How to add delay to promise inside then

fetch() {
return axios.get('/rest/foo')
//.then(response => {throw new Error(response)}) // Uncomment to test network error
//.then( <<add delay here>> ) // Uncomment to simulate network delay
}

How do I add delay in the latter then block, so it will wait specified amount of time before passing control to the fetch callers then blocks?

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最佳答案

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(...)
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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));
}

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