将javascript对象加在一起 [英] ANDing javascript objects together

问题描述

我在我们的应用程序中遇到了这段代码（已修改），并对它是如何工作感到困惑：

I ran across this chunk of code (modified) in our application, and am confused to how it works:

    function someObject()
    {
        this.someProperty = {};
        this.foo = 
        {
            bar:
            {
                baz: function() { return "Huh?" }
            }
        };

        this.getValue = function()
        {
            return (this.someProperty && this.foo.bar && this.foo.bar.baz && this.foo.bar.baz()) || null;
        }
    }

    function test()
    {
        var o = new someObject();
        var val = o.getValue();
        alert(val);
    }

当你调用test（）函数时，文字嗯？警报。我不确定getValue的结果是如何返回的，我会认为做A&& B&& C&& D会返回true，而不是D的值。

when you call the test() function, the text "Huh?" is alerted. I'm not sure how the result of getValue is returning that, I would've thought doing A && B && C && D would have returned true, rather than the value of D.

推荐答案

这是因为布尔运算符可以返回一个操作数，而不一定是 Boolean 结果，例如：

That happens because the Boolean Operators in JavaScript can return an operand, and not necessarily a Boolean result, e.g.:

Logical AND运算符（&& ）将返回值第二个操作数的第一个是 truthy ：

The Logical AND operator (&&), will return the value of the second operand if the first is truthy:

true && "foo"; // "foo"

如果是第一个操作数，它将返回第一个操作数的值 falsy ：

And it will return the value of the first operand if it is by itself falsy:

NaN && "anything"; // NaN
0 && "anything";   // 0

这就是为什么在你的例子中嗯？，因为前面的所有表达式都是 truthy ：

That's why in your example "Huh?" is returned, because all the preceding expressions are truthy:

alert("A" && "B" && "C" && "Huh?"); // "Huh?"
alert(true && true && true && "Huh?"); // "Huh?"

---

Logical OR运算符（ || ）有类似的行为，它将返回第二个操作数的值，如果第一个是 falsy ：

---

The Logical OR operator (||) has a similar behavior, it will return the value of the second operand, if the first one is falsy:

false || "bar"; // "bar"

并且它将返回第一个操作数的值，如果它本身不是-falsy：

And it will return the value of the first operand if it is by itself non-falsy:

"foo" || "anything"; // "foo"

此行为通常用于设置默认值 ，例如：

This behavior is often used to set default values, for example:

function test (arg1) {
  arg1 = arg1 || "default value";
}

注意： Falsy值是那些强制到的值 false 在布尔上下文中使用时，它们是： null ， undefined ， NaN ， 0 ，零长度字符串，当然还有 false 。其他任何东西都会强制到 true 。

Note: Falsy values are those that coerce to false when used in a boolean context, and they are: null, undefined, NaN, 0, zero-length string, and of course false. Anything else will coerce to true.

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