探索 JavaScript 定时器
setTimeout()
When writing JavaScript code, you might want to delay the execution of a function.
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:
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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:
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setTimeout returns the timer id. This is generally not used, but you can store this id, and clear it if you want to delete this scheduled function execution:
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Zero delay
If you specify the timeout delay to 0, the callback function will be executed as soon as possible, but after the current function execution:
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This code will print
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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.
Some browsers (IE and Edge) implement a setImmediate() method that does this same exact functionality, but it's not standard and unavailable on other browsers. But it's a standard function in Node.js.
setInterval()
setInterval is a function similar to setTimeout, with a difference: instead of running the callback function once, it will run it forever, at the specific time interval you specify (in milliseconds):
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The function above runs every 2 seconds unless you tell it to stop, using clearInterval, passing it the interval id that setInterval returned:
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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:
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Recursive setTimeout
setInterval starts a function every n milliseconds, without any consideration about when a function finished its execution.
If a function always takes the same amount of time, it's all fine:
Maybe the function takes different execution times, depending on network conditions for example:
And maybe one long execution overlaps the next one:
To avoid this, you can schedule a recursive setTimeout to be called when the callback function finishes:
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to achieve this scenario:
setTimeout and setInterval are available in Node.js, through the Timers module.
Node.js also provides setImmediate(), which is equivalent to using setTimeout(() => {}, 0), mostly used to work with the Node.js Event Loop.