问题
I am digging into this & Object prototype (You Don't Know JS) and I found this tricky thing that blew my mind:
I created a literal Array (JS Arrays are objects) x = ['foo', 42, 'bar'] and added a property called baz like x.baz = 'baz'. Then, in Chrome dev console, just typping x will display the following result: > (3) ["foo", 42, "bar", baz: "baz"]. If I unfold the ">", each value of the array has its own key like:
0: "foo"
1: 42
2: "bar"
baz: "baz"
What kind of JS-object-monster did I create? For and forEach loops do not count the baz property and bypass it.
Help would be very appreciated. Thank you!
回答1:
This is not a monster. It's just a JS object like any other array. Adding properties to a JS array is not a no no... You can. One good reason is to simple carry a state in a contained manner within the array. Functionally this is correct in a very loosely typed language like JS. If you chose to do this, then you also have to take precautions in order not to be bewildered in a for in loop, for instance.
回答2:
you just created an object and added a property. When you use the "for in" loop, you move on the properties of the instance of the object, not of those its prototype.
Don't worry, you haven't created a JS-object-monster! :)
来源：https://stackoverflow.com/questions/44291907/for-and-foreach-bypass-array-property