diff --git a/.mocharc.yml b/.mocharc.yml
index b846d5d7..f93fd01e 100644
--- a/.mocharc.yml
+++ b/.mocharc.yml
@@ -1,3 +1,3 @@
 reporter: spec
 opts: false
-spec: "test/*.spec.js"
+spec: "test/*.spec.ts"
diff --git a/lib/camel_case_keys.ts b/lib/camel_case_keys.ts
index 8a3a2c60..5f91d04f 100644
--- a/lib/camel_case_keys.ts
+++ b/lib/camel_case_keys.ts
@@ -27,7 +27,7 @@ function camelCaseKeys(obj: object) {
   if (typeof obj !== 'object') throw new TypeError('obj must be an object!');
 
   const keys = Object.keys(obj);
-  const result = {};
+  const result: Record<string, any> = {};
 
   for (const oldKey of keys) {
     const newKey = toCamelCase(oldKey);
diff --git a/lib/full_url_for.ts b/lib/full_url_for.ts
index 4fa63cbb..cb55c53b 100644
--- a/lib/full_url_for.ts
+++ b/lib/full_url_for.ts
@@ -2,7 +2,7 @@ import { parse } from 'url';
 import encodeURL from './encode_url';
 import prettyUrls from './pretty_urls';
 import Cache from './cache';
-const cache = new Cache();
+const cache = new Cache<string>();
 
 function fullUrlForHelper(path = '/') {
   const { config } = this;
diff --git a/lib/html_tag.ts b/lib/html_tag.ts
index 388457aa..6660dfdc 100644
--- a/lib/html_tag.ts
+++ b/lib/html_tag.ts
@@ -14,8 +14,8 @@ function encSrcset(str: string) {
   return str;
 }
 
-function htmlTag(tag: string, attrs: {
-  [key: string]: string | boolean | null | undefined;
+function htmlTag(tag: string, attrs?: {
+  [key: string]: string | boolean | number | null | undefined;
 }, text?: string, escape = true) {
   if (!tag) throw new TypeError('tag is required!');
 
diff --git a/lib/toc_obj.ts b/lib/toc_obj.ts
index e24aef9c..7c6d8c08 100644
--- a/lib/toc_obj.ts
+++ b/lib/toc_obj.ts
@@ -40,7 +40,7 @@ function tocObj(str: string, options = {}) {
 
   if (!headingsLen) return [];
 
-  const result = [];
+  const result: Result[] = [];
 
   for (let i = 0; i < headingsLen; i++) {
     const el = headings[i];
diff --git a/lib/truncate.ts b/lib/truncate.ts
index 2218f3cd..cbbb52e3 100644
--- a/lib/truncate.ts
+++ b/lib/truncate.ts
@@ -4,7 +4,7 @@ interface Options {
   separator?: string;
 }
 
-function truncate(str: string, options: Options = {}) {
+function truncate(str: string, options: Options = {}): string {
   if (typeof str !== 'string') throw new TypeError('str must be a string!');
 
   const length = options.length || 30;
diff --git a/package.json b/package.json
index 918a9a0e..ddd576ef 100644
--- a/package.json
+++ b/package.json
@@ -34,12 +34,14 @@
   ],
   "license": "MIT",
   "devDependencies": {
+    "@types/chai": "^4.3.11",
     "@types/cross-spawn": "^6.0.2",
+    "@types/mocha": "^10.0.6",
     "@types/node": "^18.11.8",
     "@types/prismjs": "^1.26.0",
+    "@types/rewire": "^2.5.30",
     "c8": "^9.1.0",
     "chai": "^4.3.6",
-    "chai-as-promised": "^7.1.1",
     "domhandler": "^5.0.3",
     "eslint": "^8.23.0",
     "eslint-config-hexo": "^5.0.0",
diff --git a/test/.eslintrc.json b/test/.eslintrc.json
index 26f258b3..658b30cf 100644
--- a/test/.eslintrc.json
+++ b/test/.eslintrc.json
@@ -1,7 +1,13 @@
 {
-  "extends": "hexo/test",
+  "extends": "hexo/ts-test",
   "rules": {
+    "@typescript-eslint/no-explicit-any": 0,
+    "@typescript-eslint/ban-ts-comment": 0,
+    "@typescript-eslint/no-non-null-assertion": 0,
+    "node/no-unsupported-features/es-syntax": 0,
     "@typescript-eslint/no-var-requires": 0,
-    "@typescript-eslint/no-empty-function": 0
+    "@typescript-eslint/no-empty-function": 0,
+    "@typescript-eslint/no-unused-vars": 0,
+    "node/no-missing-require": 0
   }
 }
\ No newline at end of file
diff --git a/test/cache.spec.js b/test/cache.spec.ts
similarity index 88%
rename from test/cache.spec.js
rename to test/cache.spec.ts
index 2864825a..716006b3 100644
--- a/test/cache.spec.js
+++ b/test/cache.spec.ts
@@ -1,14 +1,13 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import Cache from '../lib/cache';
+chai.should();
 
 describe('Cache', () => {
-  const Cache = require('../dist/cache');
-  const cache = new Cache();
+  const cache = new Cache<number>();
 
   it('get & set', () => {
     cache.set('foo', 123);
-    cache.get('foo').should.eql(123);
+    cache.get('foo')!.should.eql(123);
   });
 
   it('size', () => {
diff --git a/test/cache_stream.spec.js b/test/cache_stream.spec.ts
similarity index 71%
rename from test/cache_stream.spec.js
rename to test/cache_stream.spec.ts
index 76e7d9a0..55825549 100644
--- a/test/cache_stream.spec.js
+++ b/test/cache_stream.spec.ts
@@ -1,12 +1,10 @@
-'use strict';
+import chai from 'chai';
+import { Readable } from 'stream';
+import CacheStream from '../lib/cache_stream';
 
