).

Source (CTRL+F, "=?")

answered Jan 6, 2016 at 8:24

RiesvGeffen

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5 Comments

Shashi Over a year ago

I was trying to replicate the exception, but I donot see an error being thrown. Instead the value set is just null. JSFiddle Sample

RiesvGeffen Over a year ago

Here is an example Plunker where you get the NON_ASSIGNABLE_MODEL_EXPRESSION error.

Shashi Over a year ago

according to your example, if I add selectModel: "=?" in child1 directive scope, it should still work. Correct?

RiesvGeffen Over a year ago

@Shashi 1). It's not my example, I just quickly googled the error and gave you the first example with that error. 2). I only gave you the information that Angular has about it. - This =? expression is new for me too, I don't know anything about it.

RiesvGeffen Over a year ago

@Shashi I've found another post with a lot of information about =?, hope that answers your question a bit more.

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