Check if a Value is a Promise using JavaScript

# Check if a Value is a Promise in JavaScript

To check if a value is a Promise:

1. Check if the type of the value is an object
2. Check if the object has a property named then of type function.
3. If both conditions return true, the value is a promise.

Copied!
function isPromise(p) {
  if (typeof p === 'object' && typeof p.then === 'function') {
    return true;
  }

  return false;
}

const p1 = new Promise(resolve => {
  resolve(10);
});

console.log(isPromise(p1)); // 👉️ true
console.log(isPromise({})); // 👉️ false

We used the logical AND (&&) operator to chain 2 conditions.

All promises have a type of object, so this is what we check for first.

Copied!
const p1 = new Promise(resolve => {
  resolve(10);
});

console.log(typeof p1); // 👉️ "object"

We access the then property on the object and check if it has a type of function.

Promises have a .then() and .catch() methods.

This conditional check would fail if someone passed in a plain object with a then property of type function.

Copied!
function isPromise(p) {
  if (typeof p === 'object' && typeof p.then === 'function') {
    return true;
  }

  return false;
}

console.log(isPromise({then: () => {}})); // 👉️ true

We satisfied both conditions and got a false positive.

To make the solution more robust, you can add a check for the catch() method as well.

Copied!
function isPromise(p) {
  if (
    typeof p === 'object' &&
    typeof p.then === 'function' &&
    typeof p.catch === 'function'
  ) {
    return true;
  }

  return false;
}

console.log(isPromise({then: () => {}})); // 👉️ false

We added a condition that checks if the object contains a catch method as well.

Copied!
function isPromise(p) {
  if (
    typeof p === 'object' &&
    typeof p.then === 'function' &&
    typeof p.catch === 'function'
  ) {
    return true;
  }

  return false;
}

// ❌ Error: Cannot read property 'then' of null
console.log(isPromise(null));

Copied!
console.log(typeof null); // 👉️ "object"

We can add a condition that checks if the passed-in value is not null.

Copied!
function isPromise(p) {
  if (
    p !== null &&
    typeof p === 'object' &&
    typeof p.then === 'function' &&
    typeof p.catch === 'function'
  ) {
    return true;
  }

  return false;
}

console.log(isPromise(null)); // 👉️ false

const p1 = new Promise(resolve => {
  resolve(10);
});
console.log(isPromise(p1)); // 👉️ true

Now our solution is a bit more robust. The only way to get a false positive would be to pass an object that contains methods named then() and catch(), but is not an actual promise.

Your use case might not require this over-engineered solution. You might be just fine with checking for an object that has the then method.

I've also written an article on how to access the value of a Promise.

# Using Promise.resolve() instead

As an alternative to checking if the value is a Promise, you could directly use the Promise.resolve() method.

Copied!
const value = Promise.resolve(42);

Promise.resolve(value).then(value => {
  console.log(value); // 👉️ 42
});

The value in the example doesn't necessarily have to be a Promise.

Copied!
const value = 42;

Promise.resolve(value).then(value => {
  console.log(value); // 👉️ 42
});

Either way, our code works as expected.

The Promise.resolve() method resolves the supplied value to a Promise.

If the provided value is a Promise, then the Promise is returned, otherwise, the method returns a Promise that is resolved with the supplied value.

If the value is not a Promise, we wrap it in one to be able to use the .then() method.

If the value is a Promise, it gets returned as is and we are still able to use the .then() method.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Check if a value is a Float or an Integer in JavaScript
• Check if Value is Negative or Positive Number in JavaScript
• How to Check if a Value is a Number in JavaScript
• How to Check if a Value is an Object in JavaScript