-require('chai').should();
-
-const { Readable } = require('stream');
+chai.should();
 
 describe('CacheStream', () => {
-  const CacheStream = require('../dist/cache_stream');
-
   it('default', () => {
     const src = new Readable();
     const cacheStream = new CacheStream();
diff --git a/test/camel_case_keys.spec.js b/test/camel_case_keys.spec.ts
similarity index 89%
rename from test/camel_case_keys.spec.js
rename to test/camel_case_keys.spec.ts
index 63bff12a..2971a3d8 100644
--- a/test/camel_case_keys.spec.js
+++ b/test/camel_case_keys.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import camelCaseKeys from '../lib/camel_case_keys';
+chai.should();
 
 describe('camelCaseKeys', () => {
-  const camelCaseKeys = require('../dist/camel_case_keys');
-
   it('default', () => {
     const result = camelCaseKeys({
       foo_bar: 'test'
diff --git a/test/color.spec.js b/test/color.spec.ts
similarity index 96%
rename from test/color.spec.js
rename to test/color.spec.ts
index 093c385c..a7359195 100644
--- a/test/color.spec.js
+++ b/test/color.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import Color from '../lib/color';
+chai.should();
 
 describe('color', () => {
-  const Color = require('../dist/color');
-
   it('name', () => {
     const red = new Color('red');
     const pink = new Color('pink');
@@ -67,6 +65,7 @@ describe('color', () => {
   it('invalid color', () => {
     let color;
     try {
+      // @ts-ignore
       color = new Color(200);
     } catch (e) {
       e.message.should.eql('color is required!');
diff --git a/test/decode_url.spec.js b/test/decode_url.spec.ts
similarity index 96%
rename from test/decode_url.spec.js
rename to test/decode_url.spec.ts
index d6bc3c3e..95945475 100644
--- a/test/decode_url.spec.js
+++ b/test/decode_url.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import decodeURL from '../lib/decode_url';
+chai.should();
 
 describe('decodeURL', () => {
-  const decodeURL = require('../dist/decode_url');
-
   it('regular', () => {
     const content = 'http://foo.com/';
     decodeURL(content).should.eql(content);
diff --git a/test/deep_merge.spec.js b/test/deep_merge.spec.ts
similarity index 59%
rename from test/deep_merge.spec.js
rename to test/deep_merge.spec.ts
index a9b996d8..27f696e4 100644
--- a/test/deep_merge.spec.js
+++ b/test/deep_merge.spec.ts
@@ -1,36 +1,34 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import deepMerge from '../lib/deep_merge';
+chai.should();
 
 // The test is modified based on https://github.com/jonschlinkert/merge-deep/blob/master/test.js
 
 describe('deepMerge()', () => {
-  const deepMerge = require('../dist/deep_merge');
-
   it('should act as lodash.merge', () => {
-    const obj1 = { 'a': [{ 'b': 2 }, { 'd': 4 }] };
-    const obj2 = { 'a': [{ 'c': 3 }, { 'e': 5 }] };
+    const obj1: any = { 'a': [{ 'b': 2 }, { 'd': 4 }] };
+    const obj2: any = { 'a': [{ 'c': 3 }, { 'e': 5 }] };
 
     const expected = { 'a': [{ 'b': 2, 'c': 3 }, { 'd': 4, 'e': 5 }] };
-
-    deepMerge(obj1, obj2).should.eql(expected);
+    const result: any = deepMerge(obj1, obj2);
+    result.should.eql(expected);
   });
 
   it('should do a deep merge', () => {
-    const obj1 = {a: {b: 1, c: 1, d: {e: 1, f: 1}}};
-    const obj2 = {a: {b: 2, d: {f: 'f'} }};
+    const obj1: any = {a: {b: 1, c: 1, d: {e: 1, f: 1}}};
+    const obj2: any = {a: {b: 2, d: {f: 'f'} }};
 
     const expected = {a: {b: 2, c: 1, d: {e: 1, f: 'f'} }};
-
-    deepMerge(obj1, obj2).should.eql(expected);
+    const result: any = deepMerge(obj1, obj2);
+    result.should.eql(expected);
   });
 
   it('should not merge strings', () => {
-    const obj1 = {a: 'fooo'};
-    const obj2 = {a: {b: 2, d: {f: 'f'} }};
-    const obj3 = {a: 'bar'};
+    const obj1: any = {a: 'fooo'};
+    const obj2: any = {a: {b: 2, d: {f: 'f'} }};
+    const obj3: any = {a: 'bar'};
 
