JavaScript: Checking if a value is a Promise/thenable object

A common way to check if a value is a Promise in JavaScript is to use the instanceof operator:

obj instanceof Promise

This will only work if an object is an instance of a native Promise class, but we can use any object that implements the then method in place of a Promise, for example:

let x = {then: cb => cb(42)}
console.log(x instanceof Promise)

One option is to turn the thenable object to a native Promise with Promise.resolve, which will always return the instance of Promise:

console.log(Promise.resolve(x) instance of Promise)

Or, we can just check that the value is an object that has the then method:

function isPromise(v) {
  return typeof v == "object" && typeof v.then == "function"
}

A few tests that everything is working as expected:

assert(isPromise(Promise.resolve(14)))
assert(isPromise({then: cb => cb(42)}))


assert(!isPromise(42))
assert(!isPromise({then: 42}))

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