Can you use a trailing comma in a JSON object

Question

When manually generating a JSON object or array  it s often easier to leave a trailing comma on the last item in the object or array   For example  code to output from an array of strings might look like  in a C   like pseudocode    s append       for  i   0  i  lt  5    i        s appendF     d      i     s append         giving you a string like   0 1 2 3 4 5     Is this allowed

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Interestingly  both C  amp  C    and I think C   but I m not sure  specifically allow the trailing comma -- for exactly the reason given  It make programmaticly generating lists much easier   Not sure why JavaScript didn t follow their lead

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No   The  quot railroad diagrams quot  in https   json org are an exact translation of the spec and make it clear a   always comes before a value  never directly before     or

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According to the Class JSONArray specification    An extra    comma  may appear just before the closing bracket   The null value will be inserted when there is    comma  elision    So  as I understand it  it should be allowed to write    0 1 2 3 4 5     But it could happen that some parsers will return the 7 as item count  like IE8 as Daniel Earwicker pointed out  instead of the expected 6     Edited   I found this JSON Validator that validates a JSON string against RFC 4627  The application json media type for JavaScript Object Notation  and against the JavaScript language specification  Actually here an array with a trailing comma is considered valid just for JavaScript and not for the RFC 4627 specification   However  in the RFC 4627 specification is stated that      2 3   Arrays      An array structure is represented as square brackets surrounding zero   or more values  or elements    Elements are separated by commas   array   begin-array   value    value-separator value     end-array    To me this is again an interpretation problem  If you write that Elements are separated by commas  without stating something about special cases  like the last element   it could be understood in both ways      P S  RFC 4627 isn t a standard  as explicitly stated   and is already obsolited by RFC 7159  which is a proposed standard  RFC 7159

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From my past experience  I found that different browsers deal with trailing commas in JSON differently    Both Firefox and Chrome handles it just fine  But IE  All versions  seems to break  I mean really break and stop reading the rest of the script    Keeping that in mind  and also the fact that it s always nice to write compliant code  I suggest spending the extra effort of making sure that there s no trailing comma

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Trailing commas are allowed in JavaScript  but don t work in IE  Douglas Crockford s versionless JSON spec didn t allow them  and because it was versionless this wasn t supposed to change  The ES5 JSON spec allowed them as an extension  but Crockford s RFC 4627 didn t  and ES5 reverted to disallowing them  Firefox followed suit  Internet Explorer is why we can t have nice things

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As it s been already said  JSON spec  based on ECMAScript 3  doesn t allow trailing comma  ES    5 allows it  so you can actually use that notation in pure JS  It s been argued about  and some parsers did support it  http   bolinfest com essays json html  http   whereswalden com 2010 09 08 spidermonkey-json-change-trailing-commas-no-longer-accepted    but it s the spec fact  as shown on http   json org   that it shouldn t work in JSON  That thing said         I m wondering why no-one pointed out that you can actually split the loop at 0th iteration and use leading comma instead of trailing one to get rid of the comparison code smell and any actual performance overhead in the loop  resulting in a code that s actually shorter  simpler and faster  due to no branching conditionals in the loop  than other solutions proposed   E g   in a C-style pseudocode similar to OP s proposed code    s append          MAX    5 here  if it s constant  you can inline it below and get rid of the comparison if   MAX  gt  0         s appendF     d     0      0-th iteration     for  int i   1  i  lt  MAX    i             s appendF      d     i      i-th iteration         s append

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I usually loop over the array and attach a comma after every entry in the string  After the loop I delete the last comma again   Maybe not the best way  but less expensive than checking every time if it s the last object in the loop I guess

User · Answer

Unfortunately the JSON specification does not allow a trailing comma  There are a few browsers that will allow it  but generally you need to worry about all browsers   In general I try turn the problem around  and add the comma before the actual value  so you end up with code that looks like this   s append       for  i   0  i  lt  5    i      if  i  s append          add the comma only if this isn t the first entry   s appendF     d     i     s append         That extra one line of code in your for loop is hardly expensive     Another alternative I ve used when output a structure to JSON from a dictionary of some form is to always append a comma after each entry  as you are doing above  and then add a dummy entry at the end that has not trailing comma  but that is just lazy  -     Doesn t work well with an array unfortunately

