Understanding the Cannot read property 'valueChanges' of undefined Error in Angular

Описание: Learn how to fix the `Cannot read property 'valueChanges' of undefined` error in Angular when working with ViewChild and NgModel.
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This video is based on the question https://stackoverflow.com/q/62214770/ asked by the user 'S4L4H' ( https://stackoverflow.com/u/5537465/ ) and on the answer https://stackoverflow.com/a/62214829/ provided by the user 'Edison Augusthy' ( https://stackoverflow.com/u/6781625/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Cannot read property 'valueChanges' of undefined Error in Angular

When developing applications in Angular, you might encounter various errors that can hinder your progress. One common error that developers face is the message Cannot read property 'valueChanges' of undefined. If you are trying to implement the valueChanges observable on a ViewChild of type NgModel and receive this error during the lifecycle methods like ngOnInit, ngAfterViewInit, or ngAfterContentChecked, you are in the right place! In this post, we'll break down the causes of this error and provide an effective solution.

Understanding the Problem

The error occurs when your component tries to access the valueChanges property of an NgModel instance that has not been initialized yet. This usually happens because of the lifecycle timing of Angular components. Here's a simple illustration of the situation you might be facing:

HTML Structure

You may have an HTML form like this:

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

TypeScript Code Snippet

In your Angular component, you might have the following code:

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

When you attempt to access this.filterInput.valueChanges in ngOnInit, it triggers the error because filterInput is not yet defined.

The Solution: Adjusting ViewChild Declaration

To resolve this issue, a simple adjustment to your @ ViewChild decorator is necessary. Here’s how you can modify your code:

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

Explanation of static Parameter

static: false means that the filterInput will be initialized after the view is fully initialized. This implies that the ngOnInit lifecycle hook may run before the filterInput is populated, leading to your error.

static: true makes sure that filterInput is initialized during the ngOnInit lifecycle hook and thus available when you try to access valueChanges in it.

Summary

By changing the @ ViewChild declaration to include static: true, you ensure that filterInput is available when your component initializes. This small yet crucial change can save you from running into the Cannot read property 'valueChanges' of undefined error.

In recap, if you encounter this error while working with Angular forms, make sure to check your @ ViewChild declaration and set the static property to true. Happy coding!