Performance of assigning values to array

Question

Code optimizing is said here in SO that profiling is the first step for optimizing javascript and the suggested engines are profilers of Chrome and Firefox. The problem with

Answer 1

Assigning values to arrays is significantly slower than assigning to normal variables. Arrays are evil! This is very sad and I really don't understand why this occurs. Arrays are so important!

That's because normal variables are statically scoped and can be (and are) easily optimised. The compiler/interpreter will learn their type, and might even avoid repeated assignments of the same value.

These kind of optimisations will be done for arrays as well, but they're not so easy and will need longer to take effect. There is additional overhead when resolving the property reference, and since JavaScript arrays are auto-growing lists the length needs to be checked as well.

Prepopulating the arrays will help to avoid reallocations for capacity changes, but for your little arrays (length=10) it shouldn't make much difference.

Is there any method to get the speed of non-array-variable-assigments and the dynamics of arrays?

No. Dynamics do cost, but they are worth it - as are loops.

You hardly ever will be in the case to need such a micro-optimisation, don't try it. The only thing I can think of are fixed-sized loops (n <= 4) when dealing with ImageData, there inlining is applicable.

Push is evil!

Nope, only your test was flawed. The jsperf snippets are executed in a timed loop without tearup and -down, and only there you have been resetting the size. Your repeated pushes have been producing arrays with lengths of hundredth thousands, with correspondent need of memory (re-)allocations. See the console at http://jsperf.com/pre-filled-array/11.

Actually push is just as fast as property assignment. Good measurements are rare, but those that are done properly show varying results across different browser engine versions - changing rapidly and unexpected. See How to append something to an array?, Why is array.push sometimes faster than array[n] = value? and Is there a reason JavaScript developers don't use Array.push()? - the conclusion is that you should use what is most readable / appropriate for your use case, not what you think could be faster.