js中[object Object]表示什么？为何第一个o小写，第二个O大写？

var a = {} a.toString() 将会返回 [object Object],为何会第一个o小写，第二个大写？[object Object]到底是含义是啥？

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If this method is not overridden in a custom object, toString() returns "[object type]", where type is the object type.

MDN上的解释 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toString