-    const result = deepMerge(deepMerge(obj1, obj2), obj3);
+    const result: any = deepMerge(deepMerge(obj1, obj2), obj3);
     result.a.should.eql('bar');
   });
 
@@ -45,10 +43,11 @@ describe('deepMerge()', () => {
   });
 
   it('should not merge an objects into an array', () => {
-    const obj1 = {a: {b: 1}};
-    const obj2 = {a: ['foo', 'bar']};
+    const obj1: any = {a: {b: 1}};
+    const obj2: any = {a: ['foo', 'bar']};
 
-    deepMerge(obj1, obj2).should.eql({a: ['foo', 'bar']});
+    const result: any = deepMerge(obj1, obj2);
+    result.should.eql({a: ['foo', 'bar']});
   });
 
   it('should not affect target & source', () => {
@@ -68,10 +67,10 @@ describe('deepMerge()', () => {
   });
 
   it('should deep clone arrays during merge', () => {
-    const obj1 = {a: [1, 2, [3, 4]]};
-    const obj2 = {b: [5, 6]};
+    const obj1: any = {a: [1, 2, [3, 4]]};
+    const obj2: any = {b: [5, 6]};
 
-    const result = deepMerge(obj1, obj2);
+    const result: any = deepMerge(obj1, obj2);
 
     result.a.should.eql([1, 2, [3, 4]]);
     result.a[2].should.eql([3, 4]);
diff --git a/test/encode_url.spec.js b/test/encode_url.spec.ts
similarity index 96%
rename from test/encode_url.spec.js
rename to test/encode_url.spec.ts
index a8b70b6c..583df516 100644
--- a/test/encode_url.spec.js
+++ b/test/encode_url.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import encodeURL from '../lib/encode_url';
+chai.should();
 
 describe('encodeURL', () => {
-  const encodeURL = require('../dist/encode_url');
-
   it('regular', () => {
     const content = 'http://foo.com/';
     encodeURL(content).should.eql(content);
diff --git a/test/escape_diacritic.spec.js b/test/escape_diacritic.spec.ts
similarity index 70%
rename from test/escape_diacritic.spec.js
rename to test/escape_diacritic.spec.ts
index eaf55073..6f8441c1 100644
--- a/test/escape_diacritic.spec.js
+++ b/test/escape_diacritic.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import escapeDiacritic from '../lib/escape_diacritic';
+chai.should();
 
 describe('escapeDiacritic', () => {
-  const escapeDiacritic = require('../dist/escape_diacritic');
-
   it('default', () => {
     escapeDiacritic('Hell\u00F2 w\u00F2rld').should.eql('Hello world');
   });
diff --git a/test/escape_html.spec.js b/test/escape_html.spec.ts
similarity index 86%
rename from test/escape_html.spec.js
rename to test/escape_html.spec.ts
index cf2835cc..5f4528f5 100644
--- a/test/escape_html.spec.js
+++ b/test/escape_html.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import escapeHTML from '../lib/escape_html';
+chai.should();
 
 describe('escapeHTML', () => {
-  const escapeHTML = require('../dist/escape_html');
-
   it('default', () => {
     escapeHTML('<p class="foo">Hello "world".</p>').should.eql('&lt;p class&#x3D;&quot;foo&quot;&gt;Hello &quot;world&quot;.&lt;&#x2F;p&gt;');
   });
diff --git a/test/escape_regexp.spec.js b/test/escape_regexp.spec.ts
similarity index 69%
rename from test/escape_regexp.spec.js
rename to test/escape_regexp.spec.ts
index b0fe15ff..9ffd44d1 100644
--- a/test/escape_regexp.spec.js
+++ b/test/escape_regexp.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import escapeRegExp from '../lib/escape_regexp';
+chai.should();
 
 describe('escapeRegExp', () => {
-  const escapeRegExp = require('../dist/escape_regexp');
-
   it('default', () => {
     escapeRegExp('hello*world').should.eql('hello\\*world');
   });
diff --git a/test/full_url_for.spec.js b/test/full_url_for.spec.ts
old mode 100755
new mode 100644
similarity index 93%
rename from test/full_url_for.spec.js
rename to test/full_url_for.spec.ts
index 39afa7ac..b7b1f0d8
--- a/test/full_url_for.spec.js
+++ b/test/full_url_for.spec.ts
@@ -1,13 +1,15 @@
-'use strict';
+import chai from 'chai';
+import fullUrlForHelper from '../lib/full_url_for';
+chai.should();
 
 describe('full_url_for', () => {
-  const ctx = {
+  const ctx: any = {
     config: {
       url: 'http://example.com'
     }
   };
 
