javascript 评估

发布于 2024-08-14 11:48:23 字数 130 浏览 2 评论 0原文

eval('({"suc":true})')

上面的说法是错误的，应该是：

eval('{"suc":true}')

为什么？

原文

eval('({"suc":true})')

The above is wrong,should be:

eval('{"suc":true}')

Why?

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℡Ms空城旧梦 2024-08-21 11:48:23

当尝试评估时，解释器会看到大括号并认为它是一个块开头。将其括在括号中使其成为表达式并正确初始化对象。

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假装爱人 2024-08-21 11:48:23

我不知道你想实现什么，但从你的例子来看，第一个是正确的，第二个抛出语法错误。

eval('({"suc":true})') 与 ({"suc":true}) 相同，JavaScript 将其解释为：

( // <- this states begining of expression
    { // <- this is hash/object literal begining
        "suc": // <- this is property name, given as string
            true // <- this is value
    }
)

所以它返回具有 suc 属性和关联值 true 的新对象。

eval('{"suc":true}') 与 {"suc":true} 相同，解释为：

{ // <- this is block begining
    "suc": // <- this is label, but incorrect, as it is given as string, not literally
        true // <- this is expression
}

如果更改 "suc " 到 suc （不带括号），那么它就可以工作，但它与第一个示例不同。

更新：

至于为什么数组不需要括号：JavaSript 中除了数组之外没有其他以 [ 字符开头的构造。

如果 { 出现在需要这样的值的上下文中，那么它不会有问题：

eval('var a = {"succ": true}')

它在源代码中是相同的（因此不仅在 eval 块中）：您不能使用短创建对象符号 ({ .. }) 而不将其分配给某个变量或作为值传递（到函数、返回语句...）。

I don't know what you want to achieve, but from your examples first is correct and the second throws syntax error.

eval('({"suc":true})') is the same as ({"suc":true}) and JavaScript interprets it as:

( // <- this states begining of expression
    { // <- this is hash/object literal begining
        "suc": // <- this is property name, given as string
            true // <- this is value
    }
)

So it returns new object with suc property and associated value true.

eval('{"suc":true}') is the same as {"suc":true} and is interpreted as:

{ // <- this is block begining
    "suc": // <- this is label, but incorrect, as it is given as string, not literally
        true // <- this is expression
}

If you change "suc" to suc (without parentheses), then it would work, but it's not the same as first example.

UPDATE:

As to why array doesn't need parentheses: there is no other construct in JavaSript which would start with [ character other than array.

There would be no problem with { if it would show up in context which expects value like this:

eval('var a = {"succ": true}')

It's the same in source code (so not only in eval block): you can't create object using short notation ({ .. }) without assigning it to some variable or passing as value (to function, return statement...).

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