Unspecified binding in knockout.js

Question

Unspecified binding in knockout.js

As a newcomer to JS app development, I am currently focused on studying existing code and attempting to replicate it while playing around with variable names to test my understanding.

I have been working on this JS quiz code built with KO.js... Here is my version (with minimal changes).

http://jsfiddle.net/s6EsB/

The issue I'm facing is that when running the code, I encounter an "isCorrect is undefined" error not in my script, but within the KO library. If I comment out the lines containing isCorrect in the HTML, I then receive a "correctNbrAnswers is undefined" error, still within the KO library...

It seems like KO is having difficulty recognizing the data parameters I'm using to reference the appropriate templates...

<script id="questionView" type="text/html">
        <div>Question <span data-bind="text: index"></span></div>
        <div data-bind="visible: selectedAnswer() === undefined">
            <div data-bind="text: questionText"></div>
            <ul data-bind="template: { name: 'answerView', foreach: answers }"></ul>
        </div>
        <div data-bind="visible: selectedAnswer() !== undefined">
            <div data-bind="template: { name: 'questionResultView', data: selectedAnswer }"></div> <!-- isCorrect id defined for answers, so if KO respects this scope, this should work... But it doesn't! -->
            <a href="#" data-bind="click: $parent.nextQuestion">Next</a>
        </div>
    </script>
    <script id="questionResultView" type="text/html">
        <div data-bind="visible: isCorrect">Correct Answer!</div> <!-- Returns isCorrect is undefined (knockout.js line blablabla) -->
        <div data-bind="visible: !isCorrect">Incorrect. The correct answer was <span data-bind="text: $parent.correctAnswer.answerText"></span></div>
    </script>

If anyone could provide insights into this matter, I would greatly appreciate it...

Thank you!

Ari ;o)

javascript data-binding knockout.js undefined

Answer 1

Answer №1

The issue here lies in the fact that the selectedAnswer variable may be undefined or have a value. While this is typically okay, there seems to be a flaw in the logic of your view.

<script id="questionView" type="text/html">
    <div data-bind="visible: selectedAnswer() === undefined">
        <!-- This section will be visible only if selectedAnswer is undefined -->
    </div>
    <div data-bind="if: selectedAnswer() !== undefined">
        <!-- This section will not be rendered if selectedAnswer is undefined -->
        <div data-bind="template: { name: 'questionResultView', data: selectedAnswer }"></div>
    </div>
</script>

Please pay attention to the additional comments I've included for clarity.

The first div element will appear when selectedAnswer is found to be undefined. In this case, the content within the div will still be processed, but it won't be displayed.

The second div will remain hidden if selectedAnswer is undefined. However, its contents will still be evaluated. As the template relies on selectedAnswer, it tries to render an object that is undefined. To address this, you must prevent the evaluation of the second div's content when selectedAnswer is undefined.

There are various ways to resolve this issue, but let's focus on the simplest solution for now.

Replace the visible binding on the second div with an if binding. For more details, refer to the documentation. Essentially, the if binding functions similarly to visible, except it avoids evaluating the content if the condition isn't met – which aligns perfectly with our requirements.

Answer 2

The issue here lies in the fact that the selectedAnswer variable may be undefined or have a value. While this is typically okay, there seems to be a flaw in the logic of your view.

<script id="questionView" type="text/html">
    <div data-bind="visible: selectedAnswer() === undefined">
        <!-- This section will be visible only if selectedAnswer is undefined -->
    </div>
    <div data-bind="if: selectedAnswer() !== undefined">
        <!-- This section will not be rendered if selectedAnswer is undefined -->
        <div data-bind="template: { name: 'questionResultView', data: selectedAnswer }"></div>
    </div>
</script>

Please pay attention to the additional comments I've included for clarity.

The first div element will appear when selectedAnswer is found to be undefined. In this case, the content within the div will still be processed, but it won't be displayed.

The second div will remain hidden if selectedAnswer is undefined. However, its contents will still be evaluated. As the template relies on selectedAnswer, it tries to render an object that is undefined. To address this, you must prevent the evaluation of the second div's content when selectedAnswer is undefined.

There are various ways to resolve this issue, but let's focus on the simplest solution for now.

Replace the visible binding on the second div with an if binding. For more details, refer to the documentation. Essentially, the if binding functions similarly to visible, except it avoids evaluating the content if the condition isn't met – which aligns perfectly with our requirements.

Unspecified binding in knockout.js

Answer №1

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