function a() {

var arr = new Array(1000000);

for(var i = 0; i < arr.length;i ++) {

arr[i] = i;

}

var start1 = new Date().getTime();

for(var i = 0; i < arr.length;i ++) {

arr[i] = i+1;

}

var stop1 = new Date().getTime();

console.info(stop1-start1);

var start2 = new Date().getTime();

arr.forEach(function(value,index,array) {

arr[i] = i+1;

});

var stop2 = new Date().getTime();

console.info(stop2-start2);

var start3 = new Date().getTime();

arr.map(function(index,value,array) {

arr[i] = i+1;

});

var stop3 = new Date().getTime();

console.info(stop3-start3);

}

a();

>>>4

>>>29

>>>155

js對比for、forEach、map遍歷數組速度