为什么可以将非函数参数传递给Promise.then（）而不会导致错误？ [英] Why is it possible to pass in a non-function parameter to Promise.then() without causing an error?

问题描述

我有以下内容：

new Promise(resolve => setTimeout(resolve, 2000))
    .then(() => console.log("after 2 seconds"));

new Promise(resolve => setTimeout(resolve, 3000))
    .then(console.log("before 3 seconds (instantly)"));

产生以下输出：

> node index.js
before 3 seconds (instantly)
after 2 seconds

Promise.then（）预计一个 onFulfilled 函数，但我传入 console.log（在2秒之前（立即）），这是不是一个功能。由两部分组成的问题：

Promise.then() expects a onFulfilled function, but I passed in console.log("before 2 seconds (instantly)"), which is not a function. Two-part question:

• 为什么 console.log（在2秒之前（立即））立即执行（或根本不执行）？


• 当我没有传递函数时，为什么第二个Promise没有引发异常？

• Why does console.log("before 2 seconds (instantly)") get executed right away (or at all)?
• Why didn't the second Promise raise an exception when I didn't pass in a function?

推荐答案

代码

console.log("before 3 seconds (instantly)")

是表达式，特别是一个函数调用表达式。无论出现在哪里，它都意味着相同的东西，包括外观作为Promise的 .then（）方法的参数。与任何其他类似语言一样，函数调用中使用的表达式在函数调用之前进行评估，因此

is an expression, specifically a function call expression. Wherever that appears, it means the same thing, including an appearance as an argument to the .then() method of a Promise. As in any other similar language, an expression used in a function call is evaluated before the function call, so

.then(console.log("before 3 seconds (instantly)"))

结果在 console.log（）函数中调用 first ，然后将返回值传递给 .then（）。这就是你立即在控制台中看到消息的原因。

results in the console.log() function being called first, with the return value then passed to .then(). That's why you see the message in the console immediately.

将 undefined 传递给。然后（）是允许的，因为那是 console.log（）返回的，所以没有引发错误。

Passing undefined to .then() is allowed, and since that's what console.log() returns, there's no error raised.

如果你希望在实现Promise时发生 console.log（），你可以将它包装在一个函数中：

If you want that console.log() to happen when the Promise is fulfilled, you'd wrap it in a function:

.then(function() { console.log("after 3 seconds"); })

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