The order of looping through an object may be broken only during iteration?

Question

I guess the preferred way of looping through an object is this:

for (var prop in obj) {
  if( obj.hasOwnProperty( prop ) ) {
    console.log(\"obj.\" + prop          


        

Answer 1

The MDN page on delete states that:

...all major browsers support an iteration order based on the earliest added property coming first... However, in the case of Internet Explorer, when one uses delete on a property [and] adds back a property with the same name, the property will be iterated in its old position -- not at the end of the iteration sequence...

Illustration:

var obj = {};

obj.x = 1; 
obj.y = 2;
obj.z = 3;

var a = [];
for(var i in obj) a.push(i);

delete obj.y;
obj.y = 2;

var b = [];
for(var i in obj) b.push(i);


document.write("1:" + a + "<br>2:" + b);

Chrome/FF/Safari display 1:x,y,z 2:x,z,y, while in MSIE (and Edge) the result is 1:x,y,z 2:x,y,z.

Note that unlike ES5, ES6 mandates that the properties must be iterated in the creation order:

For each own property key P of O that is a String but is not an integer index, in property creation order...

http://www.ecma-international.org/ecma-262/6.0/#sec-ordinary-object-internal-methods-and-internal-slots-ownpropertykeys

The standard is not very clear what exactly "creation order" means. MS takes it that it's the initial creation time that matters, while others use the last creation time.