Create a delaying function in promise-style

Problem

You want to create a function that creates a promise that delays for a given number of milliseconds.

Ingredients

• a function
• the Promise object
• the setTimeout() function

Directions

1. Create a function that accepts the number of milliseconds.

   function delay(ms) {
     ...
   }

2. Let the function return a promise object.

   function delay(ms) {
     return new Promise(...);
   }

3. Add the resolve-/reject-callback to the promise.

   function delay(ms) {
     return new Promise(
       (resolve, reject) => {
         ...
       }
     );
   }

4. Inside that callback function call setTimeout().

   function delay(ms) {
     return new Promise(
       (resolve, reject) => {
         setTimeout(resolve, ms);
       }
     );
   }

5. Voilá, a perfect delaying function in promise-style.

   delay(5000).then(() => {
     console.log('Printed after 5 seconds.')
   });

Variants

• If you want to pass data to the promise:

  function delay(ms, data) {
    return new Promise(
      (resolve, reject) => {
        setTimeout(() => {
          resolve(data);
        }, ms);
      }
    );
  }

Notes

• Promises are natively supported since ES2015. If you are using ES5 you may want to use one of the many polyfill libraries for Promises.