Douglas Crockford wrote:
I am not in favor of adding := to the language (as I am not in favor of adding to the language generally), but if := is added, my practical advice will be to always use := and never use =.
I hesitate to point this out, since it will convince you that := should not be added, but := is not a substitute for =!
As proposed at http://wiki.ecmascript.org/doku.php?id=strawman:define_properties_operator by Allen, both the LHS and RHS of := are ToObject-converted (only null and undefined throw) and the result of so converting the RHS is inspected for own properties which are copied into the LHS as if by ES5 meta-object APIs that preserve accessors (vs. data props), attributes, etc.
So you can't change x = 42 into x := 42. First, x must not denote null or undefined. Second, ToObject(42) has no own properties so nothing new is defined on x. Third, if x were a primitive boolean, number, or string, I take it from Allen's proposal that a throwaway Boolean, Number, or String wrapper would be created and (if the RHS dictates) extended.
We will probably see better performance with := since it doesn't have to slide down the proto chain to see if the assignment works.
This isn't a problem in at least some modern JITs.
And I like stepping away from ='s visual ambiguity, looking too much like an equality operator. It could be argued that = is still needed for respecting some kinds of inherited stuff, but it could also be argued that those kinds of inherited stuff are just weird edge cases that can now be conveniently and efficiently avoided.
You mean every single DOM property specified by a WebIDL attribute?
So if we do add :=, we should allow it to work on variables too, for consistency.
See above -- := is not a drop-in replacement for =. Think of it more like the .{ proposal, which had at one point suggested alternative syntax: .= (with an object literal to the right, but the latest proposal relaxes that to be any expression given to ToObject).
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