Compare ++, --, +=, -=, = +, and = -, with different values.

perf.js

// Compare ++, --, +=, -=, = +, and = -, with different values.

var a = 1, p1 = 1, m1 = -1, p9 = 9, m9 = -9;

this.group(
  "inc/dec by 1",
  function baseline()            { return a; },
  function prefix_plus_plus()    { ++a; return a; },
  function postfix_plus_plus()   { a++; return a; },
  function prefix_minus_minus()  { --a; return a; },
  function postfix_minus_minus() { a--; return a; },
  function plus_eq()             { a +=  1; return a; },
  function minus_eq()            { a -= -1; return a; },
  function eq_plus()             { a = a + 1; return a; },
  function eq_minus()            { a = a - (-1); return a; }
);

this.group(
  "inc/dec by 1 (var)",
  function plus_eq_v1()          { a += p1; return a; },
  function minus_eq_v1()         { a -= m1; return a; },
  function eq_plus_v1()          { a = a + p1; return a; },
  function eq_minus_v1()         { a = a - m1; return a; }
);

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

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