Javascript 对象文字不允许引用现有值？

Question

我有一个对象，定义如下:

var obj = {
    'title': 'foo',
    'id': '123',
    'category': 'aaa',
    'meta': 'blah blah'
};

我想对其进行子集化，但是，我在 JS 中发现了一些我不理解的行为:

var foo = {obj.title: obj.id}; // doesn't work
var bar = {obj['title']: obj['id']}; // this doesn't either

var baz = {}
baz[obj.title] = obj.id // this works fine.

为什么？

Answer 1

因为这是在 ECMAScript 5, Section 11.1.5 中定义的方式:

属性名称必须是:

PropertyName :
    IdentifierName
    StringLiteral
    NumericLiteral

obj.title 既不是 identifier name ，也不是字符串或数字文字。但即使您之前将标题分配给另一个变量，您也会看到生产规则不会评估标识符:

The production PropertyName : IdentifierName is evaluated as follows:

1. Return the String value containing the same sequence of characters as the IdentifierName.

---

只是详细说明为什么它不是有效的标识符名称:full stop (.) 属于 Puncuation, other [Po]类别，这不是允许字符的一部分。其中包括:

UnicodeLetter: any character in the Unicode categories "Uppercase letter (Lu)", "Lowercase letter (Ll)", "Titlecase letter (Lt)", "Modifier letter (Lm)", "Other letter (Lo)", or "Letter number (Nl)".

UnicodeCombiningMark: any character in the Unicode categories "Non-spacing mark (Mn)" or "Combining spacing mark (Mc)"

UnicodeDigit: any character in the Unicode category "Decimal number (Nd)"

UnicodeConnectorPunctuation: any character in the Unicode category "Connector punctuation (Pc)"