Скачать или смотреть How to Fix Undefined Input Value in JavaScript

Learn how to resolve the `undefined` value issue when changing input values in JavaScript. Understand the correct usage of event handlers and functions for effective web development.
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This video is based on the question https://stackoverflow.com/q/64352411/ asked by the user 'Marihuan' ( https://stackoverflow.com/u/14265568/ ) and on the answer https://stackoverflow.com/a/64352476/ provided by the user 'Dipankar Maikap' ( https://stackoverflow.com/u/14425902/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Fix Undefined Input Value in JavaScript: A Simple Guide

If you’re delving into the world of JavaScript and web development, you might encounter some frustrating issues along the way. One common problem many developers face is trying to set an input value using buttons, only to find that the value is undefined. This guide will help you understand why this happens and how to resolve it effectively.

The Problem: What is undefined Input Value?

In your code, you attempted to create buttons that would update an input field with numbers when clicked. Here’s a snippet of the HTML structure you were using:

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

The primary issue here is that when you're trying to access the values from your buttons, the attempt yields undefined.

Why is it Undefined?

Selector Issue: The method document.querySelectorAll() uses CSS selectors. In your case, you aimed to select button IDs directly ("numb1, numb2, numb3"), but CSS selectors do not recognize IDs with a comma-separated structure.

Improper Value Passing: Your initial function number() did not accept any parameters, which means it couldn't process the button values effectively.

The Solution: A Simple Rewrite

To fix the undefined value issue, you should modify both the button’s click events and the number function. Let’s break this down:

Step 1: Update the Buttons

Change the onclick attribute of your buttons to directly pass a number to the number function when clicked:

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

Step 2: Modify the Function

Update the number function to accept and utilize the parameter passed from the button:

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

Complete Working Example

Here’s how the complete HTML and JavaScript should look:

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

Conclusion: A Clear Path Forward

With these straightforward changes, you can effectively eliminate the undefined input value issue. By passing values directly through the onclick attribute of the buttons and accepting those values in your function, your input field will update as expected.

If you ever find yourself stuck with JavaScript, remember to check your selectors and the way you're passing data into your functions. Happy coding!