🌐 Discover JavaScript Timers
setTimeout(): void
编写 JavaScript 代码时，你可能希望延迟函数的执行。
🌐 When writing JavaScript code, you might want to delay the execution of a function.
这是 setTimeout 的作用。你需要指定一个稍后执行的回调函数，以及一个用毫秒表示的延迟时间：
🌐 This is the job of setTimeout. You specify a callback function to execute later, and a value expressing how later you want it to run, in milliseconds:
setTimeout(() => { // runs after 2 seconds }, 2000); setTimeout(() => { // runs after 50 milliseconds }, 50);
此语法定义了一个新函数。你可以在其中调用任何其他函数，或者传入一个已有的函数名和一组参数：
🌐 This syntax defines a new function. You can call whatever other function you want in there, or you can pass an existing function name, and a set of parameters:
const myFunction = (firstParam, secondParam) => { // do something }; // runs after 2 seconds setTimeout(myFunction, 2000, firstParam, secondParam);
setTimeout 在 Node.js 中返回一个 Timeout 实例，而在浏览器中返回一个数字类型的定时器 ID。这个对象或 ID 可以用来取消已安排的函数执行：
const timeout = setTimeout(() => { // should run after 2 seconds }, 2000); // I changed my mind clearTimeout(timeout);
🌐 Zero delay
如果你将超时延迟指定为 0，回调函数将尽快执行，但会在当前函数执行之后：
🌐 If you specify the timeout delay to 0, the callback function will be executed as soon as possible, but after the current function execution:
setTimeout(() => { console.log('after '); }, 0); console.log(' before ');
这段代码将打印
🌐 This code will print
before after
通过在调度程序中排队函数，这特别有用，可以避免在密集任务上阻塞 CPU，并在执行繁重计算时执行其他函数。
🌐 This is especially useful to avoid blocking the CPU on intensive tasks and let other functions be executed while performing a heavy calculation, by queuing functions in the scheduler.
一些浏览器（IE 和 Edge）实现了
setImmediate()方法，它具有完全相同的功能，但它不是标准方法，并且在其他浏览器上 不可用。不过，它是 Node.js 中的标准函数。
setInterval(): void
setInterval 是一个类似于 setTimeout 的函数，它的区别在于：它不会只执行一次回调函数，而是会在你指定的时间间隔（以毫秒为单位）内一直执行该函数：
setInterval(() => { // runs every 2 seconds }, 2000);
上面的函数每两秒运行一次，除非你用 clearInterval 告诉它停止，并传入 setInterval 返回的间隔 ID:
🌐 The function above runs every 2 seconds unless you tell it to stop, using clearInterval, passing it the interval id that setInterval returned:
const timeout = setInterval(() => { // runs every 2 seconds }, 2000); clearInterval(timeout);
通常会在 setInterval 回调函数内部调用 clearInterval，以便让它自动判断是否应该再次运行或停止。例如，这段代码会一直执行某些操作，除非 App.somethingIWait 的值为 arrived：
🌐 It's common to call clearInterval inside the setInterval callback function, to let it auto-determine if it should run again or stop. For example this code runs something unless App.somethingIWait has the value arrived:
const interval = setInterval(() => { if (App.somethingIWait === 'arrived') { clearInterval(interval); } // otherwise do things }, 100);
🌐 Recursive setTimeout
setInterval 会每隔 n 毫秒启动一次函数，而不会考虑函数何时执行完成。
如果一个函数总是花费相同的时间，那就没问题：
🌐 If a function always takes the same amount of time, it's all fine:
0 1000 2000 3000 | | | | | | | | | [██████████] | | | [exe script] | | | [██████████] | | | | [██████████] | | | [exe script] | | | [██████████] | | | | [██████████] | | | [exe script] | | | [██████████] |__________|___________|___________|____
也许函数需要不同的执行时间，具体取决于网络条件，例如：
🌐 Maybe the function takes different execution times, depending on network conditions for example:
0 1000 2000 3000 | | | | | | | | | [██████████] | | | [exe script] | | | [██████████] | | | | [████████████████████] | | [ execute script ] | | [████████████████████] | | | [██████████] | | | [exe script] | | | [██████████] |__________|___________|___________|____
也许一个长时间的执行会与下一个重叠：
🌐 And maybe one long execution overlaps the next one:
0 1000 2000 3000 4000 | | | | | | [████████████████████] | | | [ execute script ] | | | [████████████████████] | | | [███████████████████████████] | | [ execute script ] | | [███████████████████████████] | | | | [██████████████] | | | [execute script] | | | [██████████████] |__________|___________|___________|___________|__
要避免这种情况，你可以安排在回调函数完成时调用递归 setTimeout：
🌐 To avoid this, you can schedule a recursive setTimeout to be called when the callback function finishes:
const myFunction = () => { // do something setTimeout(myFunction, 1000); }; setTimeout(myFunction, 1000);
要实现这种情况：
🌐 to achieve this scenario:
0 1000 2000 3000 | | | | | [█████████████] | [█████████████] | [ exec script ]>──1s──<[ exec script ] | [█████████████] | [█████████████] | | | | |__________|____________|___________|__________
setTimeout 和 setInterval 可以在 Node.js 中使用，通过 Timers 模块。
Node.js 还提供了 setImmediate()，它会安排回调在事件循环的检查阶段执行。与 setTimeout(() => {}, 0) 不同，它通常不等同，因为它们的回调在事件循环的不同阶段执行，并且它们的执行顺序取决于执行上下文。
🌐 Node.js also provides setImmediate(), which schedules a callback to execute during the check phase of the event loop. Unlike setTimeout(() => {}, 0), it is not generally equivalent because their callbacks are executed in different phases of the event loop and their execution order depends on the execution context.