什么javascript对象打印为false，是真的，并且是类型“对象”？

Question

I'm trying to debug a BIRT report where I've retrieved a paramter from the reportContext like so: var myParameter = reportContext.getParameterValue("myParameter"); BIRT has no debugger and BIRT javascript cannot access JSON.stringify, so I cannot examine what this variable is except by printing it. When I print it, it prints as "false". typeof(myParameter) is "object", myParameter === null evaluates to false, myParameter === undefined evaluates to false, myParameter == "false" evaluates to false, and myParameter is truthy (if I use it as the guard to an if statement, the if statement is executed). What the heck is this variable, and how can I determine what it is? Is there a way to stringify it without using JSON.stringify, which I cannot access in BIRT?

Answer 1

> new Boolean(false).toString()
'false'
> typeof new Boolean(false)
'object'
> !!new Boolean(false)
true
> 

To check whether this is in fact your object, new Boolean(false).constructor returns (stringified) [Function: Boolean] .

Answer 2

SLaks给出的答案非常接近，并让我弄清楚它到底是什么：BIRT javascript可以调用Java代码并使用Java对象和类 - myParameter是BIRT的Java布尔对象的Javascript版本（不是Java布尔基元，或Javascript布尔对象，或Javascript布尔基元）。

Answer 3

Consider some custom object with a custom toString() function with the latter returning false.

 function MyType() {} MyType.prototype.toString = function() { return "false"; }; var a = new MyType(); console.log( String( a ) ); console.log( typeof a ); console.log( Boolean( a ) );