How to add delay to promise inside then

Return a promise from the then handler that waits:

.then(() => new Promise(resolve => setTimeout(resolve, 1000)))

If you want to "pass through" the value of the promise, then

.then(x => new Promise(resolve => setTimeout(() => resolve(x), 1000)))

To avoid this boilerplate everywhere, write a utility function:

function sleeper(ms) {
  return function(x) {
    return new Promise(resolve => setTimeout(() => resolve(x), ms));
  };
}

then use it as in

.then(sleeper(1000)).then(...)

This is one of the rare situations you create a new promise:

fetch() {   
    return axios.get('/rest/foo')
        .then(value => new Promise(resolve => {
                setTimeout(() => {
                    resolve(value);
                }, delayInMilliseconds);
            })
        );
}

But rather than a one-off, I'd have (in fact, do have) a utility function:

function wait(ms, value) {
    return new Promise(resolve => setTimeout(resolve, ms, value));
}

Then:

fetch() {   
    return axios.get('/rest/foo')
        .then(value => wait(delayInMilliseconds, value));
}

---

Here's wait with TypeScript types (thanks MEMark!):

function wait<T>(ms: number, value: T) {
    return new Promise<T>((resolve) => setTimeout(resolve, ms, value));
}