[object Promise] in output
[object Promise] in output in JavaScript means a promise was used where its resolved value was expected — nearly always a missing await.
How do I fix [object Promise] in output?
To fix [object Promise] in output in JavaScript, locate the line causing the error and check the values right before it runs. Most occurrences happen because of missing await. Apply a defensive check or safe fallback, make sure collections or objects exist before access, and verify return values from upstream functions.
What does [object Promise] in output mean?
An async function always returns a promise. Using that promise in a string or a calculation converts it via toString, giving [object Promise]. The count or shape often looks right, which is why it slips through.
Why does [object Promise] in output happen?
1. Missing await
The promise itself flows onward instead of the value.
Breaks
const total = getTotal();
console.log(`Total: ${total}`);Works
const total = await getTotal();
console.log(`Total: ${total}`);2. map with an async callback
map gives an array of promises. The length is right, which is the trap.
Breaks
const names = ids.map(async id => fetchName(id));Works
const names = await Promise.all(ids.map(id => fetchName(id)));3. reduce with an async callback
The accumulator is a promise from the second iteration onward.
Breaks
nums.reduce(async (acc, n) => acc + await f(n), 0);Works
nums.reduce(async (acc, n) => (await acc) + await f(n), Promise.resolve(0));How to find it in your own code
If you see [object Promise], or typeof gives 'object' where you expected a number or string, look for the await you dropped. Promise.all is what you want whenever you need every result rather than the first.
Try it: fix this bug
mediumawait sumDelayed([1, 2, 3]) gives "0[object Promise][object Promise][object Promise]".
function delay(n) {
return new Promise((resolve) => setTimeout(() => resolve(n), 1));
}
async function sumDelayed(nums) {
let total = 0;
for (const n of nums) {
total += delay(n);
}
return total;
}More challenges with this error
Each of these challenges runs into this error or pattern. Find the bug and fix it in your browser — no install, no account needed.
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