英文:
Why 1[''] evaluates to undefined, but 1[] throws `Uncaught SyntaxError: Unexpected token ']`
问题
JavaScript如何解释表达式1['']为undefined，但1[]会抛出Uncaught SyntaxError: Unexpected token ']？
console.log(1['']) // 输出undefined
console.log(1[]) // 抛出Uncaught SyntaxError: Unexpected token ']
英文:
How is Javascript interpreting the expression 1[''] as undefined, yet 1[] throws Uncaught SyntaxError: Unexpected token ']?
<!-- begin snippet: js hide: false console: true babel: false -->
<!-- language: lang-js -->
console.log(1[''])
<!-- end snippet -->
<!-- begin snippet: js hide: false console: true babel: false -->
<!-- language: lang-js -->
console.log(1[])
<!-- end snippet -->
答案1
得分: 3
The subscript operator ([]) is used to access an object's properties.
1 是一个数字字面量，当后面跟着 ['objectPropertyName'] 时，它会被解释为一个 Number 对象。它没有一个叫 '' 的属性，因此当你调用 1[''] 时返回 undefined。
另一方面，1[] 只是一个语法错误 - 你不能省略你试图访问的属性。换句话说，你必须在大括号中有 某些东西。
英文:
The subscript operator ([]) is used to access an object's properties.
1 is a number literal, which is evaluated as a Number object when followed by ['objectPropertyName']. It does not have a property '', and thus returns undefined when you call 1[''].
1[], on the other hand, is just a syntax error - you can't omit the property you're trying to access. In other words, you have to have something in the braces.
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