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

Question

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.

I hate myself for saying this, but maybe eval is what you're looking for? — Emile Bergeron
– Emile Bergeron, Commented Jul 26, 2016 at 16:12
Thanks, it is indeed, I didn't think about it. Eval is too powerful :) — Jeremy Thille
– Jeremy Thille, Commented Jul 26, 2016 at 16:22

T.J. Crowder · Accepted Answer · 2016-07-26 16:18:38Z

10

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:

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

edited Jul 26, 2016 at 16:18

answered Jul 26, 2016 at 16:12

T.J. Crowder

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8 Comments

Joachim Sauer Over a year ago

That's a big "if" ;-)

Emile Bergeron Over a year ago

Also, I think it will prevent JIT optimization.

T.J. Crowder Over a year ago

@JoachimSauer: Yeah, I tried to mark it up fairly large. :-) The question suggests that it may well be the case here.

T.J. Crowder Over a year ago

@EmileBergeron: Ah, yes, it's worth calling that bit out. I'll do that. Unlikely to be an issue, but still.

user663031 Over a year ago

Didn't you answer the same question the same way a week or two ago? Or are you getting like me and finding it harder and harder to find old questions and answers, even the ones you wrote?

|

Dharman · Accepted Answer · 2021-09-09 20:15:45Z

I faced simmilar issue and solved it with the help of split and reduce.

const getObjectFromString = (stringToParse) => {
    if(typeof stringToParse === 'string'){
        let currentKey = '';
        const keyValPairArr = stringToParse.replace('{','').replace('}','').split(':');
        return keyValPairArr.reduce((obj,current,index,arr)=>{
            const previousKey = currentKey;
            const arrKeyVal = current.trim().split(',');
            currentKey = index!==arr.length-1?arrKeyVal.pop().trim():'';
            const previousVal = arrKeyVal.join(',');
            if(previousKey&&previousVal!=='')obj[previousKey]=previousVal;
            return obj;
        },{})
    }else{
        return stringToParse||{};
    }
}

// following are some results

const example1 = getObjectFromString('{first : 1, second : 2nd, third: "third, 3rd" }')
console.log(example1) // output: {first: '1', second: '2nd', third: '"third, 3rd"'}

It returns either empty object or the converted object from the passed string.

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