-  const fullUrlFor = require('../dist/full_url_for').bind(ctx);
+  const fullUrlFor: typeof fullUrlForHelper = fullUrlForHelper.bind(ctx);
 
   it('internal url - root directory', () => {
     ctx.config.url = 'https://example.com';
diff --git a/test/gravatar.spec.js b/test/gravatar.spec.ts
similarity index 84%
rename from test/gravatar.spec.js
rename to test/gravatar.spec.ts
index b350ce2c..99ce5cb3 100644
--- a/test/gravatar.spec.js
+++ b/test/gravatar.spec.ts
@@ -1,10 +1,7 @@
-'use strict';
-
-const { createHash } = require('crypto');
+import { createHash } from 'crypto';
+import gravatar from '../lib/gravatar';
 
 describe('gravatar', () => {
-  const gravatar = require('../dist/gravatar');
-
   function md5(str) {
     return createHash('md5').update(str).digest('hex');
   }
diff --git a/test/hash.spec.js b/test/hash.spec.ts
similarity index 67%
rename from test/hash.spec.js
rename to test/hash.spec.ts
index f1b50983..f4b0b6ce 100644
--- a/test/hash.spec.js
+++ b/test/hash.spec.ts
@@ -1,25 +1,21 @@
-'use strict';
-
-require('chai').should();
-const crypto = require('crypto');
+import { createHash } from 'crypto';
+import { hash, createSha1Hash } from '../lib/hash';
 
 function sha1(content) {
-  const hash = crypto.createHash('sha1');
+  const hash = createHash('sha1');
   hash.update(content);
 
   return hash.digest();
 }
 
 describe('hash', () => {
-  const hash = require('../dist/hash');
-
   it('hash', () => {
     const content = '123456';
-    hash.hash(content).should.eql(sha1(content));
+    hash(content).should.eql(sha1(content));
   });
 
   it('createSha1Hash', () => {
-    const _sha1 = hash.createSha1Hash();
+    const _sha1 = createSha1Hash();
     const content = '123456';
     _sha1.update(content);
     _sha1.digest().should.eql(sha1(content));
@@ -28,7 +24,7 @@ describe('hash', () => {
   it('createSha1Hash - streamMode', () => {
     const content1 = '123456';
     const content2 = '654321';
-    const stream = hash.createSha1Hash();
+    const stream = createSha1Hash();
     // explicit convert
     stream.write(Buffer.from(content1));
     // implicit convert
diff --git a/test/highlight.spec.js b/test/highlight.spec.ts
similarity index 96%
rename from test/highlight.spec.js
rename to test/highlight.spec.ts
index 0dec8a04..0d4ddf00 100644
--- a/test/highlight.spec.js
+++ b/test/highlight.spec.ts
@@ -1,8 +1,9 @@
-'use strict';
+import chai from 'chai';
+import hljs from 'highlight.js';
+import { encode } from 'html-entities';
+import highlight from '../lib/highlight';
 
-const should = require('chai').should(); // eslint-disable-line
-const hljs = require('highlight.js');
-const entities = require('html-entities');
+const should = chai.should(); // eslint-disable-line
 const validator = require('html-tag-validator');
 
 const testJson = {
@@ -33,7 +34,7 @@ function gutter(start, end) {
   return result;
 }
 
-function code(str, lang) {
+function code(str, lang?) {
   let data;
 
   if (lang) {
@@ -43,7 +44,7 @@ function code(str, lang) {
   } else if (lang === null) {
     data = {value: str};
   } else {
-    data = {value: entities.encode(str)};
+    data = {value: encode(str)};
   }
 
   const lines = data.value.split('\n');
@@ -68,8 +69,6 @@ function validateHtmlAsync(str, done) {
 }
 
 describe('highlight', () => {
-  const highlight = require('../dist/highlight');
-
   it('default', done => {
     const result = highlight(testString);
     assertResult(result, gutter(1, 4), code(testString));
@@ -96,7 +95,7 @@ describe('highlight', () => {
     const result = highlight(testString, {gutter: false, wrap: false});
     result.should.eql([
       '<pre><code class="highlight plaintext">',
-      entities.encode(testString),
+      encode(testString),
       '</code></pre>'
     ].join(''));
     validateHtmlAsync(result, done);
@@ -106,7 +105,7 @@ describe('highlight', () => {
     const result = highlight(testString, {gutter: false, wrap: false, hljs: true});
     result.should.eql([
       '<pre><code class="hljs plaintext">',
-      entities.encode(testString),
+      encode(testString),
       '</code></pre>'
     ].join(''));
     validateHtmlAsync(result, done);
@@ -139,7 +138,7 @@ describe('highlight', () => {
   });
 
   it('wrap: false (with mark)', done => {
-    const result = highlight(testString, {gutter: false, wrap: false, hljs: true, lang: 'json', mark: '1'});
+    const result = highlight(testString, {gutter: false, wrap: false, hljs: true, lang: 'json', mark: [1]});
     hljs.configure({classPrefix: 'hljs-'});
     result.should.eql([
       '<pre><code class="hljs json">',
@@ -229,7 +228,7 @@ describe('highlight', () => {
       '<pre>',
       `<div class="caption">${caption}</div>`,
       '<code class="highlight plaintext">',
-      entities.encode(testString),
+      encode(testString),
       '</code></pre>'
     ].join(''));
     validateHtmlAsync(result, done);
@@ -453,7 +452,7 @@ describe('highlight', () => {
     console.log(result);
     result.should.eql([
       '<pre><code class="highlight plaintext" data-language="plaintext">',
-      entities.encode(testString),
+      encode(testString),
       '</code></pre>'
     ].join(''));
     validateHtmlAsync(result, done);
diff --git a/test/html_tag.spec.js b/test/html_tag.spec.ts
similarity index 96%
rename from test/html_tag.spec.js
rename to test/html_tag.spec.ts
index ef414a10..5649dbd8 100644
--- a/test/html_tag.spec.js
+++ b/test/html_tag.spec.ts
@@ -1,11 +1,9 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import htmlTag from '../lib/html_tag';
+import encodeURL from '../lib/encode_url';
+chai.should();
 
