[英]How to write an Asynchronous Loop using Promises?
如何使用 Promises 編寫同步循環? 下面的兩個函數都沒有等待前一個循環完成,然后再開始......
(async ()=> {
let arr = [3,1,2,1,2];
const waitFor = ms => new Promise(r => setTimeout(r, ms));
// Using Promise.all
const firstFn = async () => { // doens't work
return Promise.all(arr.map(async (sec) => {
await waitFor(sec*1000);
console.log(`waited for ${sec} seconds`);
}));
}
await firstFn();
// Using new Promise
const secondFn = async () => {
arr.map(sec => {
new Promise(async (res, rej) => {
await waitFor(sec*1000);
console.log(`waited for ${sec} seconds`);
res();
});
});
}
await Promise.all(secondFn());
})();
map 在並行執行中處理承諾。 如果您想按順序使用for... of
或for
的簡單形式。 例子:
async function something () {
const arr = [3,1,2,1,2];
for (let x = 0; x < arr.length; x++) {
const sec = arr[x];
await waitFor(sec*1000);
console.log(`waited for ${sec} seconds`);
}
}
這是一個異步 function 的示例,它采用異步函數列表並按順序執行它們。 等待一個完成,然后再移動到另一個。
const wait = ms => new Promise ( resolve => setTimeout ( () => (console.log(`wait ${ms}`), resolve()), ms ) ); const async_chain = async ([fn, ...fns]) => typeof fn === 'function'? (await fn(), await async_chain(fns)): undefined; (async function main() { await async_chain ( [ async () => wait(1000), async () => wait(2000), async () => wait(3000) ] ) })();
您不必為此使用 promise。 您可以為此使用for..of
循環:
for await (const sec of arr){
await waitFor(sec*1000);
console.log(`waited for ${sec} seconds`);
}
您可以在此處了解有關 async for of 循環的更多信息。 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for-await...of
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