Why does console.log not require an argument in this case?

Question

If I use the Fetch API as follows:

fetch(url)
    .then(response => response.json())
    .then(console.log)

I understand that this would log the result of the previous "then" (the response data), but why does console.log not require any arguments in this case?

Is there any technical reasoning or documentation behind this, and can any other built-in methods be used in this nature?

Answer 1

Thats simple Javascript, in this case the console.log requires that argument, what happens here is that the then function takes a callback as its first argument and internally executes that callback with the argument returned by the last then function. So it means that you are passing the reference (or a copy, im not sure) of console.log function instead of directly executing this.

In summary, this:

function a (callback) {
  var something = 12345;
  callback(something);
}

a(console.log);

is the same as

a(function(something) {
  console.log(something);
})