 describe('htmlTag', () => {
-  const htmlTag = require('../dist/html_tag');
-  const encodeURL = require('../dist/encode_url');
-
   it('tag', () => {
     htmlTag('hr').should.eql('<hr>');
   });
@@ -58,6 +56,7 @@ describe('htmlTag', () => {
 
   it('tag is required', () => {
     try {
+      // @ts-ignore
       htmlTag();
     } catch (err) {
       err.should.have.property('message', 'tag is required!');
diff --git a/test/is_external_link.spec.js b/test/is_external_link.spec.ts
similarity index 96%
rename from test/is_external_link.spec.js
rename to test/is_external_link.spec.ts
index 2b06821a..b9ca0843 100644
--- a/test/is_external_link.spec.js
+++ b/test/is_external_link.spec.ts
@@ -1,14 +1,12 @@
-'use strict';
+import isExternalLink from '../lib/is_external_link';
 
 describe('isExternalLink', () => {
-  const ctx = {
+  const ctx: any = {
     config: {
       url: 'https://example.com'
     }
   };
 
-  const isExternalLink = require('../dist/is_external_link');
-
   it('invalid url', () => {
     isExternalLink('https://localhost:4000你好', ctx.config.url).should.eql(false);
   });
diff --git a/test/pattern.spec.js b/test/pattern.spec.ts
similarity index 94%
rename from test/pattern.spec.js
rename to test/pattern.spec.ts
index 75fa3d5a..6c779830 100644
--- a/test/pattern.spec.js
+++ b/test/pattern.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import Pattern from '../lib/pattern';
+chai.should();
 
 describe('Pattern', () => {
-  const Pattern = require('../dist/pattern');
-
   it('String - posts/:id', () => {
     const pattern = new Pattern('posts/:id');
     const result = pattern.match('/posts/89');
@@ -66,6 +64,7 @@ describe('Pattern', () => {
 
   it('rule is required', () => {
     (() => {
+      // @ts-ignore
       // eslint-disable-next-line no-new
       new Pattern();
     }).should.throw('rule must be a function, a string or a regular expression.');
diff --git a/test/permalink.spec.js b/test/permalink.spec.ts
similarity index 96%
rename from test/permalink.spec.js
rename to test/permalink.spec.ts
index 4eb47b42..a73ddb8f 100644
--- a/test/permalink.spec.js
+++ b/test/permalink.spec.ts
@@ -1,9 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import Permalink from '../lib/permalink';
+chai.should();
 
 describe('Permalink', () => {
-  const Permalink = require('../dist/permalink');
   let permalink;
 
   it('constructor', () => {
@@ -46,6 +45,7 @@ describe('Permalink', () => {
 
   it('rule is required', () => {
     try {
+      // @ts-ignore
       // eslint-disable-next-line no-new
       new Permalink();
     } catch (err) {
diff --git a/test/pretty_utls.spec.js b/test/pretty_utls.spec.ts
similarity index 93%
rename from test/pretty_utls.spec.js
rename to test/pretty_utls.spec.ts
index d56ec6a4..01b259e8 100644
--- a/test/pretty_utls.spec.js
+++ b/test/pretty_utls.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import prettyUrls from '../lib/pretty_urls';
+chai.should();
 
 describe('prettyUrls', () => {
-  const prettyUrls = require('../dist/pretty_urls');
-
   it('default', () => {
     prettyUrls('//example.com/index.html').should.eql('//example.com/index.html');
     prettyUrls('/bar/foo.html').should.eql('/bar/foo.html');
diff --git a/test/prism.spec.js b/test/prism.spec.ts
similarity index 98%
rename from test/prism.spec.js
rename to test/prism.spec.ts
index a76a777d..d98ed197 100644
--- a/test/prism.spec.js
+++ b/test/prism.spec.ts
@@ -1,9 +1,9 @@
-'use strict';
-
-require('chai').should();
-const escapeHTML = require('../dist/escape_html');
-const stripIndent = require('strip-indent');
+import chai from 'chai';
+import escapeHTML from '../lib/escape_html';
+import prismHighlight from '../lib/prism';
+import stripIndent from 'strip-indent';
 
+chai.should();
 const validator = require('html-tag-validator');
 function validateHtmlAsync(str, done) {
   validator(str, {
@@ -27,8 +27,6 @@ const lineNumberStartTag = '<span aria-hidden="true" class="line-numbers-rows">'
 const highlightToken = '<span class="token ';
 
 describe('prismHighlight', () => {
-  const prismHighlight = require('../dist/prism');
-
   it('default (plain text)', done => {
     const input = `
     {
diff --git a/test/relative_url.spec.js b/test/relative_url.spec.ts
similarity index 94%
rename from test/relative_url.spec.js
rename to test/relative_url.spec.ts
index 86513963..08824704 100644
--- a/test/relative_url.spec.js
+++ b/test/relative_url.spec.ts
@@ -1,8 +1,6 @@
-'use strict';
+import relativeURL from '../lib/relative_url';
 
