Stringify only a field in JavaScript

Question

I would want that at passing an object at JSON.Stringify, it checks if it has a field like "val" for stringify only that field, else been stringfied everything.

Is possible to change JSON.Stringify to stringy only a determined field?

And what JavaScript are you using? What's the relevant/[SSCCE](http://sscce.org/) HTML? Can we see a [demo](http://jsfiddle.net/) of what you've got so far? — David Thomas, Nov 24 '12 at 12:33

score 1 · Answer 1 · edited May 23 '17 at 11:43

1

You need to watch out for falsy values:

function my_json_stringify(obj) {
  return JSON.stringify(obj.hasOwnProperty("val") ? obj.val : obj);
}

Otherwise, it is going to provide wrong results for things like

{
    val: ""
}

You may need to include some cross-browser solution for hasOwnProperty as shown here.

edited May 23 '17 at 11:43

Community

1
1

answered Nov 24 '12 at 12:46

phant0m

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Pavel Strakhov · Accepted Answer · 2012-11-24T13:10:55.597

0

It's quite simple:

function my_json_stringify(obj) {
  return JSON.stringify("val" in obj ? obj.val : obj);
}

console.log(my_json_stringify({ a: 1, b: 2})); // => {"a":1,"b":2}
console.log(my_json_stringify({ val: { a : 3, b: 4 }, other: 5}));
  // => {"a":3,"b":4}

You generally must not modify system functions. It's very bad idea. But if you have to do it, it can be done like that:

JSON.original_stringify = JSON.stringify;

JSON.stringify = function(obj) {
  return JSON.original_stringify(obj.val ? obj.val : obj);
}

At least, it works in Firefox. But I don't know if it will work on any other JS implementation or not.

edited Nov 24 '12 at 13:10

answered Nov 24 '12 at 12:36

Pavel Strakhov

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2

`obj.val ? ...` will be `false` for a variety of defined values (false, null, 0, ''), which is probably not what the OP wants. Better to use `'val' in obj` – broofa Nov 24 '12 at 13:09

KooiInc · Answer 3 · 2012-11-24T13:34:17.180

0

You can use a replacer method.

var myObj = {a:1,b:2,c:3,d:4,e:5}
   ,myObjStr = JSON.stringify(myObj,censor);

//=> myObjStr now: "{"a":1,"b":2,"c":3}"

function censor(key,val){
  return !key ? val : val>3 ? undefined : val;
}

If I understood your question right, in your case you could use something like:

var myObj = {a:{val:1},b:{val:2},c:3,d:null,e:'noval'}
   ,myObjStr = JSON.stringify(
                       myObj,
                       function(key,val){ 
                         return !key 
                                ? val 
                                : val && val.val 
                                  ? JSON.stringify(val) 
                                  : undefined;
                       });
 //=> myObjStr: {"a":"{\"val\":1}","b":"{\"val\":2}"}

See also

edited Nov 24 '12 at 13:34

answered Nov 24 '12 at 12:37

KooiInc

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You cannot use the `replacer` method with such a simplistic implementation. You would need to keep around some global state to make it work! – phant0m Nov 24 '12 at 12:52
@phant0m: que? Could you elaborate? – KooiInc Nov 24 '12 at 13:02
Sure. There are two cases that need to be distinguished: `myObj` has an attribute called `val` and the case where it does not. (I made an error with the mention of global state, the docs aren't entirely clear about the workings, which is why I made an invalid assumption). Anyway, the `censor` function *needs* to be implemented like this: `if(key === "" && value.hasOwnProperty("val")) { return value.val; } return value;` --- `censor` gets called on the each attribute of the result of `censor("", myObj")` which leads to the above implementation. – phant0m Nov 24 '12 at 13:11

Stringify only a field in JavaScript

3 Answers3