javascript 这是什么？ 这是不好的做法吗？

Question

Right now i'm following a tutorial, and the guy who does the tutorial does this

let ball = ballObject = new AmmobtTransform(); 

inside a function to make "ball" a local variable, and "ballObject" a global variable at the same time. Now, he calls the "= ballObject" bit a 'handler', but I cant seem to find documentation of that (i find JavaScript event handlers eg touchdown events instead) so what is this, and is it an old/new feature? A link to documentation would be fine. Thank you.

Answer 1

It's a chained assignment. An assignment expression ( a = b ) has a result, which is the value that was assigned. So by doing let ball = ballObject = new AmmobtTransform(); the author is doing (effectively¹) this:

ballObject = new AmmobtTransform();
let ball = ballObject;

It's something that's reasonable to do in some situations. Like anything, it can also be misused. But:

...to make "ball" a local variable, and "ballObject" a global variable at the same time...

If you mean that ballObject is not declared anywhere, and the author is using that statement to create a new global variable, rather than just assigning to a variable that is already declared in some containing scope, that is definitely poor practice. It relies on what I call The Horror of Implicit Globals . In general, use strict mode so that doing that is the error it always should have been. :-)

...he calls the "= ballObject" bit a 'handler'...

That seems odd. The constructor name AmmobtTransform doens't immediately suggest to me that it's any kind of "handler," and if it were, it wouldn't just apply to ballObject , but to ball as well.

To sum up:

1. It's a chained assignment ( a = b = c )
2. Chained assignments are occasionally appropriate
3. Relying on The Horror of Implicit Globals is never appropriate; use strict mode to avoid doing it by accident
4. Calling ballobject a "handler" seems odd

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¹ Okay, so it's really a bit more like:

let ball;
const tmp =  new AmmobtTransform();
ballObject = tmp;
ball = tmp;

...(with tmp under the covers and gone by the end of it) but that's just pedantry on my part. ;-)