setTimeout() 和 setInterval() 函数是异步的吗？

Question

I'm new for front end. In Javascript, are setTimeout() and setInterval() functions async? I read some tutorial that use these two functions to imitate async, but I don't know if they really are.

Answer 1

They are asynchronous and run in the JavaScript event loop . However they are not async , which is a syntactic sugar around functions that work with Promises .

We can provide a concrete example of how setTimeout is used to easily leverage async and await . Click run snippet to verify the results in your own browser -

 const sleep = ms => new Promise(r => setTimeout(r, ms)) // <- Promise around setTimeout async function main() { console.log("powering on. please wait...") await sleep(1000) console.log("initializing system resources...") await sleep(1500) console.log("133 MHZ CPU deteced") await sleep(1200) console.log("64 KB RAM available") await sleep(2000) console.log("120 MB hard drive deteced") await sleep(500) console.log("registering PCI devices...") await sleep(1000) console.log("starting MS-DOS...") return "ready" } main().then(console.log, console.error)

powering on. please wait ...
initializing system resources...
133 MHZ CPU deteced
64 KB RAM available
120 MB hard drive deteced
registering PCI devices...
starting MS-DOS...
ready

Answer 2

Both take a function as a parameter and schedule that function to be executed later. So, one could say they are "async" in a sense. However, in JavaScript the "async" word has a very specific meaning, since async is a keyword used to declare a function, and in this sense they are not async, since they do not return a Promise.

Answer 3

quick answer javascript is synchronous but some callbacks act async.

the setTimeout() function will be triggered in the stack, then continue on with what comes after even though it has not finished its timer.

Javascript is naturally synchronous, but some callbacks are async like setTimeout and setInterval, now if you want to wait for something, consider using a promise instead new Promise(() => console.log() . - I gave bad information, fixed it now

If you write a simple block of code like so, you'll notice that non timeout will print then interval then timeout even though it's laid in the opposite order.

setTimeout(() => console.log('timeout'), 3000)
setInterval(() => console.log('interval'), 2000)
console.log('non timeout')