※This article is a reprint from the Innovator Japan Engineers’ Blog.

What is a Promise

A Promise is...

The Promise object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value. MDN - Promise

That's what it is.

In summary, a Promise is an object that nicely handles asynchronous operations and their results.

Using Promises provides the following main benefits:

Reduces nesting
Improves readability
Allows passing the result of one operation to the next
Enables catching exceptions

Let's look at some examples of using Promises.

Asynchronous Processing Without Promises

Here is an example of asynchronous processing using callbacks without Promises.

// Example of a higher-order function without Promises
const asyncSayHi = (greet, callback) => {
    setTimeout(function () {
    callback(greet);
  }, 1000);
};

asyncSayHi('Hello', (value) => {
	console.log(value);
});
// Output: Hello

When you want to call asyncSayHi consecutively, it becomes a so-called callback hell like this.

// Callback hell
asyncSayHi('Hello', (value) => {
	console.log(value);
    asyncSayHi('こんにちは', (value) => {
        console.log(value);
        asyncSayHi('你好', (value) => {
    	    console.log(value);
            // callback loop is forever...
        });
    });
});
// Output: Hello こんにちは 你好

Asynchronous Processing With Promises

Rewriting the previous example of asynchronous processing using callbacks with Promises looks like this.

// Promise implementation
const asyncPromiseSayHi = function (greet) {
	return new Promise((resolve, reject) => {
  	if (greet) {
    	resolve(greet);
    } else {
    	reject('Please greet');
    }
  })
};

// Execute asynchronous processing
asyncPromiseSayHi('Hello').then((value) => {
	console.log(value);
}).catch((error) => {
	console.log(error);
});
// Hello

// Execute asynchronous processing consecutively
asyncPromiseSayHi('Hello').then((value) => {
	console.log(value);
  return new asyncPromiseSayHi(value);
}).then((value) => {
	console.log(value);
  return new asyncPromiseSayHi(value);
}).then((value) => {
	console.log(value);
	return new asyncPromiseSayHi(value);
}).catch((error) => {
	console.log(error);
});
// Hello Hello Hello

If you want to execute multiple processes in parallel, a method called Promise.all is available.

const asyncPromiseSayHi = function (greet) {
	return new Promise((resolve, reject) => {
  	if (greet) {
    	resolve(greet);
    } else {
    	reject('Please greet');
    }
  })
};

const asyncPromiseSalute = function (salute) {
	return new Promise((resolve, reject) => {
  	if (salute) {
    	resolve(salute);
    } else {
    	reject('Please salute');
    }
  });
};

// Execute asynchronous processing consecutively
Promise.all(['asyncPromiseSayHi', 'asyncPromiseSalute']).then((value) => {
   asyncPromiseSayHi('Hello').then((value) => {
       console.log(value);
   });
   asyncPromiseSalute('Attention').then((value) => {
       console.log(value);
   });
});
// Hello Attention