 describe('relative_url', () => {
-  const relativeURL = require('../dist/relative_url');
-
   it('from root', () => {
     relativeURL('', 'css/style.css').should.eql('css/style.css');
     relativeURL('index.html', 'css/style.css').should.eql('css/style.css');
diff --git a/test/slugize.spec.js b/test/slugize.spec.ts
similarity index 92%
rename from test/slugize.spec.js
rename to test/slugize.spec.ts
index 5b36315a..a6e829cc 100644
--- a/test/slugize.spec.js
+++ b/test/slugize.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import slugize from '../lib/slugize';
+chai.should();
 
 describe('slugize', () => {
-  const slugize = require('../dist/slugize');
-
   it('spaces', () => {
     slugize('Hello World').should.eql('Hello-World');
   });
diff --git a/test/spawn.spec.js b/test/spawn.spec.js
deleted file mode 100644
index 0a6476b9..00000000
--- a/test/spawn.spec.js
+++ /dev/null
@@ -1,125 +0,0 @@
-'use strict';
-
-require('chai').use(require('chai-as-promised')).should();
-const { join } = require('path');
-const { writeFile, unlink } = require('fs');
-const rewire = require('rewire');
-
-const isWindows = process.platform === 'win32';
-const catCommand = isWindows ? 'type' : 'cat';
-
-describe('spawn', () => {
-  const spawn = require('../dist/spawn');
-  const CacheStream = require('../dist/cache_stream');
-  const fixturePath = join(__dirname, 'spawn_test.txt');
-  const fixture = 'test content';
-
-  before(done => {
-    writeFile(fixturePath, fixture, done);
-  });
-
-  after(done => {
-    unlink(fixturePath, done);
-  });
-
-  it('default', () => spawn(catCommand, [fixturePath]).should.become(fixture));
-
-  it('default - string', () => spawn(catCommand, fixturePath).should.become(fixture));
-
-  it('default - empty argument and options', async () => {
-    if (isWindows) {
-      const out = await spawn('ver');
-      out.trim().startsWith('Microsoft Windows').should.eql(true);
-    } else {
-      const out = await spawn('uname');
-      ['Linux', 'Darwin'].includes(out.trim()).should.eql(true);
-    }
-  });
-
-  it('default - options and empty argument', async () => {
-    if (isWindows) {
-      const out = await spawn('chdir', { cwd: __dirname });
-      out.trim().should.eql(__dirname);
-    } else {
-      const out = await spawn('pwd', { cwd: __dirname });
-      out.trim().should.eql(__dirname);
-    }
-  });
-
-  it('command is required', () => {
-    spawn.should.throw('command is required!');
-  });
-
-  it('error', () => {
-    const promise = spawn(catCommand, ['nothing']);
-    if (isWindows) {
-      return promise.should.rejectedWith('spawn type ENOENT').and.eventually.have.property('code', 'ENOENT');
-    }
-    return promise.should.rejectedWith('cat: nothing: No such file or directory').and.eventually.have.property('code', 1);
-  });
-
-  it('verbose - stdout', () => {
-    const spawn = rewire('../dist/spawn');
-    const stdoutCache = new CacheStream();
-    const stderrCache = new CacheStream();
-    const content = 'something';
-
-    spawn.__set__('process', Object.assign({}, process, {
-      stdout: stdoutCache,
-      stderr: stderrCache
-    }));
-
-    return spawn('echo', [content], {
-      verbose: true
-    }).then(() => {
-      const result = stdoutCache.getCache().toString('utf8').trim();
-      if (isWindows) {
-        result.should.match(new RegExp(`^(["']?)${content}\\1$`));
-      } else {
-        result.should.eql(content);
-      }
-    });
-  });
-
-  it('verbose - stderr', () => {
-    const spawn = rewire('../dist/spawn');
-    const stdoutCache = new CacheStream();
-    const stderrCache = new CacheStream();
-
-    spawn.__set__('process', Object.assign({}, process, {
-      stdout: stdoutCache,
-      stderr: stderrCache,
-      removeListener: () => {},
-      on: () => {}
-    }));
-
-    return spawn(catCommand, ['nothing'], {
-      verbose: true
-    }).should.rejected.then(() => {
-      const stderrResult = stderrCache.getCache();
-      if (isWindows) {
-        // utf8 support in windows shell (cmd.exe) is difficult.
-        Buffer.byteLength(stderrResult, 'hex').should.least(1);
-      } else {
-        stderrResult.toString('utf8').should.with.match(/^cat: nothing: No such file or directory\n?$/);
-      }
-    });
-  });
-
-  it('custom encoding', () => {
-    return spawn(catCommand, [fixturePath], {encoding: 'hex'}).should.become(Buffer.from(fixture).toString('hex'));
-  });
-
-  it('encoding = null', () => {
-    return spawn(catCommand, [fixturePath], {encoding: null}).should.become(Buffer.from(fixture));
-  });
-
-  it('stdio = inherit', () => spawn('echo', ['something'], { stdio: 'inherit' }));
-
-  it('exit with code', () => {
-    if (isWindows) {
-      return spawn('cmd.exe', ['/c', 'exit', '1'], { stdio: 'inherit' }).should.rejectedWith('Spawn failed');
-    }
-    return spawn('sh', ['/c', 'exit', '1'], { stdio: 'inherit' }).should.rejectedWith('Spawn failed');
-  });
-});
diff --git a/test/spawn.spec.ts b/test/spawn.spec.ts
new file mode 100644
