fix: hotfix preserve final newline

Author: zardoy

src/index.ts

@@ -98,7 +98,7 @@ export const modifyJsonFile: ModifyJsonFileGenericFunction = async (path, modify
         removeJsonc = false,
     } = options
     try {
-        let { json, indent } = await loadJsonFile(path, { encoding, tabSize, removeJsonc })
+        let { json, indent, hasFinalNewline } = await loadJsonFile(path, { encoding, tabSize, removeJsonc })
         if (typeof modifyProperties === 'function') {
             // TODO why arg is never
             json = await (modifyProperties as any)(json)
@@ -120,7 +120,10 @@ export const modifyJsonFile: ModifyJsonFileGenericFunction = async (path, modify
             }
         }
 
-        await fs.promises.writeFile(path, JSON.stringify(json, undefined, indent))
+        // TODO don't use this fix
+        let string = JSON.stringify(json, undefined, indent)
+        if (hasFinalNewline) string += '\n'
+        await fs.promises.writeFile(path, string)
     } catch (err) {
         if (throws) throw err
     }

src/loadJsonFile.ts

@@ -19,5 +19,6 @@ export const loadJsonFile = async (filePath: string, { encoding, tabSize, remove
     return {
         json: parseJson(contents, filePath) as JsonRoot,
         indent: tabSize === 'preserve' ? detectIndent(contents).indent : tabSize === 'hard' ? '\t' : tabSize === null ? undefined : ' '.repeat(tabSize),
+        hasFinalNewline: contents.split('\n').slice(-1)[0]! === '',
     }
 }

tests/index.test.ts

@@ -190,6 +190,47 @@ test("Don't throw error if option is provided on missing property in input", asy
     )
 })
 
+test('Preserves empty newline', async () => {
+    expect.assertions(1)
+    prepare({
+        data: `
+{
+    "author": "eldar",
+    "bin": "src/bin.ts",
+    "dependencies": {
+        "fdir": ">=2",
+        "type-fest": "*"
+    }
+}
+`,
+        json: false,
+        writeCallback(data) {
+            expect(data).toMatchInlineSnapshot(`
+"{
+    \\"author\\": \\"eldar\\",
+    \\"bin\\": \\"build/bin.js\\",
+    \\"dependencies\\": {
+        \\"fdir\\": \\">=2\\",
+        \\"type-fest\\": \\"*\\"
+    },
+    \\"somethingWeird\\": \\"new-item\\"
+}
+"
+`)
+        },
+    })
+    await modifyJsonFile(
+        '',
+        {
+            bin: 'build/bin.js',
+            somethingWeird: () => 'new-item',
+        },
+        {
+            ifPropertyIsMissingForSetter: 'pass',
+        },
+    )
+})
+
 test('loader removes comments and trailing commas', async () => {
     const { json } = await loadJsonFile(join(__dirname, './tsconfig.fixture.json'), { encoding: 'utf-8', removeJsonc: true, tabSize: 'preserve' })
     expect(json).toMatchInlineSnapshot(`