Скачать или смотреть How to Fix undefined Props in Child Components in Angular

Learn how to troubleshoot and resolve the issue of `undefined` properties in child components when passing data from a parent component in Angular.
---
This video is based on the question https://stackoverflow.com/q/64696240/ asked by the user 'Sunstrike527' ( https://stackoverflow.com/u/12157144/ ) and on the answer https://stackoverflow.com/a/64697050/ provided by the user 'Michael D' ( https://stackoverflow.com/u/6513921/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Why prop is undefined in child component Angular?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding undefined Props in Child Components in Angular

When working with Angular, you might encounter issues when passing properties between parent and child components. One common problem is finding that a prop is undefined in the child component, which can lead to frustration and the need for urgent debugging. In this article, we'll explore why this happens when trying to pass a size for an SVG image from a parent component to its child, and how to troubleshoot and resolve this issue effectively.

The Problem: Passing Props in Angular

Imagine you have a parent component that looks something like this:

[[See Video to Reveal this Text or Code Snippet]]

When you check the size input in the app-svg-icon component, you might see that it's undefined, which is not what you expect. Additionally, if you try passing the property without square brackets, you end up with a string representation of the object: '{width: 40, height: 40}'. This can lead to further confusion when you attempt to use it in your component.

Why Does This Happen?

The issue typically arises due to the way data-binding works in Angular:

Square Brackets [] are used for property binding, allowing Angular to pass an actual object reference rather than just a string.

Interpolation {{}} treats the value as a string and can lead to undesired results, for instance, converting objects to strings like "[Object, Object]".

Key Observations

In your child component, you might have an @ Input() like this:

[[See Video to Reveal this Text or Code Snippet]]

This logs undefined when you check for the size property during initialization. Let's break down how to resolve this problem in two main scenarios.

Solutions to the Problem

Option 1: Using a Variable in the Parent Component

If you're managing the size object in the parent component, the best approach is to use a variable for property binding like so:

Parent Component (Controller)

[[See Video to Reveal this Text or Code Snippet]]

Parent Component (Template)

[[See Video to Reveal this Text or Code Snippet]]

This way, Angular correctly passes the size object to the child component as intended.

Option 2: Passing the Object Directly in the Template

If you prefer to bind the object directly in the template without using a variable, you can do this by passing a string as follows:

Parent Component (Template)

[[See Video to Reveal this Text or Code Snippet]]

Child Component (Controller)

To ensure it can handle the string properly, update your child component as shown:

[[See Video to Reveal this Text or Code Snippet]]

Handling JSON Parsing

When passing data as a string, make sure that it is properly formatted and that your quotes are correctly matched to avoid any parsing errors. The way the string is defined in the parent template is crucial for the child component to interpret it correctly.

Conclusion

When encountering undefined props in child components in Angular, it is essential to understand how property binding works and how to effectively use variables versus string literals. By following the outlined options above, you can troubleshoot this common issue and ensure your data flows smoothly through your components.

In summary, whether you use a variable in the parent component or pass the object directly in the template, it’s crucial to ensure that the data is passed in a compatible format for the child component to recognize and use without returning undefined.