index 00000000..0019eda5
--- /dev/null
+++ b/test/spawn.spec.ts
@@ -0,0 +1,154 @@
+import chai from 'chai';
+import { join } from 'path';
+import { writeFile, unlink } from 'fs';
+import rewire from 'rewire';
+import spawn from '../lib/spawn';
+import CacheStream from '../lib/cache_stream';
+
+chai.should();
+
+const isWindows = process.platform === 'win32';
+const catCommand = isWindows ? 'type' : 'cat';
+
+describe('spawn', () => {
+  const fixturePath = join(__dirname, 'spawn_test.txt');
+  const fixture = 'test content';
+
+  before(done => {
+    writeFile(fixturePath, fixture, done);
+  });
+
+  after(done => {
+    unlink(fixturePath, done);
+  });
+
+  it('default', async () => {
+    const out = await spawn(catCommand, [fixturePath]) as string;
+    out.should.eql(fixture);
+  });
+
+  it('default - string', async () => {
+    const out = await spawn(catCommand, fixturePath) as string;
+    out.should.eql(fixture);
+  });
+
+  it('default - empty argument and options', async () => {
+    if (isWindows) {
+      const out = await spawn('ver') as string;
+      out.trim().startsWith('Microsoft Windows').should.eql(true);
+    } else {
+      const out = await spawn('uname') as string;
+      ['Linux', 'Darwin'].includes(out.trim()).should.eql(true);
+    }
+  });
+
+  it('default - options and empty argument', async () => {
+    if (isWindows) {
+      const out = await spawn('chdir', { cwd: __dirname }) as string;
+      out.trim().should.eql(__dirname);
+    } else {
+      const out = await spawn('pwd', { cwd: __dirname }) as string;
+      out.trim().should.eql(__dirname);
+    }
+  });
+
+  it('command is required', () => {
+    spawn.should.throw('command is required!');
+  });
+
+  it('error', async () => {
+    try {
+      await spawn(catCommand, ['nothing']);
+      // should not reach here
+      true.should.eql(false);
+    } catch (e) {
+      if (isWindows) {
+        e.message.should.eql('spawn type ENOENT');
+        e.should.have.property('code', 'ENOENT');
+      } else {
+        e.message.should.with.match(/^cat: nothing: No such file or directory\n?$/);
+        e.should.have.property('code', 1);
+      }
+    }
+  });
+
+  it('verbose - stdout', () => {
+    const _spawn = rewire<typeof spawn>('../dist/spawn');
+    const stdoutCache = new CacheStream();
+    const stderrCache = new CacheStream();
+    const content = 'something';
+
+    _spawn.__set__('process', Object.assign({}, process, {
+      stdout: stdoutCache,
+      stderr: stderrCache
+    }));
+
+    return _spawn('echo', [content], {
+      verbose: true
+    }).then(() => {
+      const result = stdoutCache.getCache().toString('utf8').trim();
+      if (isWindows) {
+        result.should.match(new RegExp(`^(["']?)${content}\\1$`));
+      } else {
+        result.should.eql(content);
+      }
+    });
+  });
+
+  it('verbose - stderr', () => {
+    const _spawn = rewire<typeof spawn>('../dist/spawn');
+    const stdoutCache = new CacheStream();
+    const stderrCache = new CacheStream();
+
+    _spawn.__set__('process', Object.assign({}, process, {
+      stdout: stdoutCache,
+      stderr: stderrCache,
+      removeListener: () => {},
+      on: () => {}
+    }));
+
+    return _spawn(catCommand, ['nothing'], {
+      verbose: true
+    }).then(() => {
+      // should not reach here
+      true.should.eql(false);
+    }, e => {
+      return e;
+    }).then(() => {
+      const stderrResult = stderrCache.getCache();
+      if (isWindows) {
+        // utf8 support in windows shell (cmd.exe) is difficult.
+        Buffer.byteLength(stderrResult, 'hex').should.least(1);
+      } else {
+        stderrResult.toString('utf8').should.with.match(/^cat: nothing: No such file or directory\n?$/);
+      }
+    });
+  });
+
+  it('custom encoding', async () => {
+    const out = await spawn(catCommand, [fixturePath], {encoding: 'hex'}) as Buffer;
+    out.should.eql(Buffer.from(fixture).toString('hex'));
+  });
+
+  it('encoding = null', async () => {
+    // @ts-ignore
+    const out = await spawn(catCommand, [fixturePath], {encoding: null}) as Buffer;
+    out.should.eql(Buffer.from(fixture));
+  });
+
+  it('stdio = inherit', () => spawn('echo', ['something'], { stdio: 'inherit' }));
+
+  it('exit with code', async () => {
+    try {
+      if (isWindows) {
+        await spawn('cmd.exe', ['/c', 'exit', '1'], { stdio: 'inherit' });
+      } else {
+        await spawn('sh', ['/c', 'exit', '1'], { stdio: 'inherit' });
+      }
+      // should not reach here
+      true.should.eql(false);
+    } catch (error) {
+      error.message.should.equal('Spawn failed');
+    }
+  });
+});
diff --git a/test/strip_html.spec.js b/test/strip_html.spec.ts
similarity index 95%
rename from test/strip_html.spec.js
rename to test/strip_html.spec.ts
index a57786f5..9495a3d1 100644
--- a/test/strip_html.spec.js
+++ b/test/strip_html.spec.ts
@@ -1,6 +1,4 @@
-'use strict';
-
-const stripHTML = require('../dist/strip_html');
+import stripHTML from '../lib/strip_html';
 
