str-concat2.js

 const N = 1000000;
 const STR = 'abcdefghijkl';

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

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

 const M = 12;

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

     for (let i = 0; i < N; i++) {
         buf += STR;
         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 +=');