How to Check if a Function is Async in JavaScript

Check if a Function is Async in JavaScript #

To check if a function is async, access the constructor.name property on the function and check if the value is equal to AsyncFunction, e.g. myFunction.constructor.name === 'AsyncFunction'. If the equality check returns true, then the function is async.

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const sum = async (a, b) => {
  return a + b;
};

if (sum.constructor.name === 'AsyncFunction') {
  // 👇️ this runs
  console.log('✅ function is async');
} else {
  console.log('⛔️ function is NOT async');
}

When access on a function, the constructor property returns:

• the Function constructor for non-async functions
• the AsyncFunction constructor for async functions

Every async function in JavaScript is an AsyncFunction object.

If the condition passes, then we have an async function.

If we access the name property on a non-async function, the string "Function" is returned.

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function example() {}

console.log(example.constructor.name); // 👉️ "Function"

Different constructor functions are used to create async and non-async functions, so we can easily determine if a function is async.

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// 👇️ non-async returns Promise
function example() {
  return new Promise(resolve => {
    resolve(100);
  });
}

console.log(example()); // 👉️ Promise {}

console.log(example.constructor.name); // 👉️ "Function"

The example shows a non-async function, which returns a promise.

You can check if the return value of a function is a promise in the following way.

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function example() {
  return new Promise(resolve => {
    resolve(100);
  });
}

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

  return false;
}

console.log(isPromise(example())); // 👉️ true
console.log(isPromise(() => {})); // 👉️ false

We check if the passed in value is an object and it contains a property named then that is of type function.

Most of the time you can't just call functions to check what their return value is, because they might be mutating state, e.g. in a database.

However, if you could call the function without mutating any state, you can combine these 2 conditions.

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if (
  example.constructor.name === 'AsyncFunction' ||
  (typeof example === 'function' && isPromise(example()))
) {
  console.log('✅ Function returns promise');
} else {
  console.log('⛔️ Function does NOT return promise');
}

Our if statement checks if the function is async or it returns a promise. If either check passes, the if block runs.

Further Reading #

• Check if a Value is a Promise using JavaScript
• How to Rename an Object's Key in JavaScript