从 setTimeout 获取返回值

我只想得到 setTimeout的返回值但我得到的是函数的整个文本格式？

function x () {
setTimeout(y = function () {
return 'done';
}, 1000);
return y;
}


console.log(x());

151879 次浏览

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You can't get a return value from the function you pass to setTimeout.

The function that called setTimeout (x in your example) will finish executing and return before the function you pass to setTimeout is even called.

Whatever you want to do with the value you get, you need to do it from the function you pass to setTimeout.

In your example, that would be written as:

function x () {
setTimeout(function () {
console.log("done");
}, 1000);
}


x();

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Better to take a callback for function x and whatever task you want to perform after that timeout send in that callback.

function x (callback) {
setTimeout(function () {
callback("done");
}, 1000);
}


x(console.log.bind(console)); //this is special case of console.log
x(alert)

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最佳答案

You need to use Promises for this. They are available in ES6 but can be polyfilled quite easily:

function x() {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('done!');
});
});
}


x().then((done) => {
console.log(done); // --> 'done!'
});

With async/await in ES2017 it becomes nicer if inside an async function:

async function() {
const result = await x();
console.log(result); // --> 'done!';
}

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I think you want have flag to know event occured or no. setTimeout not return a value. you can use a variable to detect event occured or no

var y="notdone";
setTimeout(function () {
y="done";
}, 1000);

You can access variable y after timeout occured to know done or not

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You can use a combination of Promise and setTimeOut like the example below

let f1 = function(){
return new Promise(async function(res,err){
let x=0;
let p = new Promise(function(res,err){
setTimeout(function(){
x= 1;
res(x);
},2000)
})
p.then(function(x){
console.log(x);
res(x);
})




});
}