JavaScript: Return true if an object looks like a Promise, false otherwise

JavaScript fundamental (ES6 Syntax): Exercise-189 with Solution

Check If Looks Like Promise

Write a JavaScript program that returns true if an object looks like a Promise, false otherwise.

• Check if the object is not null, its typeof matches either object or function and if it has a .then property, which is also a function.

Sample Solution:

JavaScript Code:

// Define a function 'isPromiseLike' that checks if the given object 'obj' is similar to a Promise
const isPromiseLike = obj =>
  // Check if the object is not null and has a 'then' method, typically present in Promise objects
  obj !== null &&
  (typeof obj === 'object' || typeof obj === 'function') &&
  typeof obj.then === 'function';

// Test cases to check if the objects are promise-like
console.log(isPromiseLike({
  then: function() {
    return '';
  }
})); // true (has 'then' method)
console.log(isPromiseLike(null)); // false (null)
console.log(isPromiseLike({})); // false (object without 'then' method)

Output:

true
false
false

Flowchart:

Live Demo:

See the Pen javascript-basic-exercise-189-1 by w3resource (@w3resource) on CodePen.

For more Practice: Solve these Related Problems:

• Write a JavaScript program that returns true if an object has a then method and appears to be a Promise.
• Write a JavaScript function that checks if an input is “thenable” by testing for the presence of a callable then property.
• Write a JavaScript program that inspects an object’s structure to determine if it follows the Promise interface.
• Write a JavaScript function that uses duck typing to verify if an object behaves like a Promise.

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