标签 javascript
<分区>
如标题所说，为什么我不能映射包含undefined项的数组？
var foo = new Array(3);
// [ , , ]
var bar = [null, null, null];
// [null, null, null]
foo.map(function(val){return 'test'});
// [ , , ]
bar.map(function(val){return 'test'});
// ['test', 'test', 'test']
也许是愚蠢的问题，但我真的很想知道原因。
谢谢。
最佳答案
callbackis invoked only for indexes of the array which have assigned values, includingundefined. It is not called for missing elements of the array (that is, indexes that have never been set, which have been deleted or which have never been assigned a value).
它不起作用，因为尚未为元素分配值。
如果你使用
foo[0] = undefined;
foo[1] = undefined;
foo[2] = undefined;
然后它会起作用。
关于javascript - 为什么不能映射 'undefined' 数组，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/35407021/