JavaScript Internals (521 Qs)•Part 7: Execution Context, Functional JS & V8 Engine (404 - 463)
How do you check an object is a promise or not
If you don't know if a value is a promise or not, wrapping the value as Promise.resolve(value) which returns a promise
javascript
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
javascript
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