User · Answer

Unfortunately the JSON specification does not allow a trailing comma  There are a few browsers that will allow it  but generally you need to worry about all browsers   In general I try turn the problem around  and add the comma before the actual value  so you end up with code that looks like this   s append       for  i   0  i  lt  5    i      if  i  s append          add the comma only if this isn t the first entry   s appendF     d     i     s append         That extra one line of code in your for loop is hardly expensive     Another alternative I ve used when output a structure to JSON from a dictionary of some form is to always append a comma after each entry  as you are doing above  and then add a dummy entry at the end that has not trailing comma  but that is just lazy  -     Doesn t work well with an array unfortunately

User · Answer

No   The JSON spec  as maintained at http   json org  does not allow trailing commas   From what I ve seen  some parsers may silently allow them when reading a JSON string  while others will throw errors   For interoperability  you shouldn t include it   The code above could be restructured  either to remove the trailing comma when adding the array terminator or to add the comma before items  skipping that for the first one

User · Answer

I keep a current count and compare it to a total count   If the current count is less than the total count  I display the comma   May not work if you don t have a total count prior to executing the JSON generation   Then again  if your using PHP 5 2 0 or better  you can just format your response using the JSON API built in

User · Answer

No   The  quot railroad diagrams quot  in https   json org are an exact translation of the spec and make it clear a   always comes before a value  never directly before     or

User · Answer

No   The JSON spec  as maintained at http   json org  does not allow trailing commas   From what I ve seen  some parsers may silently allow them when reading a JSON string  while others will throw errors   For interoperability  you shouldn t include it   The code above could be restructured  either to remove the trailing comma when adding the array terminator or to add the comma before items  skipping that for the first one

User · Answer

According to the Class JSONArray specification    An extra    comma  may appear just before the closing bracket   The null value will be inserted when there is    comma  elision    So  as I understand it  it should be allowed to write    0 1 2 3 4 5     But it could happen that some parsers will return the 7 as item count  like IE8 as Daniel Earwicker pointed out  instead of the expected 6     Edited   I found this JSON Validator that validates a JSON string against RFC 4627  The application json media type for JavaScript Object Notation  and against the JavaScript language specification  Actually here an array with a trailing comma is considered valid just for JavaScript and not for the RFC 4627 specification   However  in the RFC 4627 specification is stated that      2 3   Arrays      An array structure is represented as square brackets surrounding zero   or more values  or elements    Elements are separated by commas   array   begin-array   value    value-separator value     end-array    To me this is again an interpretation problem  If you write that Elements are separated by commas  without stating something about special cases  like the last element   it could be understood in both ways      P S  RFC 4627 isn t a standard  as explicitly stated   and is already obsolited by RFC 7159  which is a proposed standard  RFC 7159

User · Answer

Since a for-loop is used to iterate over an array  or similar iterable data structure  we can use the length of the array as shown   awk -v header  FirstName LastName DOB      BEGIN       FS            print           columns   split header  column names                print             for  i   1  i  lt  columns  i            printf         s      s    n   column names i     i              printf         s      s   n   column names i     i        print               END   print           datafile txt   With datafile txt containing    Angela Baker 2010-05-23  Betty Crockett 1990-12-07  David Done 2003-10-31

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As stated it is not allowed  But in JavaScript this is   var a   Array   for let i 1  i lt  5  i          a push i    var s         a join               works fine in Firefox  Chrome  Edge  IE11  and without the let in IE9  8  7  5

User · Answer

It is not recommended  but you can still do something like this to parse it    x000D   x000D  jsonStr     0 1 2 3 4 5     x000D  let data  x000D  eval  data       jsonStr   x000D  console log data  x000D   x000D   x000D

User · Answer

PHP coders may want to check out implode    This takes an array joins it up using a string   From the docs      array   array  lastname    email    phone    echo implode       array      lastname email phone

User · Answer

Interestingly  both C  amp  C    and I think C   but I m not sure  specifically allow the trailing comma -- for exactly the reason given  It make programmaticly generating lists much easier   Not sure why JavaScript didn t follow their lead

User · Answer

I usually loop over the array and attach a comma after every entry in the string  After the loop I delete the last comma again   Maybe not the best way  but less expensive than checking every time if it s the last object in the loop I guess

User · Answer

Trailing commas are allowed in JavaScript  but don t work in IE  Douglas Crockford s versionless JSON spec didn t allow them  and because it was versionless this wasn t supposed to change  The ES5 JSON spec allowed them as an extension  but Crockford s RFC 4627 didn t  and ES5 reverted to disallowing them  Firefox followed suit  Internet Explorer is why we can t have nice things

