🔍 JavaScript Promise Methods - Explained with Examples

📌 then() - Handling Success

Definition: The then() method is used to define what to do when a Promise is fulfilled (i.e., successful).

When to use: When you want to handle the result after a successful async task.

const promise = Promise.resolve("Data received");

promise.then(result => {
  console.log("Success:", result);
});
      

⚠️ catch() - Handling Errors

Definition: The catch() method handles rejected Promises (errors).

When to use: When you want to deal with failures like network errors or invalid operations.

const promise = Promise.reject("Something went wrong");

promise
  .then(data => console.log(data))
  .catch(error => console.error("Error caught:", error));
      

✅ finally() - Always Runs

Definition: finally() runs after the promise settles, no matter if it was resolved or rejected.

Use case: Cleanup, hiding loaders, resetting states.

fetch("https://jsonplaceholder.typicode.com/posts/1")
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error(err))
  .finally(() => console.log("✅ Request completed"));
      

🔗 Promise.all() - Wait for All

Definition: Promise.all() waits for all promises to succeed or fails if any one fails.

When to use: When you need all operations to complete before proceeding (e.g., multiple API calls).

const p1 = Promise.resolve("🟢 Task 1");
const p2 = Promise.resolve("🟢 Task 2");

Promise.all([p1, p2])
  .then(results => console.log("All done:", results))
  .catch(err => console.error("At least one failed:", err));
      

⚡ Promise.race() - First to Settle

Definition: Promise.race() returns the first settled promise (fulfilled or rejected).

Use case: Timeout handling or picking the fastest response.

const fast = new Promise(res => setTimeout(() => res("Fast!"), 500));
const slow = new Promise(res => setTimeout(() => res("Slow"), 2000));

Promise.race([fast, slow]).then(result => console.log("First done:", result));
      

🔍 Promise.allSettled() - Get All Results

Definition: This method returns results of all promises whether they are resolved or rejected.

Use case: When you want to know the outcome of every promise without short-circuiting.

const p1 = Promise.resolve("OK");
const p2 = Promise.reject("Failed");

Promise.allSettled([p1, p2]).then(results => console.log(results));
      

🎯 Promise.any() - First Success

Definition: Returns the first fulfilled promise. Ignores any rejections.

Use case: Get the first successful result, even if others fail.

const fail1 = Promise.reject("Fail 1");
const fail2 = Promise.reject("Fail 2");
const pass = Promise.resolve("Success!");

Promise.any([fail1, fail2, pass])
  .then(result => console.log("First fulfilled:", result))
  .catch(err => console.error("All failed:", err));
      

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