concat creates a new array. [google](https://www.google.fr/search?q=javascript+concat+array+prototype.push+jsperf&ie=utf-8&oe=utf-8&rls=org.mozilla:fr:official&client=firefox-a&gws_rd=cr&ei=1UxUUtjDEKTI0QWAiYCgCQ) finds a number of jsperf's comparing concat to prototype.push.apply if you're interested.