Why is Boolean() so slow in Javascript?

Question

According to the ECMAScript specification, both the unary logical NOT operator (!) and the Boolean() function use the internal function ToBoolean(), and the NOT operator also does a few checks to reverse the result. So why is a double logical NOT operation much faster than running the Boolean() function?

I used the following piece of code to test which was faster:

function logicalNotOperator() {
  var start = performance.now();
  for (var i = 0; i < 9999999; i++) !!Math.random();
  return 0.001 * (performance.now() - start);
}
 
function booleanFunc() {
  var start = performance.now();
  for (var i = 0; i < 9999999; i++) Boolean(Math.random());
  return 0.001 * (performance.now() - start);
}

var logicalNotOperatorResult = logicalNotOperator();
var booleanFuncResult = booleanFunc();
var diff = booleanFuncResult - logicalNotOperatorResult;

console.log('logicalNotOperator:', logicalNotOperatorResult);
console.log('booleanFunc:', booleanFuncResult);
console.log('diff:', diff);

Note: I am not referring to the new Boolean() constructor, but the Boolean() function that coerces the argument it's given to a boolean.

Answer 1

While Boolean will call the function (internally optimized), most JITs will inline the double not to use XOR which is far faster (source code reference - JägerMonkey).

And the JSperf: http://jsperf.com/bool-vs-doublenot