JSON Remove trailing comma from last object

Question

This JSON data is being dynamically inserted into a template I'm working on. I'm trying to remove the trailing comma from the list of objects.

The CMS I'm working in uses Velocity, which I'm not too familiar with yet. So I was looking to write a snippet of JavaScript that detects that trailing comma on the last object (ITEM2) and removes it. Is there a REGEX I can use to detect any comma before that closing bracket?

[
    {
        "ITEM1":{
            "names":[
            "nameA"
            ]
        }
    },
    {
        "ITEM2":{
            "names":[
            "nameB",
            "nameC"
            ]
        }
    }, // need to remove this comma!
]

it's not legal JSON - whatever produced it needs fixing. Not fixing that will just accrue technical debt for the next user of that output. — Alnitak
– Alnitak, Commented Dec 17, 2015 at 21:15

Parzh · Accepted Answer · 2022-10-28 09:24:15Z

43

You need to find ,, after which there is no any new attribute, object or array.
New attribute could start either with quotes (" or ') or with any word-character (\w).
New object could start only with character {.
New array could start only with character [.
New attribute, object or array could be placed after a bunch of space-like symbols (\s).

So, the regex will be like this:

const regex = /\,(?!\s*?[\{\[\"\'\w])/g;

Use it like this:

// javascript
const json = input.replace(regex, ''); // remove all trailing commas (`input` variable holds the erroneous JSON)
const data = JSON.parse(json); // build a new JSON object based on correct string

Try the first regex.

Another approach is to find every ,, after which there is a closing bracket.
Closing brackets in this case are } and ].
Again, closing brackets might be placed after a bunch of space-like symbols (\s).

Hence the regexp:

const regex = /\,(?=\s*?[\}\]])/g;

Usage is the same.

Try the second regex.

edited Oct 28, 2022 at 9:24

answered Dec 18, 2015 at 2:09

Parzh

10.2k4 gold badges48 silver badges83 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

bob Over a year ago

This also replaces comma's inside strings, e.g. {"letters" : "a, b, c"}. The following just checks against trailing commas: fix.replace(/(.*?),\s*(\}|])/g, "$1$2");

3limin4t0r Over a year ago

@bob That statement requires an *. The code provided still removes a comma in string context given the following JSON { "chars": "1,2,3,},]" }. The resulting output would be { "chars": "1,2,3}]" }. This scenario is less common, so it's an improvement, but depending on the context this might or might not be an issue.

Parzh Over a year ago

More generally, you will always have false positives in this case, because a JSON can contain another JSON (which can contain another JSON, and so on). That's why the issue must be dealt with on the generation stage: don't produce malformed JSON, and assume that if the input is a JSON, it is valid.

Yuvaraj S · Accepted Answer · 2021-05-11 12:11:00Z

3

Consider the Json input = [{"ITEM1":{"names":["nameA"]}},{"ITEM2":{"names":["nameB","nameC"]}},] without whitespaces. I suggest a simple way using substring.

input = input.substring(0, input.length-2);
input = input + "]";

answered May 11, 2021 at 12:11

Yuvaraj S

3113 silver badges16 bronze badges

Comments

Federico Piazza · Accepted Answer · 2015-12-17 21:58:37Z

1

For your specific example, you can do a simple search/replace like this:

,\n]$

Replacement string:

\n]

Working demo

Code

var re = /,\n]$/; 
var str = '[  \n   {  \n      "ITEM1":{  \n         "names":[  \n            "nameA"\n         ]\n      }\n   },\n   {  \n      "ITEM2":{  \n         "names":[  \n            "nameB",\n            "nameC"\n         ]\n      }\n   },\n]';
var subst = '\n]'; 

var result = str.replace(re, subst);

answered Dec 17, 2015 at 21:58

Federico Piazza

31.2k15 gold badges91 silver badges133 bronze badges

Comments

Adam Wise · Accepted Answer · 2024-04-01 14:43:54Z

1

For the general case where you need to process JSON that is malformed in ways that are otherwise accepted as JavaScript, the JSON5 library works great. It would work for this specific case as well.

See https://www.npmjs.com/package/json5

Once used in your project, the trailing comma would be handled by a single call like so:

input = JSON5.parseText(input)

answered Apr 1, 2024 at 14:43

Adam Wise

2,34121 silver badges19 bronze badges

Comments

Amar Das · Accepted Answer · 2020-09-01 13:48:29Z

0

I developped a simple but useful logic for this purpose - you can try this.

Integer Cnt = 5;
        String StrInput = "[";
        
        for(int i=1; i<Cnt; i++){
            StrInput +="   {"  
                     + "     \"ITEM"+i+"\":{ " 
                     + "      \"names\":["  
                              + "   \"nameA\""
                                + "]"
                  +"}";
                            
            if(i ==(Cnt-1)) { 
                StrInput += "}";            
            } else {
                StrInput += "},";
            }
        
        }
        StrInput +="]";
        
        System.out.println(StrInput);

answered Sep 1, 2020 at 13:48

Amar Das

138 bronze badges

Comments

Eric · Accepted Answer · 2023-03-19 08:28:25Z

0

You can remove trailing commas in a complex json string using regular expression

  const regex = /,\s*([\]}])/g;
  const fixedJson = jsonString.replace(regex, "$1");
  const json = JSON.parse(fixedJson);
``

answered Mar 19, 2023 at 8:28

Eric

1,7951 gold badge18 silver badges37 bronze badges

Comments

Stéphane Laurent · Accepted Answer · 2023-05-24 08:22:56Z

0

The proposed regex approaches have a problem. If the JSON string you want to normalize is

var jstring = '{"a":"5,}", "z":{"x":2,}, "b":4,}';

then applying jstring.replace(regex, '') will also remove the comma after 5 in the string jstring.a = "5,}". This is not desirable.

Another approach, not resorting to regex, is:

eval(`var result=${jstring}`)
JSON.stringify(result)

answered May 24, 2023 at 8:22

Stéphane Laurent

85.3k18 gold badges141 silver badges263 bronze badges

1 Comment

martin.malek Over a year ago

I would expect that the wrong json string is from some source, not part of the application. In that case, never use eval. If it's in the application, then fix the string

Collectives™ on Stack Overflow

JSON Remove trailing comma from last object

7 Answers 7

3 Comments

Comments

Comments

Comments

Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

7 Answers 7

3 Comments

Comments

Comments

Comments

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related