基本 promise 问题:
console.log('Promise START');
function makeFullJSON(time) {
return new Promise((resolve, reject) => {
setTimeout(resolve, time, [time]);
})
}
var p1 = makeFullJSON(1000);
var p2 = makeFullJSON(500);
var p3 = makeFullJSON(750);
p1.then(array => {
console.log('Promise 1 complete', array);
});
p2.then(array => {
console.log('Promise 2 complete', array);
});
p3.then(array => {
console.log('Promise 3 complete', array);
});
Promise.all([p1, p2, p3]).then(arrayOfAllResolvedValues => {
console.log('Array of resolved values:', arrayOfAllResolvedValues);
});
console.log('Promise END');
代码输出为:
Promise START
Promise END
Promise 2 complete [ 500 ]
Promise 3 complete [ 750 ]
Promise 1 complete [ 1000 ]
Array of resolved values: [ [ 1000 ], [ 500 ], [ 750 ] ]
如何重写代码,使输出为:
Promise START
Promise 2 complete [ 500 ]
Promise 3 complete [ 750 ]
Promise 1 complete [ 1000 ]
Array of resolved values: [ [ 1000 ], [ 500 ], [ 750 ] ]
Promise END
最佳答案
完成后你想要发生的任何事情都在你传递给 then 的箭头函数中。
console.log('Promise START')
function makeFullJSON(time) {
return new Promise((resolve, reject) => {
setTimeout(resolve, time, [time])
})}
var p1 = makeFullJSON(1000)
var p2 = makeFullJSON(500)
var p3 = makeFullJSON(750)
p1.then(array => {
console.log('Promise 1 complete', array)})
p2.then(array => {
console.log('Promise 2 complete', array)})
p3.then(array => {
console.log('Promise 3 complete', array)})
Promise.all([p1, p2, p3]).then(arrayOfAllResolvedValues => {
console.log('Array of resolved values:', arrayOfAllResolvedValues)
console.log('Promise END')
})
为了放弃立即执行程序,并开始编写只有在所有 3 个 promise 都解决后才会发生的代码,因为听起来你想要发生,那么我建议直接在你的代码下面创建一个新函数,以包含代码您希望在解析后发生,并传递该函数,如:Promise.all([p1, p2, p3]).then(newFunctionName)
。以这种方式想象它可能更容易,至少在您习惯于思考它的精确工作原理之前是这样。
关于javascript - 如何在函数返回之前等待 promise 完成,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/42685257/