JS perf quiz

results.md

Similar solutions with very different execution times. What's the reason? Why solution #1 is too slow?

Hint: access to .length property and to variables is not an issue.

Based on a real case in CSSTree

Node v6.11.3

N	Solution #1	Solution #2
10000	107 ms	5 ms
20000	287 ms	3 ms
30000	665 ms	3 ms
40000	1183 ms	4 ms
50000	1880 ms	4 ms
60000	2793 ms	6 ms
70000	7985 ms	16 ms
80000	14322 ms	20 ms
90000	21804 ms	21 ms
100000	29559 ms	23 ms
150000	83309 ms	42 ms

Node v8.6.0

N	Solution #1	Solution #2
10000	98 ms	6 ms
20000	328 ms	5 ms
30000	708 ms	4 ms
40000	1272 ms	4 ms
50000	1946 ms	4 ms
60000	5943 ms	8 ms
70000	10793 ms	7 ms
80000	16535 ms	10 ms
90000	23096 ms	9 ms
100000	31433 ms	14 ms
150000	82313 ms	21 ms

solution1.js

function updatePos(buffer) {
    for (; bufferPos < buffer.length; bufferPos++) {
        if (buffer.charCodeAt(bufferPos) === 48) {
            stubCounter++;
        }
    }
}

const N = 50000;
let buffer = '';
let bufferPos = 0;
let stubCounter = 0;

for (let i = 0, add; i < N; i++) {
    add = '0123456789';
    buffer += add;
    updatePos(buffer);
}

console.log(stubCounter, buffer.length); // 50000 500000

solution2.js

function updatePos(str) {
    for (let i = 0; i < str.length; i++) {
        if (str.charCodeAt(i) === 48) {
            stubCounter++;
        }
    }
}

const N = 50000;
let buffer = '';
let stubCounter = 0;

for (let i = 0, add; i < N; i++) {
    add = '0123456789';
    updatePos(add);
    buffer += add;
}

console.log(stubCounter, buffer.length); // 50000 500000