TypeError: Promise.then is not a function in JavaScript

Just when you’re feeling confident in your javascript coding prowess, an error message like “TypeError: Promise.then is not a function” jumps out at you, leaving you scratching your head.

What does “TypeError: Promise.then is not a function in JavaScript” mean?

Let’s try to understand what the error message says.

“TypeError: Promise.then is not a function” seems daunting at first glance, but in simpler terms, it’s saying that you’re trying to use the .then() method on something that’s not a promise.


In JavaScript, promises are like those little notes you pass in class, promising to do something in the future. The .then() method is like your way of saying, “Once this promise is fulfilled, do this.”

// creating a promise
const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
  }, 1000);

// Trying to use .then() correctly
myPromise.then(result => {
  console.log(result); // Output: Success!

// But what if we do this?
const notAPromise = "I'm just a string, not a promise";

notAPromise.then(result => {
  console.log(result); // Error: TypeError: notAPromise.then is not a function

Possible Reasons why this error occurs:

Now that we’ve seen the error in action, let’s talk about the reasons behind its appearance.

  1. Wrong Variable Type: f you try to call .then() on something that’s not a promise (like a regular string, number, or object), JavaScript will throw this error message.
  2. Forgetting to Return a Promise: If you’re writing your own promise-returning function, make sure you actually return a promise. If you forget the return keyword, you’ll end up trying to use .then() on undefined.
  3. Promises Not Chaining Correctly: Promises can be chained together using .then(), forming a sequence of actions. If you mess up the chaining order or syntax, you might run into this error.
  4. This could be an issue with the library you are using probably, very rare but who knows!!

I hope you find this helpful.

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