When is the finally block in JavaScript Promise executed? I was asked this question recently at a seminar and I wanted to write a small article to relieve some confusion.
The answer is, to quote Snape:
……always.
The basic structure is as follows:
try {
// I'll try to execute some code for you} catch (error) {
// I will deal with any errors that appear in the process} finally {
// I will execute anyway}
For example, this is a Chuck Norris joke generator example, which comes from the Chuck Norris database API. (By the way, I found this API from Todd Motto's list of awesome open APIs, which is great for presentations and side projects.)
async function getData() {
try {
let chuckJokes = await fetch(`https://api.chucknorris.io/jokes/random`)
.then(res => res.json());
console.log('I got some data for you!');
document.getElementById("quote").innerHTML = chuckJokes.value;
} catch (error) {
console.warn(`There is an error here: ${error}`);
} finally {
console.log('Everything, finally executes!');
}
}
Console output: (Console output that successfully obtained the joke should be displayed here)
Now, let's introduce a typo in the URL of the API and accidentally add a bunch of <var>r</var> . This will cause our try statement to fail, which means that catch will now throw an error.
async function getData() {
try {
// Let's mess it up a little let chuckJokes = await fetch(`https://api.chucknorrrrris.io/jokes/random`)
.then(res => res.json());
console.log('I got some data for you!');
document.getElementById("quote").innerHTML = chuckJokes.value;
} catch (error) {
console.warn(`There is an error here: ${error}`);
} finally {
console.log('Everything, finally executes!');
}
}
Console output: (Error message and finally block output should be displayed here)
A very important aspect is that what is not explained in the example is that even if return or break statement stops the code in try or catch block, the finally block will run .
When to use finally ?
I found it particularly useful in two different situations, although I believe there are other situations:
- When I need to repeat the code in
tryandcatchblocks. Here is an example from the Vue recipe I wrote. I closed the loading state infinallyblock. This includes, like the example above, I need to change the UI in any case. - When some cleaning work is needed. Oracle mentions this in its documentation. It is Java, but the premise is the same.
finally is not as useful as try and catch , but it is worth noting in some cases. Hope this clarifies your question!
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