 describe('stripHTML', () => {
   it('should not strip invalid tags', () => {
@@ -55,6 +53,7 @@ describe('stripHTML', () => {
     const html = ['X'];
     const text = '';
 
+    // @ts-ignore
     stripHTML(html).should.eql(text);
   });
 });
diff --git a/test/strip_indent.spec.js b/test/strip_indent.spec.ts
similarity index 70%
rename from test/strip_indent.spec.js
rename to test/strip_indent.spec.ts
index 90d3065b..fd929ec2 100644
--- a/test/strip_indent.spec.js
+++ b/test/strip_indent.spec.ts
@@ -1,6 +1,4 @@
-'use strict';
-
-const stripIndent = require('../dist/strip_indent');
+import stripIndent from '../lib/strip_indent';
 
 describe('stripIndent', () => {
   it('default', () => {
diff --git a/test/toc_obj.spec.js b/test/toc_obj.spec.ts
similarity index 98%
rename from test/toc_obj.spec.js
rename to test/toc_obj.spec.ts
index d8caba52..40ad65c1 100644
--- a/test/toc_obj.spec.js
+++ b/test/toc_obj.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import tocObj from '../lib/toc_obj';
+chai.should();
 
 describe('tocObj', () => {
-  const tocObj = require('../dist/toc_obj');
-
   const html = [
     '<h1 id="title_1">Title 1</h1>',
     '<div id="title_1_1"><h2>Title 1.1</h2></div>',
diff --git a/test/truncate.spec.js b/test/truncate.spec.ts
similarity index 90%
rename from test/truncate.spec.js
rename to test/truncate.spec.ts
index 5f51cfd9..f180ee3f 100644
--- a/test/truncate.spec.js
+++ b/test/truncate.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import truncate from '../lib/truncate';
+chai.should();
 
 describe('truncate', () => {
-  const truncate = require('../dist/truncate');
-
   it('default', () => {
     truncate('Once upon a time in a world far far away')
       .should.eql('Once upon a time in a world...');
diff --git a/test/unescape_html.spec.js b/test/unescape_html.spec.ts
similarity index 75%
rename from test/unescape_html.spec.js
rename to test/unescape_html.spec.ts
index d4b965dd..16fd3175 100644
--- a/test/unescape_html.spec.js
+++ b/test/unescape_html.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import unescapeHTML from '../lib/unescape_html';
+chai.should();
 
 describe('unescapeHTML', () => {
-  const unescapeHTML = require('../dist/unescape_html');
-
   it('default', () => {
     unescapeHTML('&lt;p class&#x3D;&quot;foo&quot;&gt;Hello &quot;world&quot;.&lt;&#x2F;p&gt;').should.eql('<p class="foo">Hello "world".</p>');
   });
diff --git a/test/url_for.spec.js b/test/url_for.spec.ts
similarity index 97%
rename from test/url_for.spec.js
rename to test/url_for.spec.ts
index c3f3e707..da2e6a27 100644
--- a/test/url_for.spec.js
+++ b/test/url_for.spec.ts
@@ -1,13 +1,13 @@
-'use strict';
+import urlForHelper from '../lib/url_for';
 
 describe('url_for', () => {
-  const ctx = {
+  const ctx: any = {
     config: {
       url: 'http://example.com'
     }
   };
 
-  const urlFor = require('../dist/url_for').bind(ctx);
+  const urlFor: typeof urlForHelper = urlForHelper.bind(ctx);
 
   it('should encode path', () => {
     ctx.config.root = '/';
diff --git a/test/word_wrap.spec.js b/test/word_wrap.spec.ts
similarity index 90%
rename from test/word_wrap.spec.js
rename to test/word_wrap.spec.ts
index a6d9fee8..d039cda1 100644
--- a/test/word_wrap.spec.js
+++ b/test/word_wrap.spec.ts
@@ -1,10 +1,8 @@
-'use strict';
-
-require('chai').should();
+import chai from 'chai';
+import wordWrap from '../lib/word_wrap';
+chai.should();
 
 describe('wordWrap', () => {
-  const wordWrap = require('../dist/word_wrap');
-
   it('default', () => {
     wordWrap('Once upon a time').should.eql('Once upon a time');
   });
diff --git a/tsconfig.json b/tsconfig.json
index 0c39663c..995b4c6f 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -6,7 +6,7 @@
     "outDir": "dist",
     "declaration": true,
     "esModuleInterop": true,
-    "types": ["node"],
+    "types": ["node", "mocha"],
     "lib": ["ES2020", "ES2019"]
   },
   "include": ["lib/**.ts"],