看各类框架源码淘来的一些JavaScript技巧

// 创建定长20的JavaScript数组，并把每个项的值设为null
Array.apply(null, { length: 20 }).map(function(v,k,ar){
   return ar[k] = null;
})

// 创建定长20的JavaScript数组，并把每个项的值设为undefined
Array.apply(null, { length: 20 }).map(function(){})

var arr = [[], {}, function(){}, "", null, undefined, 0, true ];

for(var i in arr){
    console.log(Object.prototype.toString.call(arr[i]));
}

/*
结果为：

"[object Array]"
"[object Object]"
"[object Function]"
"[object String]"
"[object Null]"
"[object Undefined]"
"[object Number]"
"[object Boolean]"

*/