Скачать или смотреть Understanding the undefined Issue in JavaScript Function Calls

Discover why `undefined` appears in your JavaScript code and learn how to avoid this issue when working with functions.
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This video is based on the question https://stackoverflow.com/q/62822765/ asked by the user 'Fidel Sebastian' ( https://stackoverflow.com/u/13901180/ ) and on the answer https://stackoverflow.com/a/62823087/ provided by the user 'smwhr' ( https://stackoverflow.com/u/3228127/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Why undefined Shows Up in Your JavaScript

As a beginner coder, encountering unexpected outputs in your code can be frustrating. One common issue faced by many is the appearance of the word undefined in the console output when executing function calls in JavaScript. The question at hand dives into this problem and seeks to understand why undefined surfaces between two desired results, particularly when using a function designed to compute runtime. Let's explore this further.

The Code Snippet in Question

Here’s the original code snippet causing confusion:

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

Analyzing the Output

When you run this code, the following happens:

First log output: When you call timeFuncRuntime(addOneToOne()), the function addOneToOne() is executed first, which returns the value 2. Hence, 2 gets printed on the first log line.

Second log output: When you call console.log(timeFuncRuntime()), you pass no arguments to timeFuncRuntime(). As a result, funcParameter inside the function is undefined, and thus the console outputs undefined.

Third log output: This comes from the return value of timeFuncRuntime(), which measures the execution time but returns the time difference, which is typically 0 because of how quickly the function runs.

Understanding the Function Call

Why the Parentheses Matter

The main reason you see undefined appears due to the incorrect use of parentheses. When you write timeFuncRuntime(addOneToOne()), you're actually invoking the function addOneToOne and passing its return value (2) to timeFuncRuntime. However, when you write timeFuncRuntime(), you're calling the function without passing anything, resulting in undefined.

How to Correctly Handle Function Runtime Measurement

If your goal is to measure how long a function takes to execute and also want to ensure it doesn't return undefined, you might want to rewrite your function. Here’s an improved version of the code:

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

Key Takeaways

Always ensure that when you pass a function to another, you're either passing the function itself (no parentheses) or executing it but knowing what to expect (the return value).

When debugging, be mindful of whether you're passing a function reference or the result of a function call.

If a function is expected to return a value, ensure that it's properly invoked to avoid returning undefined.

Understanding how functions work in JavaScript may seem complex at first, but with patience and practice, these concepts will become clearer. Happy coding!