Jeremy Thille Jeremy Thille - 4 months ago 8
JSON Question

JS : how to convert a string to JS Object (**NOT** to JSON)?

I know how to parse a valid JSON string :

JSON.parse('{"key" : "value"}')
.

But what about a valid JS object, but invalid JSON, like :
JSON.parse("{ key : 'value'}")
?
The example above throws :


Uncaught SyntaxError: Unexpected token k in JSON at position 2


My actual goal is even trickier. I want to parse a string of a JS object containing RegEx (unsupported by JSON but supported by JS) into a JS object :

'{ key1 : /val1/g , key2 : /val2/i }'


I eventually want to use this object with Mongoose and find documents with it :

Model.find({
key1 : /val1/g ,
key2 : /val2/i
})


I have tried applying a fairly complex RegEx to my String, replacing
/val1/g
with
new RegEx("val1","i")
:

str = str.replace( /\/(.+?)\/(g?i?).+?(?=,|})/g , "new RegExp(`$1`,`$2`)" )


The
.replace()
operation works and modifies the string the way I want it. It yields :

{ key1 : new RegExp("val1","g") , key2 : new RegExp("val2","i") }


But when I try to apply
JSON.parse
to it, it still fails because
new RegEx("val1","i")
is not a valid value.

Answer

If you control and can trust the text you're converting, you can use eval:

var str = '{ key1 : /val1/g , key2 : /val2/i }';
var obj = eval("(" + str + ")");
console.log(obj.key1);

Note that when doing the eval, since your expression starts with {, you have to wrap it in () so the parser knows that's starting an object initializer, not a block.

A couple of notes about eval:

  1. It allows arbitrary code execution. So you really have to trust the text you're evaling. Don't eval user input.

  2. Code in an execution context directly containing a call to eval basically cannot be optimized, because the JavaScript engine can't know, when parsing the code, what the string will contain. So while I suppose technically it's premature optimization, I'd tuck the above off in a function that you call from your main logic rather than embedding it in your main logic directly.

Comments