javascript - 为什么|| {} 在父[当前部分] = 父[当前部分] || {}

Question

这个 JavaScript 语法是什么？ 父级[当前部分] = 父级[当前部分] || {}；

特别是这部分|| {}

取自此 JavaScript 代码(位于 http://elegantcode.com/2011/01/26/basic-javascript-part-8-namespaces/ )

// Creates a namespace
function namespace(namespaceString) {
    var parts = namespaceString.split('.'),
        parent = window,
        currentPart = '';

    var length = parts.length;
        for (var i = 0; i < length; i++) {
            currentPart = parts[i];
            parent[currentPart] = parent[currentPart] || {};
            parent = parent[currentPart];
         }

     return parent;
 }

Answer 1

JavaScript 中的 || 运算符的工作方式与许多其他语言略有不同。在 JavaScript 中，它计算出第一个“真实”值，允许“失败”行为。

示例:

var a = false;
var b = "asdf";
alert(a || b); //alert box with "asdf" since a was false

var c = true;
var d = "asdf";
var e = false;
alert(c || d || d); //alert box with true. d and e were never evaluated, so "asdf" isn't returned. This is called "short-circuiting" operation.

&& 运算符的工作原理类似，如果一切都为 true，它会计算第一个“falsey”值或最后一个“truthy”值:

var a = true;
var b = "asdf";
alert(a && b); //alert box with "asdf"
alert(b && a): //alert box with true

var c = 6;
var d = 0;
alert(c && d); //alert box with 0
alert(d && c); //alert box with 0