AngularJS - Component that allows access to a text value

There is an issue with my service where it doesn't update the exposed string value externally. The service itself knows that the value has changed, but externally it remains the same. When I try nesting the string inside an object, it works fine, but I prefer not to do that.

I'm perplexed as to why this is happening. It seems like it should work and I feel like I am overlooking something fundamental.

Service:

myApp.service('neverChanges', function () {
    var id = 'Hello';
    var changeId = function () {
        console.log('pre change:' + id);
        id = 'World';
        console.log('post change:' + id);
    };

    return {
        id: id,
        changeId: changeId
    };
});

Controller:

myApp.controller('Controller1', ['neverChanges', function (neverChanges) {
    this.idValue = function() {
        return neverChanges.id;
    }
    this.clickHandler = function () {
        console.log('Trust me, I did fire...');
        neverChanges.changeId();
        console.log('external post change:' + neverChanges.id);
    };
}]);

Markup:

<div ng-app="myApp">
    <div ng-controller="Controller1 as vm">
         <h3>This will never change:</h3>
        <button ng-click="vm.clickHandler()">Click Me!</button>
        <p>Values:</p>
        <p>id: {{vm.idValue()}}</p>
</div>

Fiddle showing the two scenarios: http://jsfiddle.net/KyleMuir/2nhoc2rz/

Answer №1

In order to achieve the desired result, you must implement the following code snippet:

var changeId = function () {
    console.log('pre change:' + id);
    this.id = 'World';
    console.log('post change:' + id);
};

Answer №2

The issue arises from the presence of a local variable called id: var id = 'Hello';

Within the function, you are then transferring the value of this local variable to an object that is being returned:

return {
        id: id,
        changeId: changeId
    };

As a result, the returned object now contains a property id which is a mere COPY of the original id variable, while the changeId function only modifies the local variable, not the copy.

To address this issue, you should maintain a reference to the returned object, like so:

var result = {id:'Hello'};
result.changeId = function () {
    console.log('pre change:' + result.id);
    result.id = 'World';
    console.log('post change:' + result.id);
};
return result;

View the working version here: http://jsfiddle.net/y4mxazqh/

This approach eliminates the need for the local variable id and allows you to modify the object that was returned.

Although the return statement still creates a reference to your local variable result, both references point to the same object, enabling you to alter the object's content and reflect those changes in both references.

EDIT:

Essentially, originof's answer tackles the same issue but through a different method: By invoking vm.clickHandler(), the function clickHandler() has its this set to vm and vm is the object that was returned. Therefore, you can access the returned object. However, if you were to execute code like this:

var func = vm.clickHandler();
func();

it would not produce the same result. In this scenario, this would not be set to vm and you would encounter issues. It's important to be mindful of this distinction when choosing the this-based approach.

Answer №3

When you pass a function to another object, the function's scope can change. To resolve this, consider using:

this.id = "World"

instead of

id = "World"

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