A benchmark that demonstrates something is wrong with v8's concatenation performance

str-concat.js

const N = 1000000;

function simpleConcat() {
    let str = '';
    for (let i = 0; i < N; i++) {
        str += 'a';
    }
    return str;
}

function joinConcat() {
    let arr = [];
    for (let i = 0; i < N; i++) {
        arr.push('a');
    }
    return arr.join('');
}

const M = 12;

function bufferedConcat() {
    let str = '';
    let buf = '';

    for (let i = 0; i < N; i++) {
        buf += 'a';
        if (buf.length >= M) {
            str += buf;
            buf = '';
        }
    }
    str += buf;

    return str;
}

// warmup
let str1 = simpleConcat();
let str2 = joinConcat();
let str3 = bufferedConcat();

if (str3 !== str1) throw new Error('Bad buffered concat.');

console.time('+=');
for (let i = 0; i < 10; i++) str1 = simpleConcat();
console.timeEnd('+=');

console.time('push/join');
for (let i = 0; i < 10; i++) str2 = joinConcat();
console.timeEnd('push/join');

console.time('buffered +=');
for (let i = 0; i < 10; i++) str3 = bufferedConcat();
console.timeEnd('buffered +=');

Interesting, I guess this is the answer: https://softwareengineering.stackexchange.com/a/90231

+= is only slower when N = 1000000!

+=: 2503.689ms
push/join: 1331.478ms
buffered +=: 627.932ms
+= x 4.27 ops/sec ±14.43% (15 runs sampled)
push(/join x 8.28 ops/sec ±11.51% (25 runs sampled)
buffered x 13.65 ops/sec ±14.12% (28 runs sampled)

lowering N to a more realistic 1000, changes the results to following on node 10.2.1

+=: 0.650ms
push/join: 8.361ms
buffered +=: 2.534ms
+= x 53,101 ops/sec ±2.03% (76 runs sampled)
push(/join x 17,733 ops/sec ±2.72% (80 runs sampled)
buffered x 32,264 ops/sec ±1.47% (81 runs sampled)