diff --git a/lib/workers/dmpWorker.ts b/lib/workers/dmpWorker.ts
index 982555bb..825a8616 100644
--- a/lib/workers/dmpWorker.ts
+++ b/lib/workers/dmpWorker.ts
@@ -4,6 +4,7 @@ import DiffMatchPatch from "@hackmd/diff-match-patch";
 // core
 import config from "../config";
 import {logger} from "../logger";
+import {Revision} from "../models";
 
 const dmp = new DiffMatchPatch()
 process.on('message', function (data) {
@@ -57,25 +58,31 @@ process.on('message', function (data) {
   return null
 })
 
-function createPatch(lastDoc, currDoc) {
+function createPatch(lastDoc: string, currDoc: string): string {
   const msStart = (new Date()).getTime()
   const diff = dmp.diff_main(lastDoc, currDoc)
-  let patch = dmp.patch_make(lastDoc, diff)
-  patch = dmp.patch_toText(patch)
+  const patch = dmp.patch_make(lastDoc, diff)
+  const patchText = dmp.patch_toText(patch)
   const msEnd = (new Date()).getTime()
   if (config.debug) {
-    logger.info(patch)
+    logger.info(patchText)
     logger.info((msEnd - msStart) + 'ms')
   }
-  return patch
+  return patchText
 }
 
-function getRevision(revisions, count) {
+interface DiffRevision {
+  content: string,
+  patch: Patch[],
+  authorship: string
+}
+
+function getRevision(revisions: Revision[], count: number): DiffRevision {
   const msStart = (new Date()).getTime()
   let startContent = null
-  let lastPatch = []
+  let lastPatch = ""
   let applyPatches = []
-  let authorship = []
+  let authorship = ""
   if (count <= Math.round(revisions.length / 2)) {
     // start from top to target
     for (let i = 0; i < count; i++) {
diff --git a/tsconfig.json b/tsconfig.json
index 29e1f16a..e06f932a 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,12 +1,16 @@
 {
   "compileOnSave": true,
   "compilerOptions": {
+    "baseUrl": ".",
     "allowJs": false,
     "target": "ES2019",
     "outDir": "./dist/",
     "module": "CommonJS",
     "esModuleInterop": true,
-    "typeRoots": ["./typings", "./node_modules/@types"]
+    "typeRoots": ["./typings", "./node_modules/@types"],
+    "paths": {
+      "@hackmd/diff-match-patch": ["./typings/diff-match-patch/"]
+    }
   },
   "include": [
     "./lib/**/*"
diff --git a/typings/diff-match-patch/index.d.ts b/typings/diff-match-patch/index.d.ts
new file mode 100644
index 00000000..7e6b5415
--- /dev/null
+++ b/typings/diff-match-patch/index.d.ts
@@ -0,0 +1,82 @@
+declare global {
+  module diffMatchPatch {
+    export const DIFF_DELETE = -1;
+    export const DIFF_INSERT = 1;
+    export const DIFF_EQUAL = 0;
+  }
+
+  enum DiffType {
+    DIFF_DELETE = -1,
+    DIFF_INSERT = 1,
+    DIFF_EQUAL = 0
+  }
+
+  type Diff = {
+    [0]: DiffType
+    [1]: string
+  }
+
+  interface Patch {
+    diffs: Diff[];
+
+    start1?: number
+    start2?: number
+
+    length1: number
+    length2: number
+
+    toString(): string
+  }
+
+  class diffMatchPatch {
+    diff_main(text1: string, text2: string, opt_checklines?: boolean,
+              opt_deadline?: number): Diff[]
+
+    diff_cleanupSemantic(diffs: Diff[])
+
+    diff_cleanupEfficiency(diffs: Diff[])
+
+    diff_cleanupSemanticLossless(diffs: Diff[])
+
+    diff_levenshtein(diffs: Diff[]): number
+
+    diff_prettyHtml(diffs: Diff[]): string
+
+    diff_xIndex(diffs: Diff[], loc: number): number
+
+    diff_cleanupMerge(diffs: Diff[])
+
+    match_main(text: string, pattern: string, loc: number): number
+
+    diff_fromDelta(text1: string, delta: string): Diff[]
+
+    diff_toDelta(diffs: Diff[]): string
+
+    diff_levenshtein(diffs: Diff[]): number
+
+    diff_text1(diffs: Diff[]): string
+
+    diff_text2(diffs: Diff[]): string
+
+    patch_make(a: string, opt_b: string): Patch[]
+    patch_make(a: Diff[]): Patch[]
+    patch_make(a: string, opt_b: Diff[]): Patch[]
+    patch_make(a: string, opt_b: string, opt_c: Diff[]): Patch[]
+
+    patch_splitMax(patches: Patch[])
+
+    patch_addPadding(patches: Patch[]): string
+
+    patch_deepCopy(patches: Patch[]): Patch[]
+
+    patch_toText(patches: Patch[]): string
+
+    patch_fromText(textline: string): Patch[]
+
+    patch_apply(patches: Patch[], text: string): {
+      [0]: string
+      [1]: boolean[]
+    }
+  }
+}
+export = diffMatchPatch