feat: Allow trimming trailing commas in arrays and objects (JSON5)

Author: prantlf

README.md

@@ -103,6 +103,11 @@ By default, `jsonlint` will either report a syntax error with details or pretty-
       --prune-comments             omit comments from the prettified output
       --strip-object-keys          strip quotes from object keys if possible
                                    (JSON5)
+      --enforce-double-quotes      surrounds all strings with double quotes
+      --enforce-single-quotes      surrounds all strings with single quotes
+                                   (JSON5)
+      --trim-trailing-commas       omit trailing commas from objects and arrays
+                                   (JSON5)
       -v, --version                output the version number
       -h, --help                   output usage information
 
@@ -204,6 +209,7 @@ The [`print`](#pretty-printing) method accepts an object `options` as the second
 | `stripObjectKeys` | will not print quotes around object keys which are JavaScript identifier names |
 | `enforceDoubleQuotes`       | will surround all strings with double quotes            |
 | `enforceSingleQuotes`       | will surround all strings with single quotes            |
+| `trimTrailingCommas`        | will omit all trailing commas after the last object entry or array item |
 
 ```js
 // Just concatenate the tokens to produce the same output as was the input.
@@ -223,7 +229,11 @@ print(tokens, { indent: '  ', pruneComments: true })
 // Print to multiple lines with indentation enabled and JSON5 object keys.
 print(tokens, { indent: '\t', stripObjectKeys: true })
 // Print to multiple lines with indentation enabled, unify JSON5 formatting.
-print(tokens, { indent: '    ', enforceDoubleQuotes: true })
+print(tokens, {
+  indent: '    ',
+  enforceDoubleQuotes: true,
+  trimTrailingCommas: true
+})
 ```
 
 ### Tokenizing

lib/cli.js

@@ -36,6 +36,7 @@ var options = require('commander')
   .option('--strip-object-keys', 'strip quotes from object keys if possible (JSON5)')
   .option('--enforce-double-quotes', 'surrounds all strings with double quotes')
   .option('--enforce-single-quotes', 'surrounds all strings with single quotes (JSON5)')
+  .option('--trim-trailing-commas', 'omit trailing commas from objects and arrays (JSON5)')
   .version(pkg.version, '-v, --version')
   .on('--help', () => {
     console.log()
@@ -90,7 +91,8 @@ function parse (source, file) {
         pruneComments: options.pruneComments,
         stripObjectKeys: options.stripObjectKeys,
         enforceDoubleQuotes: options.enforceDoubleQuotes,
-        enforceSingleQuotes: options.enforceSingleQuotes
+        enforceSingleQuotes: options.enforceSingleQuotes,
+        trimTrailingCommas: options.trimTrailingCommas
       })
     }
     if (options.sortKeys) {

lib/jsonlint.d.ts

@@ -92,6 +92,7 @@ declare module '@prantlf/jsonlint/lib/printer' {
     stripObjectKeys?: boolean
     enforceDoubleQuotes?: boolean
     enforceSingleQuotes?: boolean
+    trimTrailingCommas?: boolean
   }
 
   /**

lib/printer.js

@@ -50,6 +50,7 @@
     var stripObjectKeys = options.stripObjectKeys
     var enforceDoubleQuotes = options.enforceDoubleQuotes
     var enforceSingleQuotes = options.enforceSingleQuotes
+    var trimTrailingCommas = options.trimTrailingCommas
 
     var outputString = ''
     var foundLineBreak, addedLineBreak, needsLineBreak
@@ -61,6 +62,16 @@
     var tokenCount = tokens.length
     var tokenIndex, token, tokenType, tokenContent
 
+    function peekAtNextToken () {
+      var nextTokenIndex = tokenIndex
+      var nextToken
+      do {
+        nextToken = tokens[++nextTokenIndex]
+      } while (nextToken && (nextToken.type === 'whitespace' ||
+                             nextToken.type === 'comment'))
+      return nextToken
+    }
+
     var addIndent
     if (prettyPrint && indentString) {
       addIndent = function () {
@@ -222,6 +233,12 @@
     }
 
     function addComma () {
+      if (trimTrailingCommas) {
+        var nextToken = peekAtNextToken()
+        if (nextToken && nextToken.type === 'symbol') {
+          return tryAddingInlineComment()
+        }
+      }
       addDelayedSpaceOrLineBreak()
       outputString += ','
       tryAddingInlineComment()

test/print.js

@@ -161,4 +161,9 @@ addTest('enforce double quotes, but strip quotes from object keys', function ()
   assert.equal(output, '{/* String parameter */key:"value",}')
 })
 
+addTest('trim trailing commas', function () {
+  var output = print(stringTokens, { trimTrailingCommas: true })
+  assert.equal(output, '{/* String parameter */"key":\'value\'}')
+})
+
 if (require.main === module) { require('test').run(exports) }

test/typings.test.ts

@@ -67,6 +67,7 @@ addTest('print', () => {
   print(tokens, { stripObjectKeys: true })
   print(tokens, { enforceDoubleQuotes: true })
   print(tokens, { enforceSingleQuotes: true })
+  print(tokens, { trimTrailingCommas: true })
   assert.ok(true)
 })
 

web/jsonlint.html

@@ -144,6 +144,10 @@ <h1>JSON Lint</h1>
               <input type="checkbox" id="enforce-single-quotes">
               <label for="enforce-single-quotes">Enforce single quotes</label>
             </div>
+            <div style="display: none">
+              <input type="checkbox" id="trim-trailing-commas">
+              <label for="trim-trailing-commas">Trim trailing commas</label>
+            </div>
           </div>
           <div>
             <span>Standard:</span>
@@ -231,8 +235,8 @@ <h2>Result</h2>
         ensureOneSelected(selected, other)
         var forFormat = ['sort-object-keys']
         var forPrettyPrint = [
-          'prune-comments', 'strip-object-keys',
-          'enforce-double-quotes', 'enforce-single-quotes'
+          'prune-comments', 'strip-object-keys', 'enforce-double-quotes',
+          'enforce-single-quotes', 'trim-trailing-commas'
         ]
         swapWrapperVisibility(document.getElementById('pretty-print').checked,
           forPrettyPrint, forFormat)
@@ -304,7 +308,8 @@ <h2>Result</h2>
             pruneComments: document.getElementById('prune-comments').checked,
             stripObjectKeys: document.getElementById('strip-object-keys').checked,
             enforceDoubleQuotes: document.getElementById('enforce-double-quotes').checked,
-            enforceSingleQuotes: document.getElementById('enforce-single-quotes').checked
+            enforceSingleQuotes: document.getElementById('enforce-single-quotes').checked,
+            trimTrailingCommas: document.getElementById('trim-trailing-commas').checked
           })
         }