JS_while代替for循环

/*
 * i++; 0
 * --j; 4
 */
var arr = ['a1','a2','a3','a4','a5'];
var i = 0,j=arr.length,val;
while(val = arr[i++]){
    console.log(val)
}
while(val = arr[--j]){
    console.log(val)
}