javascript - for 循环中的 if 条件 - 性能差异

Question

条件1

for( var i = 0; i < 1000; i++ ) {
    if( i != 0 ) {
        console.log("i is not zero.!");
    } else {
        console.log("i is zero.!");
    }
}

条件2

for( var i = 0; i < 1000; i++ ) {
    if( i == 0 ) {
        console.log("i is zero.!");     
    } else {
        console.log("i is not zero.!");
    }
}

1. 哪种方法更可取(我的意思是更合适)？

2. 这两者之间有性能差异吗？

Answer 1

尝试一下!

还有另一种方法可以做到这一点:

console.log( i==0 ? "i is zero.!" : ""i is not zero.! );

有一个小长凳:

function run(e) {
  var start=1*new Date();
  var loop=loopcnt.value;
  if (e.target.id=="==") {
    for( var i = 0; i < loop; i++ ) {
      if( i == 0 ) {
        var log="i is zero.!";       
      } else {
        var log="i is not zero.!";
      }
    }
  } else if (e.target.id=="!=") {
    for( var i = 0; i < loop; i++ ) {
      if( i != 0 ) {
        var log="i is not zero.!";       
      } else {
        var log="i is zero.!";
      }
    }
  } else if (e.target.id=="(=") {
    for( var i = 0; i < loop; i++ ) {
      var log=i==0?"i is zero.!":"i is not zero.!";
    }      
  } else {
    for( var i = 0; i < loop; i++ ) {
      var log=i!=0?"i is not zero.!":"i is zero.!";
    }      
  };
  elapsed.innerHTML=(1*new Date()-start).toFixed(0)+"ms."
}

body,button,input{font-family:sans;font-size:8pt;padding:0pt}div{font-size:12pt;}

Loop: <input id=loopcnt value="3000000" size="8" />
<button id="==" onclick="run(event)">if == {} else {}</button>
<button id="!=" onclick="run(event)">if != {} else {}</button>
<button id="(=" onclick="run(event)">( == ? "" : "" )</button>
<button id="(!" onclick="run(event)">( != ? "" : "" )</button>
<div id="elapsed"></div>

通过使用firefox或chrome，没有明显的区别，但是Spidermonkey使用似乎做得更快一些! = 比使用 ==:

time smjs <<<'
    for( var i = 0; i < 10000000; i++ ) {
        if ( i == 0 ) { var log="i is zero.!"; }
        else { var log="i is not zero.!"; } } '
real    0m0.054s
user    0m0.044s
sys     0m0.004s

time smjs <<<'
    for( var i = 0; i < 10000000; i++ ) {
       if ( i != 0 ) { var log="i is not zero.!"; }
       else { var log="i is zero.!"; } } '
real    0m0.043s
user    0m0.040s
sys     0m0.000s

time smjs <<<'
for( var i = 0; i < 10000000; i++ ) {
       var log=(i==0?"i is zero.!":"i is not zero.!"); } '
real    0m0.051s
user    0m0.048s
sys     0m0.000s


time smjs <<<'
    for( var i = 0; i < 10000000; i++ ) {
       var log=(i!=0?"i is not zero.!":"i is zero.!"); } '
real    0m0.049s
user    0m0.040s
sys     0m0.008s

但这还没有经过充分测试(在真正免费的主机上，通过进行大量测试和完整统计)。