what's the meaning of '=?' in angularJS directive isolate scope declaration?

Question

Does the question mark after equals have special meaning? ie:

scope: {foo: '=?'}

does the above mean 'do not raise error if 'foo' cannot be resolved?

Solution

Yes:

The 'isolate' scope takes an object hash which defines a set of local scope properties derived from the parent scope. These local properties are useful for aliasing values for templates. Locals definition is a hash of local scope property to its source:

= 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.

It should trigger the expected error on every digest that affects the scope property:

parentSet = parentGet.assign || function() {
// reset the change, or we will throw this exception on every $digest
lastValue = scope[scopeName] = parentGet(parentScope);
     throw Error(NON_ASSIGNABLE_MODEL_EXPRESSION + attrs[attrName] +
     ' (directive: ' + newScopeDirective.name + ')');
};

//...


if (parentValue !== scope[scopeName]) {
    // we are out of sync and need to copy
    if (parentValue !== lastValue) {
        // parent changed and it has precedence
        lastValue = scope[scopeName] = parentValue;
    } else {
        // if the parent can be assigned then do so
        parentSet(parentScope, lastValue = scope[scopeName]);
    }
}

Comments

[Nikita]

Makes sense but why doesn't this directive throw exception. parent scope property doesn't exist and scope assignment is NOT using a '=?'

[Matt Zeunert]

It seems to only throw the error when the value is set, like here: plnkr.co/edit/OSpaC6sPE0hY9yAeFghr?p=preview

[Jason Axelson]

@cebor It's currently linked in the answer, but here is a more direct link: docs.angularjs.org/api/ng/service/…

[Jason Axelson]

Although personally I wish it were documented in the scope section directly and not in $compile.

[wilblack]

Thanks for the answer, I have been using angular for well over a year and never found the "=?" option on a directive. You just made my day ;-)