Handle future results with then and catch.
✍️ 3 exercises + code challenge in this lessonA Promise represents a value that will be available in the future, like the result of a network request.
A promise is in one of three states: pending, fulfilled (success) or rejected (error).
.then(result => ...): runs when it succeeds..catch(error => ...): runs when it fails..finally(): runs either way.Promise.all([...]): waits for several promises.Mastering JavaScript Promises helps you write clean and reliable JavaScript code. Follow standard conventions, test edge cases, and keep logic simple. Try running the code example below and complete the exercises to build confidence.
<pre id="out"></pre>
<script>
const log = (...a) => document.getElementById("out").textContent += a.join(" ") + "\n";
function orderPizza(flavour) {
return new Promise((resolve, reject) => {
log("Ordering", flavour, "...");
setTimeout(() => {
if (flavour === "pineapple") reject(new Error("Out of pineapple!"));
else resolve(`${flavour} pizza is ready 🍕`);
}, 1000);
});
}
orderPizza("tikka")
.then(msg => log(msg))
.then(() => orderPizza("pineapple"))
.then(msg => log(msg))
.catch(err => log("Problem:", err.message))
.finally(() => log("Done ordering."));
</script>Check what you learned in “JavaScript Promises”.
Handle the successful result of a promise.
fetchData().(data => log(data));
Which method handles a rejected promise?
Create a promise that resolves with "Ready" and log the value with then().
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