User · Answer

Use JSON5  Don t use JSON     Objects and arrays can have trailing commas Object keys can be unquoted if they re valid identifiers Strings can be single-quoted Strings can be split across multiple lines Numbers can be hexadecimal  base 16  Numbers can begin or end with a  leading or trailing  decimal point  Numbers can include Infinity and -Infinity  Numbers can begin with an explicit plus     sign  Both inline  single-line  and block  multi-line  comments are allowed    http   json5 org   https   github com aseemk json5

User · Answer

Unfortunately the JSON specification does not allow a trailing comma  There are a few browsers that will allow it  but generally you need to worry about all browsers   In general I try turn the problem around  and add the comma before the actual value  so you end up with code that looks like this   s append       for  i   0  i  lt  5    i      if  i  s append          add the comma only if this isn t the first entry   s appendF     d     i     s append         That extra one line of code in your for loop is hardly expensive     Another alternative I ve used when output a structure to JSON from a dictionary of some form is to always append a comma after each entry  as you are doing above  and then add a dummy entry at the end that has not trailing comma  but that is just lazy  -     Doesn t work well with an array unfortunately

User · Answer

No   The JSON spec  as maintained at http   json org  does not allow trailing commas   From what I ve seen  some parsers may silently allow them when reading a JSON string  while others will throw errors   For interoperability  you shouldn t include it   The code above could be restructured  either to remove the trailing comma when adding the array terminator or to add the comma before items  skipping that for the first one

User · Answer

PHP coders may want to check out implode    This takes an array joins it up using a string   From the docs      array   array  lastname    email    phone    echo implode       array      lastname email phone

User · Answer

There is a possible way to avoid a if-branch in the loop    s append           there is a space after the left bracket for  i   0  i  lt  5    i      s appendF     d      i      always add comma   s back             modify last comma  or the space  to right bracket

User · Answer

Interestingly  both C  amp  C    and I think C   but I m not sure  specifically allow the trailing comma -- for exactly the reason given  It make programmaticly generating lists much easier   Not sure why JavaScript didn t follow their lead

User · Answer

Simple  cheap  easy to read  and always works regardless of the specs    delimiter       for             print  delimiter  whatever      delimiter            The redundant assignment to  delim is a very small price to pay  Also works just as well if there is no explicit loop but separate code fragments

User · Answer

As it s been already said  JSON spec  based on ECMAScript 3  doesn t allow trailing comma  ES    5 allows it  so you can actually use that notation in pure JS  It s been argued about  and some parsers did support it  http   bolinfest com essays json html  http   whereswalden com 2010 09 08 spidermonkey-json-change-trailing-commas-no-longer-accepted    but it s the spec fact  as shown on http   json org   that it shouldn t work in JSON  That thing said         I m wondering why no-one pointed out that you can actually split the loop at 0th iteration and use leading comma instead of trailing one to get rid of the comparison code smell and any actual performance overhead in the loop  resulting in a code that s actually shorter  simpler and faster  due to no branching conditionals in the loop  than other solutions proposed   E g   in a C-style pseudocode similar to OP s proposed code    s append          MAX    5 here  if it s constant  you can inline it below and get rid of the comparison if   MAX  gt  0         s appendF     d     0      0-th iteration     for  int i   1  i  lt  MAX    i             s appendF      d     i      i-th iteration         s append

User · Answer

I usually loop over the array and attach a comma after every entry in the string  After the loop I delete the last comma again   Maybe not the best way  but less expensive than checking every time if it s the last object in the loop I guess

User · Answer

I usually loop over the array and attach a comma after every entry in the string  After the loop I delete the last comma again   Maybe not the best way  but less expensive than checking every time if it s the last object in the loop I guess

User · Answer

From my past experience  I found that different browsers deal with trailing commas in JSON differently    Both Firefox and Chrome handles it just fine  But IE  All versions  seems to break  I mean really break and stop reading the rest of the script    Keeping that in mind  and also the fact that it s always nice to write compliant code  I suggest spending the extra effort of making sure that there s no trailing comma

User · Answer

Use JSON5  Don t use JSON     Objects and arrays can have trailing commas Object keys can be unquoted if they re valid identifiers Strings can be single-quoted Strings can be split across multiple lines Numbers can be hexadecimal  base 16  Numbers can begin or end with a  leading or trailing  decimal point  Numbers can include Infinity and -Infinity  Numbers can begin with an explicit plus     sign  Both inline  single-line  and block  multi-line  comments are allowed    http   json5 org   https   github com aseemk json5

User · Answer

Interestingly  both C  amp  C    and I think C   but I m not sure  specifically allow the trailing comma -- for exactly the reason given  It make programmaticly generating lists much easier   Not sure why JavaScript didn t follow their lead

