为什么 Boolean() 在 Javascript 中这么慢？

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

Answer 2

I don't know how Javascript JIT compiler executed internally. Also right now the Boolean function works faster in Chrome at 2020. But if there is some different browsers, different versions or different JS engines !! operator works faster I think I know the answer reason why. When you call a function there is extra work inside memory for push stack and pop stack. When you use ! (NOT operator) ! (NOT operator) there is no need to create extra work inside memory for push/pop stack. That is why NOT operator works faster.