length_chack.js

gistfile1.js

var array = [];
var l = 100000;
for(var i = 0; i < l; i++) {
 array.push(i);
}

// Pattern A
console.time("for文で毎回array.lengthを呼ぶ");
var count=0;
for(var i = 0; i< array.length; i++) {
    count += i;
}
console.timeEnd("for文で毎回array.lengthを呼ぶ");

// Pattern B
console.time("for文でarray.lengthをキャッシュ");
var count=0;
for(var i = 0, len = array.length; i < len; i++) {
    count += i;    
}
console.timeEnd("for文でarray.lengthをキャッシュ");

// [result]
// Firefox3.6         A:190ms B:181ms on Firebug
// Chrome v9.0.597.44 A:219ms B:224ms on Developper Tool