Duplicate keys in JSON

Question

How can i parse and show error if given JSON contains duplicate keys. JSON.parse just ignores it & pick last key value.Also let me know if any sort of npm lib available for the same.

{
  "name":"mohit",
  "name":"verma"
}

thats fine, but what if i get json with duplicate keys. Even JSON.parse does not throw error. — Mohit Verma, Dec 04 '16 at 06:51
http://stackoverflow.com/questions/21832701/does-json-syntax-allow-duplicate-keys-in-an-object — Mahi, Dec 04 '16 at 06:52
okay, that we know. But i need to get exception for duplicate keys while parsing — Mohit Verma, Dec 04 '16 at 06:57
Fix the "system" creates such an abomination - that would be the easiest "fix" — Jaromanda X, Dec 04 '16 at 07:10
Sadly, the `reviver` parameter to `JSON.parse` does not seem to be called multiple times if there are duplicate keys. — , Dec 04 '16 at 09:22

code_monk · Answer 1 · 2016-12-04T23:46:31.097

5

If you can predict how the JSON will be formatted*, you can compare to the text to the result of parsing and re-stringifying the object:

See comments below for a description of cases where this would fail

const hasDuplicateKey = (j) => {
    let k = JSON.stringify(JSON.parse(j));
    let h = JSON.stringify(JSON.parse(j),null,"  ");
    return !(j === h || j === k);
};

let json1 = `{"name":"bob","name":"alice","age":7}`;
let json2 = `{"name":"bob","age":7}`;

let json3 = `{
  "name": "mohit",
  "name": "verma"
}`;

let json4 = `{
  "name": "mohit",
  "age": 107
}`;

console.log( hasDuplicateKey(json1) );
console.log( hasDuplicateKey(json2) );
console.log( hasDuplicateKey(json3) );
console.log( hasDuplicateKey(json4) );

edited Dec 04 '16 at 23:46

answered Dec 04 '16 at 07:13

code_monk

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1

That doesn't work. Passing JSON through parse and then stringify will normalise things like white space. You've got `k` and `h` to handle two different sets of normalisation, but that only works if the initial data meets those normalisation patterns. Change one bit of it to `let json2 = `{"name": "bob","age":7}`;` (with extra spaces) and you get false positives. – Quentin Dec 04 '16 at 07:50
that is why i qualified with "if you can predict how the JSON will be formatted" – code_monk Dec 04 '16 at 07:59
@code_monk: If if one can "predict" how the JSON is formatted, it still would have to follow the same formatting rules as `JSON.stringify`. So why not just say that? *"If the JSON is formatted the same way as the output produced by `JSON.stringify`, ...."*. – Felix Kling Dec 04 '16 at 08:02
i added said clarification – code_monk Dec 04 '16 at 08:24
1

This will fail on an input of `'{"a":1,"1":22}'` (in at least some native `JSON.parse/JSON.stringify` implementations, due to special rules for ordering numeric keys). Your condition of "works with" is too weak--it needs to say "if it is **guaranteed** that `JSON.stringify` on the result of `JSON.parse` is byte-for-byte **identical** with the original input to `JSON.parse`". However, this condition never holds for any known JSON libraries. Hence, this solution at best works only in limited situations. – Dec 04 '16 at 09:17
i added said clarification – code_monk Dec 04 '16 at 23:46
1

can we remove whitespace from json string, then parse and stringify it & match with string ( in which white space was removed ) – Mohit Verma Dec 10 '16 at 20:57

Duplicate keys in JSON

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