# Creation of an fulfilled promise in JavaScript

Sometimes you may need to create an already fulfilled promise. There are two methods for this: the `Promise.resolve` method creates a resolved promise, `Promise.reject` creates a rejected promise. As a parameter, these methods receive what will become the result or error of the promise, respectively.

When might we need an already fulfilled promise? Let's look at an example. Suppose we have some function that takes a number as a parameter, does something with it asynchronously and returns a promise with the result:

`function func(num) { return new Promise(function(resolve) { setTimeout(function() { resolve(num * num); }, 3000); }); }`

We will use our function as follows:

`func(5).then(function(res) { console.log(res); // shows 25 });`

Let us now decide that we will perform our asynchronous operation only if a number greater than zero is passed. Otherwise, the result of the function must be zero:

`function func(num) { if (num > 0) { return new Promise(function(resolve) { setTimeout(function() { resolve(num * num); }, 3000); }); } else { return 0; // a result is 0 } }`

However, now it turns out that the function returns either a promise or a number. Because of this, we can no longer apply the `then` method to the result of the function, since if the function returns a number, we will get an error:

`func(0).then(function(res) { // an error, we apply the then method to 0 });`

`Promise.resolve` will help us fix the problem:

`function func(num) { if (num > 0) { return new Promise(function(resolve) { setTimeout(function() { resolve(num * num); }, 3000); }); } else { return Promise.resolve(0); // returns a promise, not a number } }`

Let us now decide that for the passed zero we should return zero, and for numbers less than zero - an exception. The `Promise.reject` method will help us with this:

`function func(num) { if (num > 0) { return new Promise(function(resolve) { setTimeout(function() { resolve(num * num); }, 3000); }); } else if (num === 0) { return Promise.resolve(0); } else { return Promise.reject('incorrect number'); // returns a rejected promise } }`
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