How do you check an object is a promise or not?
Interviewer Expectations for this Question
01
Core Competency
Assesses fundamental understanding of JavaScript conventions, runtime behavior, and memory/performance considerations.
02
Evaluation Criteria
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Comprehensive Model Answer
Verified Solution
If you don't know if a value is a promise or not, wrapping the value as Promise.resolve(value) which returns a promise
function isPromise(object) {
if (Promise && Promise.resolve) {
return Promise.resolve(object) == object;
} else {
throw "Promise not supported in your environment";
}
}
var i = 1;
var promise = new Promise(function (resolve, reject) {
resolve();
});
console.log(isPromise(i)); // false
console.log(isPromise(promise)); // true
Another way is to check for .then() handler type
function isPromise(value) {
return Boolean(value && typeof value.then === "function");
}
var i = 1;
var promise = new Promise(function (resolve, reject) {
resolve();
});
console.log(isPromise(i)); // false
console.log(isPromise(promise)); // true
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.