Error Handling with Promises in JavaScript

Handling errors properly in Promises ensures that issues are caught and handled gracefully without breaking the execution flow.

🔹 Basic Error Handling in Promises

If a Promise rejects, the .catch() method captures the error.

new Promise((resolve, reject) => {
    setTimeout(() => reject("❌ Something went wrong!"), 2000);
})
    .then((result) => {
        console.log("This will never execute", result);
    })
    .catch((error) => {
        console.log("Caught Error:", error);
    });

✅ Output (after 2 sec):

Caught Error: ❌ Something went wrong!

✔ Once an error occurs, no further .then() executes.

🔹 Catching Errors Anywhere in the Chain

Errors propagate down the chain until caught by .catch().

new Promise((resolve, reject) => {
    setTimeout(() => resolve(10), 1000);
})
    .then((result) => {
        console.log("Step 1:", result);
        throw new Error("Something broke!");
    })
    .then((result) => {
        console.log("This won't run", result);
    })
    .catch((error) => {
        console.log("❌ Caught Error:", error.message);
    });

✅ Output (after 1 sec):

Step 1: 10
❌ Caught Error: Something broke!

✔ Any error thrown in a .then() jumps to .catch().

🔹 Using `.finally()` for Cleanup

The .finally() method always executes whether the Promise resolves or rejects.

new Promise((resolve, reject) => {
    setTimeout(() => reject("Network error!"), 1500);
})
    .then((result) => console.log("Success:", result))
    .catch((error) => console.log("❌ Caught:", error))
    .finally(() => console.log("✅ Cleanup completed!"));

✅ Output (after 1.5 sec):

❌ Caught: Network error!
✅ Cleanup completed!

✔ Use .finally() for UI cleanup, logging, etc.

🔹 Handling Errors in `async/await`

With async/await, use try...catch to handle errors.

async function fetchData() {
    try {
        let response = await fetch("https://invalid-url.com");
        let data = await response.json();
        console.log(data);
    } catch (error) {
        console.log("❌ Fetch Error:", error.message);
    } finally {
        console.log("✅ Request completed!");
    }
}

fetchData();

✅ Output (if the URL fails to fetch):

❌ Fetch Error: Failed to fetch
✅ Request completed!

✔ Error handling is clearer with try...catch.

🔹 Catching Errors from Multiple Promises

Using Promise.all(), if one fails, the entire Promise.all() fails.

Promise.all([
    fetch("https://jsonplaceholder.typicode.com/posts/1").then((res) => res.json()),
    fetch("https://invalid-url.com").then((res) => res.json()), // Invalid URL
])
    .then((data) => console.log("✅ Data:", data))
    .catch((error) => console.log("❌ Error in one of the promises:", error.message));

✅ Output (if one request fails):

❌ Error in one of the promises: Failed to fetch

✔ If one request fails, Promise.all() rejects immediately.

🔹 Using `Promise.allSettled()` to Handle Errors Separately

Unlike Promise.all(), Promise.allSettled() waits for all promises and returns results for both success and failure.

Promise.allSettled([
    fetch("https://jsonplaceholder.typicode.com/posts/1").then((res) => res.json()),
    fetch("https://invalid-url.com").then((res) => res.json()), // Failing request
])
    .then((results) => console.log("Results:", results));

✅ Output (success & failure info):

[
    { status: "fulfilled", value: { ... } },
    { status: "rejected", reason: "Failed to fetch" }
]

✔ Safer when handling multiple requests!

🔹 Summary

✔ .catch() captures errors anywhere in the chain
✔ .finally() runs always (cleanup)
✔ async/await uses try...catch for better readability
✔ Promise.all() fails if one fails
✔ Promise.allSettled() handles both success & failure

🚀 Want more details? Let me know! 😊

Souy Soeng

Error Handling with Promises in JavaScript