2.1.77 [ECMA-262-1999] Section 15.4.5.2, length

V0123:

The length property of this Array object is always numerically greater than the name of every property whose name is an array index.

The length property has the attributes { DontEnum, DontDelete }. __However, for JScript 5.7 the length property in addition has the ReadOnly attribute.__

The existence of the ReadOnly attribute for JScript 5.7 does not prevent modification of the value of the length property of array instances because the [[Put]] method for array objects as defined in [ECMA-262-1999] section 15.4.5.1 does not call [[CanPut]] for the length property. However, the existence of the ReadOnly attribute does affect the result of the [[CanPut]] method in any situations where it is actually called. In particular, any object that inherits its length property from an array instance has a ReadOnly length property.