Best of Modern JavaScript — Resolve and Reject Promises

Best of Modern JavaScript — Resolve and Reject Promises

How to handle promises.

Since 2015, JavaScript has improved immensely.

It’s much more pleasant to use it now than ever.

In this article, we’ll look at JavaScript promises.

Other Ways of Creating Promises

Other than using the Promise constructor, we can create promises in other ways.

One way is the Promise.resolve method.

It takes the resolved value of the promise as its argument.

And it returns a promise.

For instance, we can write:

Promise.resolve(1)
  .then(x => console.log(x));

x is the resolved value from Promise.resolve .

If x is a promise whose construction is the receiver, then the promise is unchanged.

For instance, if we have:

const p = new Promise(() => null);
console.log(Promise.resolve(p) === p);

then we get true from the console log.

If the argument is a thenable and then the then method in the argument is a function, then the resolved value with Promise.resolve is the argument that we call the then parameter with.

For example, if we have:

const thenableObj = {
  then(reaction) {
    reaction('foo');
  }
};
const promise = Promise.resolve(thenableObj);
console.log(promise instanceof Promise); 
promise.then(x => console.log(x));

We have a thenableObj , which has the then method.

It takes a reaction function and we called it with 'foo' .

Then we pass in an object to the Promise.resolve method, which returns a promise.

If we check if promise is an instance of a Promise , then that returns true .

We can also call then on it with a callback to get the 'foo' value, which is assigned to x .

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