Set newline and indentation to defaults for empty objects/arrays

Noticed that in my tests when I do "echo {} > package.json", I was then
getting an unformatted package.json after doing installs, which is
unexpected.  Treat empty objects as if they had the default indentation,
since that is the result "JSON.stringify({}, null, 2)".

Author: isaacs

index.js

@@ -66,7 +66,10 @@ const kIndent = Symbol.for('indent')
 const kNewline = Symbol.for('newline')
 // only respect indentation if we got a line break, otherwise squash it
 // things other than objects and arrays aren't indented, so ignore those
+// Important: in both of these regexps, the $1 capture group is the newline
+// or undefined, and the $2 capture group is the indent, or undefined.
 const formatRE = /^\s*[{\[]((?:\r?\n)+)([\s\t]*)/
+const emptyRE = /^(?:\{\}|\[\])((?:\r?\n)+)?$/
 
 const parseJson = (txt, reviver, context) => {
   const parseText = stripBOM(txt)
@@ -77,7 +80,11 @@ const parseJson = (txt, reviver, context) => {
     // otherwise, pick the indentation of the next line after the first \n
     // If the pattern doesn't match, then it means no indentation.
     // JSON.stringify ignores symbols, so this is reasonably safe.
-    const [, newline, indent] = parseText.match(formatRE) || [, '', '']
+    // if the string is '{}' or '[]', then use the default 2-space indent.
+    const [, newline = '\n', indent = '  '] = parseText.match(emptyRE) ||
+      parseText.match(formatRE) ||
+      [, '', '']
+
     const result = JSON.parse(parseText, reviver)
     if (result && typeof result === 'object') {
       result[kNewline] = newline

test/index.js

@@ -47,6 +47,25 @@ t.test('preserves indentation and newline styles', t => {
   t.end()
 })
 
+t.test('indentation is the default when object/array is empty', t => {
+  const kIndent = Symbol.for('indent')
+  const kNewline = Symbol.for('newline')
+  const obj = '{}'
+  const arr = '[]'
+  for (const newline of ['', '\n', '\r\n', '\n\n', '\r\n\r\n']) {
+    const expect = newline || '\n'
+    for (const str of [obj, arr]) {
+      t.test(JSON.stringify({str, newline, expect}), t => {
+        const res = parseJson(str + newline)
+        t.equal(res[kNewline], expect, 'got expected newline')
+        t.equal(res[kIndent], '  ', 'got expected default indentation')
+        t.end()
+      })
+    }
+  }
+  t.end()
+})
+
 t.test('parses JSON if it is a Buffer, removing BOM bytes', t => {
   const str = JSON.stringify({
     foo: 1,