Question 14

Question

How would you check if a value is a promise in JavaScript?

Answer

Here's how you can determine if a value is a Promise in JavaScript:

function isPromise(value) {
  return typeof value === 'object' && value !== null && 
         typeof value.then === 'function';
}

Explanation:

1. typeof value === 'object' && value !== null: This part ensures we're dealing with an object, not a primitive type like a number or string. It also excludes null.

2. typeof value.then === 'function': The key check! Promises have a .then() method that is used to handle their resolution or rejection. If this method exists and is a function, we've likely found a Promise.

Important Notes:

• This method works reliably for standard Promises created with new Promise().

• Some libraries might implement custom Promise-like objects. The above check should catch most common cases, but it might not cover every edge case.