proposal-for-in-order ) ¶

Specifying for-in enumeration order ¶

(Partially.)

ECMA-262 leaves the order of for (a in b) … almost totally unspecified, but real engines tend to be consistent in at least some cases.

Furthermore, over the years implementations have observed that anyone who wants to run code on the web needs to follow some constraints not captured by the spec.

This is a stage 4 (finished) proposal to begin fixing that.

for-in mechanics ( https://www.martinmck.com/posts/es2020-everything-you-need-to-know/ ) ¶

for (x in obj) … is a super useful syntax for many things - mainly iterating over the keys of an object.

           for (let key in obj) {
  console.log(key);
}

          

This proposal is related to the order and semantics of which elements are iterated in a for..in loop.

Before this proposal, most JavaScript engines had already applied common sense - currently all major browsers loop over the properties of an object in the order in which they were defined.

There were some nuances, however.

These mainly involved more advanced features like proxies. for..in loop semantics have historically been left out the JavaScript spec, but this proposal ensures everyone has a consistent point of reference as to how for..in should work.

for-in Partially specifying object enumeration order in JavaScript ( https://github.com/tc39/proposal-for-in-order ) ¶

Specifying for-in enumeration order ¶

for-in mechanics ( https://www.martinmck.com/posts/es2020-everything-you-need-to-know/ ) ¶

Articles ¶

https://www.freecodecamp.org ¶