为什么JS eval（）仅返回对象的值

Question

If I do this:

var x = eval('{a:"b"});
console.log(x); // -> "b"

all I get is the value in the object ("b"), not the key/property, or the whole object itself, which is weird.

but when I do this:

var x = eval('(function self(){return {a:"b"}})()');
console.log('x'); //  -> {a:'b'}

now it seems to give me what I would expect, the whole object. But why is this? Why do I need to wrap it in a (self-executing) function?

I am thinking of using eval to create some objects from strings, but need to know better how this works.

Answer 1

That's because {a:"b"} statement as-is represents the following:

1. A code block delimited by { ... }
2. A label a:
3. A string literal "b"

The latter being the only expression produces the result.

And the same in the AST explorer: https://astexplorer.net/#/gist/909bebf...

To return an object you need to turn it into an expression first, like: wrap it in parentheses: eval('({a:"b"})')

But the whole idea of "I am thinking of using eval to create some objects from strings" sounds suspicious.