使用内置的JavaScript对象和构造函数，我注意到有点奇怪的东西。

有时，通过调用不带new的构造函数可以获得新对象。 例如：

但有时这不起作用：

是否在ECMAScript规范中的任何位置定义了非新构造函数内置函数的行为？ 请注意，此行为实际上很有用; 我可以通过调用Array.apply(arr)来创建一个非稀疏的数组副本，但是如果它是跨平台的话，我只会感到很自在。

是的，ECMA-262(我使用5.1版作为参考)确实定义了在使用或不使用new关键字调用时对象构造函数应该如何表现。

对于Array：

15.4.1作为函数调用的数组构造函数：

When Array is called as a function rather than as a constructor, it creates and initialises a new Array object. Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with the same arguments.

15.4.2数组构造函数：

When Array is called as part of a new expression, it is a constructor: it initialises the newly created object.

对于Date：

15.9.2日期构造函数称为函数：

When Date is called as a function rather than as a constructor, it returns a String representing the current time (UTC).
The function call Date(…) is not equivalent to the object creation expression new Date(…) with the same arguments.

15。9。3日期构造函数：

When Date is called as part of a new expression, it is a constructor: it initialises the newly created object.

本机方法的行为取决于EcmaScript规范。

对于Date，规范说：

When Date is called as a function rather than as a constructor, it
returns a String representing the current time (UTC).

NOTE : The function call Date(…) is not equivalent to the object creation expression new Date(…) with the same arguments.

对于Array规范说

When Array is called as a function rather than as a constructor, it
creates and initialises a new Array object.

Thus the function call
Array(…) is equivalent to the object creation expression new Array(…)
with the same arguments.

因此，无论是否使用new关键字，它的工作原理完全取决于您使用的方法，以及在没有new关键字的情况下调用时应该发生的规范。

例如，Math对象再次不同

The Math object does not have a [[Construct]] internal property; it is
not possible to use the Math object as a constructor with the new
operator.