Respect package.json indentation when updating the lockfile

npm_and_yarn/lib/dependabot/npm_and_yarn/file_updater/npm_lockfile_updater.rb

@@ -476,6 +476,7 @@ def write_temporary_dependency_files(update_package_json: true)
             updated_content = lock_deps_with_latest_reqs(updated_content)
 
             updated_content = sanitized_package_json_content(updated_content)
+
             File.write(file.name, updated_content)
           end
         end
@@ -495,6 +496,16 @@ def write_lockfiles
           end
         end
 
+        # Takes a JSON string and detects if it is spaces or tabs and how many
+        # levels deep it is indented.
+        def detect_indentation(json)
+          indentation = json.scan(/^\s+/).min_by(&:length)
+          indentation_size = indentation.length
+          indentation_type = indentation.scan(/\t/).any? ? "\t" : " "
+
+          indentation_type * indentation_size
+        end
+
         def lock_git_deps(content)
           return content if git_dependencies_to_lock.empty?
 
@@ -508,7 +519,8 @@ def lock_git_deps(content)
             end
           end
 
-          json.to_json
+          indent = detect_indentation(content)
+          JSON.pretty_generate(json, indent: indent)
         end
 
         def git_dependencies_to_lock
@@ -549,7 +561,8 @@ def lock_deps_with_latest_reqs(content)
             end
           end
 
-          json.to_json
+          indent = detect_indentation(content)
+          JSON.pretty_generate(json, indent: indent)
         end
 
         def replace_ssh_sources(content)

npm_and_yarn/spec/fixtures/projects/npm7/subdependency_update_tab_indentation/package.json

@@ -1,14 +1,14 @@
 {
-	"name": "npm-sub-dep-test",
-	"version": "1.0.0",
-	"description": "",
-	"main": "index.js",
-	"scripts": {
-		"test": "echo \"Error: no test specified\" && exit 1"
-	},
-	"author": "",
-	"license": "ISC",
-	"dependencies": {
-		"test-old-npm-sub-dependency": "github:dependabot-fixtures/test-old-npm-sub-dependency"
-	}
+		"name": "npm-sub-dep-test",
+		"version": "1.0.0",
+		"description": "",
+		"main": "index.js",
+		"scripts": {
+				"test": "echo \"Error: no test specified\" && exit 1"
+		},
+		"author": "",
+		"license": "ISC",
+		"dependencies": {
+				"test-old-npm-sub-dependency": "github:dependabot-fixtures/test-old-npm-sub-dependency"
+		}
 }