javascript - Why is it possible to pass in a non-function parameter to Promise.then() without causing an error?

Question

I have the following:

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)"));

which produces the following output:

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

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

• 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?

Answer 1

The code

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

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)"))

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.

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

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"); })