Javascript. Elements of array initialization order

Question

One day I saw certain way to swap elements in array. Like this:

var arr = [1,2,3];
arr[0] = [arr[1], arr[1] = arr[0]][0];

Obviously this method assume that first value of temporary array arr[1] will be calculated before second arr[1] = arr[0].
Otherway both values will be the same. Synthetic example:

var x = 1;
var arr = [x, x += 1, x += 1];
arr; // [1,2,3] or [3,3,2] or maybe [3,2,3]?

Is there any guarantees that javascript interpreter will not act in this manner?

Answer 1

JS array initializers are guaranteed to be evaluated left-to-right

ArrayAccumulation(ElementList : ElementList , Elision[opt] AssignmentExpression):

.1. Let postIndex be the result of performing ArrayAccumulation for ElementList with arguments array and nextIndex...

.4. Let initResult be the result of evaluating AssignmentExpression.

@http://www.ecma-international.org/ecma-262/6.0/#sec-runtime-semantics-arrayaccumulation

It's better not to rely on that though.

• Bergi: "It's better not to rely on that though." - Huh? You absolutely can rely on it.