One of the most commonly used methods for debugging and troubleshooting in JavaScript is the console.log method. Developers use it to output data and messages to the browser console to gain insights into the state of their application.
However, there are times when executing console.log() results in "undefined" being printed to the console, even though the method is being called with the correct arguments. In this article, we will explore the reasons why the console returns "undefined" when using console.log() and how to avoid this issue.
console.log() is a void function
The console.log() method does not return any value. It is a void function that is designed to output data to the console for debugging purposes. This means that when you call console.log() with an argument, it will print the argument to the console but will not return anything.
For example:
console.log("Hello World!"); // Output: "Hello World!"
const x = console.log("Hello World!"); // Output: "Hello World!"
console.log(x); // Output: undefined
In the example above, we can see that the console.log() method outputs "Hello World!" to the console, but the value of the variable "x" is undefined. This is because console.log() does not return a value.
not returning a value from a function
Another reason why you may see "undefined" in the console is when you forget to return a value from a function. In JavaScript, if a function does not explicitly return a value, it will return "undefined" by default.
For example:
function add(x, y) {
const result = x + y;
console.log(result);
}
const sum = add(1, 2); // Output: 3
console.log(sum); // Output: undefined
In the example above, the add() function calculates the sum of two numbers and outputs it to the console. However, we did not explicitly return the result from the function. Therefore, when we call add(1, 2) and assign the result to "sum", it returns undefined because the function did not explicitly return a value.
asynchronous execution
When executing code asynchronously, console.log() may not output the expected value because it is being called before the code that generates the value has had a chance to complete. This can happen when working with APIs or other remote services.
For example:
function getData() {
fetch('https://jsonplaceholder.typicode.com/todos/1')
.then(response => response.json())
.then(data => console.log(data))
}
getData(); // Output: {"userId":1,"id":1,"title":"delectus aut autem","completed":false}
console.log("After calling getData"); // Output: "After calling getData"
In the example above, the getData() function is calling an API and outputting the result to the console. However, because the API call is asynchronous, console.log() is being called before the API has had a chance to return the data. This can lead to the console outputting "undefined" instead of the expected data.
To avoid this, we can use async/await or Promises to ensure that the console.log() method is called after the data has been returned.
conclusion
In conclusion, the console returning "undefined" when executing console.log() is a common issue that developers face while debugging their code. This can happen when calling a void function, forgetting to return a value from a function, or when working with asynchronous code. By understanding these common reasons, we can easily avoid the issue and gain valuable insights into the state of our application.
references
Follow me on Twitter for more such blogs.
Cheers 🍻