Compare +=, -=, = +, and = -, with 9.

perf.js

// Compare +=, -=, = +, and = -, with 9.

var a = 1, p9 = 9, m9 = -9;

this.group(
  "inc/dec by 9 (var)",
  function eq_minus_v9()         { a = a - m9; return a; },
  function eq_plus_v9()          { a = a + p9; return a; },
  function minus_eq_v9()         { a -= m9; return a; },
  function plus_eq_v9()          { a += p9; return a; }
);

Benchmark it with http://www.perfjs.com/