User · Answer

PHP coders may want to check out implode    This takes an array joins it up using a string   From the docs      array   array  lastname    email    phone    echo implode       array      lastname email phone

User · Answer

Rather than engage in a debating club  I would apply Defensive Programming  As a developer of an app that receives json data  I d allow the trailing comma  When developing an app that writes json  I d use one of the clever techniques of the other answers to only add commas between items  There are bigger problems to be solved

User · Answer

With Relaxed JSON  you can have trailing commas  or just leave the commas out   They are optional   There is no reason at all commas need to be present to parse a JSON-like document   Take a look at the Relaxed JSON spec and you will see how  noisy  the original JSON spec is   Way too many commas and quotes     http   www relaxedjson org  You can also try out your example using this online RJSON parser and see it get parsed correctly   http   www relaxedjson org docs converter html source  5B0 2C1 2C2 2C3 2C4 2C5 2C 5D

User · Answer

No   The JSON spec  as maintained at http   json org  does not allow trailing commas   From what I ve seen  some parsers may silently allow them when reading a JSON string  while others will throw errors   For interoperability  you shouldn t include it   The code above could be restructured  either to remove the trailing comma when adding the array terminator or to add the comma before items  skipping that for the first one

User · Answer

Simple  cheap  easy to read  and always works regardless of the specs    delimiter       for             print  delimiter  whatever      delimiter            The redundant assignment to  delim is a very small price to pay  Also works just as well if there is no explicit loop but separate code fragments

User · Answer

It is not recommended  but you can still do something like this to parse it    x000D   x000D  jsonStr     0 1 2 3 4 5     x000D  let data  x000D  eval  data       jsonStr   x000D  console log data  x000D   x000D   x000D

User · Answer

PHP coders may want to check out implode    This takes an array joins it up using a string   From the docs      array   array  lastname    email    phone    echo implode       array      lastname email phone

User · Answer

Since a for-loop is used to iterate over an array  or similar iterable data structure  we can use the length of the array as shown   awk -v header  FirstName LastName DOB      BEGIN       FS            print           columns   split header  column names                print             for  i   1  i  lt  columns  i            printf         s      s    n   column names i     i              printf         s      s   n   column names i     i        print               END   print           datafile txt   With datafile txt containing    Angela Baker 2010-05-23  Betty Crockett 1990-12-07  David Done 2003-10-31

User · Answer

I keep a current count and compare it to a total count   If the current count is less than the total count  I display the comma   May not work if you don t have a total count prior to executing the JSON generation   Then again  if your using PHP 5 2 0 or better  you can just format your response using the JSON API built in

User · Answer

Unfortunately the JSON specification does not allow a trailing comma  There are a few browsers that will allow it  but generally you need to worry about all browsers   In general I try turn the problem around  and add the comma before the actual value  so you end up with code that looks like this   s append       for  i   0  i  lt  5    i      if  i  s append          add the comma only if this isn t the first entry   s appendF     d     i     s append         That extra one line of code in your for loop is hardly expensive     Another alternative I ve used when output a structure to JSON from a dictionary of some form is to always append a comma after each entry  as you are doing above  and then add a dummy entry at the end that has not trailing comma  but that is just lazy  -     Doesn t work well with an array unfortunately

User · Answer

With Relaxed JSON  you can have trailing commas  or just leave the commas out   They are optional   There is no reason at all commas need to be present to parse a JSON-like document   Take a look at the Relaxed JSON spec and you will see how  noisy  the original JSON spec is   Way too many commas and quotes     http   www relaxedjson org  You can also try out your example using this online RJSON parser and see it get parsed correctly   http   www relaxedjson org docs converter html source  5B0 2C1 2C2 2C3 2C4 2C5 2C 5D

User · Answer

There is a possible way to avoid a if-branch in the loop    s append           there is a space after the left bracket for  i   0  i  lt  5    i      s appendF     d      i      always add comma   s back             modify last comma  or the space  to right bracket

User · Answer

As stated it is not allowed  But in JavaScript this is   var a   Array   for let i 1  i lt  5  i          a push i    var s         a join               works fine in Firefox  Chrome  Edge  IE11  and without the let in IE9  8  7  5

User · Answer

Rather than engage in a debating club  I would apply Defensive Programming  As a developer of an app that receives json data  I d allow the trailing comma  When developing an app that writes json  I d use one of the clever techniques of the other answers to only add commas between items  There are bigger problems to be solved

[json] Can you use a trailing comma in a JSON object?

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