should be the reason for the keyword

js keyword and cannot be used as variable, function name, procedure, and object name.
originally, he is also right, but you don't know what kind of mistake will happen when you use it with other things.
for example, when some tools are compressed. It's over.
this makes it much safer.

I don't have much knowledge, but the author just mixes it at will.
you see that the implementation of this promise is Promise.prototype.finally
https://github.com/then/promi.

agree with upstairs.

in fact, there is no problem with this way of writing at 99%.

another 1% chance that the compiler will compile your code into something else will be a problem. I yy an example:

// 


Promise.prototype = function catch(onReject) {};