Are you stuck in a scenario in your JavaScript code where you have to check if an Object in your code is a Promise object or not?

Here are some methods to check if an Object is a Promise object.

Method 1 : Checking if Object.prototype.toString.call(p) === "[object Promise]"

This is the process of checking if the string version of prototype of the object p is "[object Promise]"

function isPromise(p) {
  return p && Object.prototype.toString.call(p) === "[object Promise]"
}

Here are some examples of various types of data and we see that only a JavaScript promise object returns a true on calling the isPromise() function.

// Tests
var pi = 3.14
var name = "John"
var obj = { site: "debugpointer.com" }
var numbers = [1, 2, 3]
var prom = new Promise(function (resolve, reject) {
  resolve()
})

isPromise(pi) // false
isPromise(name) // false
isPromise(obj) // false
isPromise(numbers) // false
isPromise(prom) // true

Method 2 : Checking if typeof value.then === 'function'

In this method, we will check if .then exists and if its a function.

function isPromise(value) {
  return Boolean(value && typeof value.then === "function")
}

Now, let's use the above isPromise() method.

// Tests
var pi = 3.14
var name = "Jogn"
var obj = { site: "debugpointer.com" }
var numbers = [1, 2, 3]
var prom = new Promise(function (resolve, reject) {
  resolve()
})

isPromise(pi) // false
isPromise(name) // false
isPromise(obj) // false
isPromise(numbers) // false
isPromise(prom) // true

Source - https://github.com/graphql/graphql-js/blob/master/src/jsutils/isPromise.js

Method 3 : Checking if Promise.resolve(object) == object

Here is a very modular way where we check if .resolve() is an object.

function isPromise(object) {
  if (Promise && Promise.resolve) {
    return Promise.resolve(object) == object
  } else {
    throw "Promise not supported in your environment" // Most modern browsers support Promises
  }
}

Now, let's use the above isPromise() method.

// Tests
var pi = 3.14
var name = "Satvik"
var obj = { site: "debugpointer.com" }
var numbers = [1, 2, 3]
var prom = new Promise(function (resolve, reject) {
  resolve()
})

isPromise(pi) // false
isPromise(name) // false
isPromise(obj) // false
isPromise(numbers) // false
isPromise(prom) // true

Credits - https://gist.github.com/MarkoCen/ec27b8cd42855fde8a245d43b7b081d0

Method 4 : Checking if obj is Object & obj.then is a function

In this method we conditionally check if obj is an object and it has a function obj.then.

function isPromise(obj) {
  return (
    !!obj &&
    (typeof obj === "object" || typeof obj === "function") &&
    typeof obj.then === "function"
  )
}

Now, let's use the above isPromise() method.

// Tests
var pi = 3.14
var name = "Satvik"
var obj = { site: "debugpointer.com" }
var numbers = [1, 2, 3]
var prom = new Promise(function (resolve, reject) {
  resolve()
})

isPromise(pi) // false
isPromise(name) // false
isPromise(obj) // false
isPromise(numbers) // false
isPromise(prom) // true

To summarize, we have seem 4 ways to check if an Object is a Promise in Javascript. All of them can be used in your code. You can instead use TypeScript which should help with types as you code.