How to use Promise's reject? - ionic2

Question

As the documentation says
resolve is
The Promise.resolve(value) method returns a Promise object that is
resolved with the given value.
reject is
The Promise.reject(reason) method returns a Promise object that is
rejected with the given reason.
I understand the uses of resolve but what will be the uses of reject and when to use it ?

Answer 1

Promise.reject is promise's way of throwing errors. Usually you would have a condition inside of your promise:
const user = {
name: 'John',
age: 17
}
const p = new Promise((resolve, reject) => {
if (user.age > 18) {
resolve('Welcome!');
} else {
reject(new Error('Too young!'));
}
});
You can then chain then and catch methods to handle the results of resolve and reject respectively.
p.then(message => {
console.log(message); // 'Welcome!', if promise resolves, won't work with age of 17
})
.catch(err => {
console.error(err); // 'Too young!', because promise was rejected
});

Answer 2

Here are few examples this statement can be used for:
Function defined to return a Promise, however you perform some sync checks and would like to return an error:
function request(data) {
if (!data) return Promise.reject("Empty data!");
// other logic
}
Unit tests, for example you would like to test that default data is used if service returns error (rejected promise):
const mockService = mock(Service);
// mock request method to return rejected promise
when(mockService.performRequest()).thenReturn(Promise.reject("Failed!"));
// inject mock instance and check that default data used if service failed
const sut = new ClassUnderTest(mockService);
expect(sut.getData()).to.eq("Default data");