为什么我不能将函数调用（而不是函数引用或匿名函数）传递给setTimeout（）？ [英] Why can I not pass a function call (rather than a function reference or an anonymous function) to setTimeout()?

问题描述

请忽略这样一个事实：这段代码什么都没有，并为可能是一个无聊的问题道歉！

Please ignore the fact that this code achieves nothing and apologies for what's probably an inane question!

我知道我无法将函数调用传递给 setTimeout（）作为第一个参数，但为什么我不能这样做？

I understand that I cannot pass a function call into setTimeout() as the first argument, but why can I not do that?

let names = ['Andy', 'Ross', 'David'];

function printer (name) {
 console.log(name);
}

names.forEach(name => setTimeout(printer(name), 1000);

结果：

Andy
timers.js:327
    throw new TypeError('"callback" argument must be a function');
    ^

我可以通过使用对 printer 的引用并使用 bind（）来发送名称以及它，但为什么我必须采取这些额外的步骤？

I can solve the problem by instead using a reference to printer and using bind() to send name along with it, but why must I take these extra steps?

let names = ['Andy', 'Ross', 'David'];

function printer (name) {
  console.log(name);
}

names.forEach(name => setTimeout(printer.bind(null, name), 1000));

结果：

Andy
Ross
David

推荐答案

这是因为执行的顺序。如果你将函数调用传递给setTimeout，函数将会马上执行，我e。该函数立即放在javascript的执行堆栈上。

This is because of the order of execution. If you pass a function call to setTimeout, the function will be executed immediately, i.e. the function is put on javascript's execution stack immediately.

如果传递函数名称，即对函数的引用，则只有在计时器结束后，该函数才会被放入javascript线程的执行堆栈中。

If you pass a function name, i.e. a reference to a function, the function is only put in the javascript thread's execution stack once the timer finishes.

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