Question

A simple implementation for reversing an array is twice as fast compared to the built in function in Javascript, when tested in Chrome. What’s V8 doing? Here is the test:

var newArr = [];
var newArrDefault = [];
for(var i = 0; i < 10000000; i++){
    newArr[i] = i;
    newArrDefault[i] = i;
}


var startDefault = new Date();
newArrDefault.reverse();
console.log("Built in method took " + (new Date().getTime() - startDefault.getTime()));


var start = new Date();
for(var i = 0; i < newArr.length / 2; i++){
    var tmp = newArr[i];
    newArr[i] = newArr[newArr.length-i-1];
    newArr[newArr.length-i-1] = tmp;
}
console.log("Custom method took " + (new Date().getTime() - start.getTime()));

Results on Version 20.0.1132.47 Ubuntu 12.04 (144678):

Built in method took 149
Custom method took 71

Answer 1

For the fun of it, I implemented the specification like so:

var upper, upperExists, lowerExists, lowerValue, upperValue;

for(var lower = 0, len = newArr.length >>> 0, middle = Math.floor(len / 2); lower != middle; ++lower) {
    upper = len - lower - 1;
    lowerValue = newArr[lower];
    upperValue = newArr[upper];
    lowerExists = newArr.hasOwnProperty(lower);
    upperExists = newArr.hasOwnProperty(upper);

    if (lowerExists && upperExists) {
        newArr[lower] = upperValue;
        newArr[upper] = lowerValue;
    } else if (upperExists) {
        newArr[lower] = upperValue;
        delete newArr[upper];
    } else if (lowerExists) {
        newArr[upper] = lowerValue;
        delete newArr[lower];
    }
}

The jsperf can be found here.

It includes a whole bunch of code to deal with missing entries, which is why it’s so much slower than both the native and your code (some optimizations may be possible, but it won’t affect the performance enough). The performance difference between your code and the native implementation wasn’t very conclusive though.

Under most circumstances arrays are a contiguous block of values with no gaps in between, so you should be safe with that kind of code; as long as you know the difference 🙂