Do not write package.json if nothing changed

lib/update-root-package-json.js

@@ -15,11 +15,18 @@ const depTypes = new Set([
   'peerDependencies',
 ])
 
+const parseJsonSafe = json => {
+  try {
+    return parseJSON(json)
+  } catch (er) {
+    return null
+  }
+}
+
 const updateRootPackageJson = async tree => {
   const filename = resolve(tree.path, 'package.json')
-  const originalContent = await readFile(filename, 'utf8')
-    .then(data => parseJSON(data))
-    .catch(() => null)
+  const originalJson = await readFile(filename, 'utf8').catch(() => null)
+  const originalContent = parseJsonSafe(originalJson)
 
   const depsData = orderDeps({
     ...tree.package,
@@ -52,7 +59,8 @@ const updateRootPackageJson = async tree => {
   const content = (JSON.stringify(packageJsonContent, null, format) + '\n')
     .replace(/\n/g, eol)
 
-  return writeFile(filename, content)
+  if (content !== originalJson)
+    return writeFile(filename, content)
 }
 
 module.exports = updateRootPackageJson

test/update-root-package-json.js

@@ -1,4 +1,5 @@
 const {resolve} = require('path')
+const {statSync} = require('fs')
 
 const t = require('tap')
 
@@ -29,6 +30,33 @@ t.test('missing package.json', async t => {
   )
 })
 
+t.test('invalid package.json', async t => {
+  const path = t.testdir({
+    'package.json': 'this! is! not! json!',
+  })
+  await updateRootPackageJson({
+    path: path,
+    package: {
+      name: 'invalid-package-json-test',
+      version: '1.0.0',
+      dependencies: {
+        abbrev: '^1.0.0',
+      },
+    },
+  })
+  t.match(
+    require(resolve(path, 'package.json')),
+    {
+      name: 'invalid-package-json-test',
+      version: '1.0.0',
+      dependencies: {
+        abbrev: '^1.0.0',
+      },
+    },
+    'should write new package.json with tree data'
+  )
+})
+
 t.test('existing package.json', async t => {
   const path = t.testdir({
     'package.json': JSON.stringify({
@@ -62,6 +90,48 @@ t.test('existing package.json', async t => {
   )
 })
 
+t.test('unchanged package.json', async t => {
+  const path = t.testdir({
+    'package.json': JSON.stringify({
+      name: 'existing-package-json-test',
+      version: '1.0.0',
+      bin: './file.js',
+      funding: 'http://example.com',
+      dependencies: {
+        abbrev: '^1.0.0',
+      },
+    }, null, 2) + '\n',
+  })
+  const { mtime } = statSync(path + '/package.json')
+  await updateRootPackageJson({
+    path: path,
+    package: {
+      name: 'existing-package-json-test',
+      version: '1.0.0',
+      bin: './file.js',
+      funding: 'http://example.com',
+      dependencies: {
+        abbrev: '^1.0.0',
+      },
+    },
+  })
+  t.match(
+    require(resolve(path, 'package.json')),
+    {
+      name: 'existing-package-json-test',
+      version: '1.0.0',
+      bin: './file.js',
+      funding: 'http://example.com',
+      dependencies: {
+        abbrev: '^1.0.0',
+      },
+    },
+    'should write new package.json with tree data'
+  )
+  const { mtime: newMtime } = statSync(path + '/package.json')
+  t.equal(newMtime.toISOString(), mtime.toISOString(), 'mtime not changed')
+})
+
 t.test('existing package.json with optionalDependencies', async t => {
   const path = t.testdir({
     'package.json': JSON.stringify({