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

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?