Question

How do you check an object is a promise or not Javascript?

Answer 1

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