diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 8720f41b..437f4ec3 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -29,6 +29,7 @@ jobs:
             ${{ runner.os }}-node-${{ env.NODE }}-${{ hashFiles('package-lock.json') }}
       - run: npm ci
       - run: npm run lint
+      - run: npm run typecheck
   test:
     name: Node.js ${{ matrix.node-version }}
     runs-on: ubuntu-latest
diff --git a/lib/css-tools.js b/lib/css-tools.js
index 715f696c..ba8708bc 100644
--- a/lib/css-tools.js
+++ b/lib/css-tools.js
@@ -8,7 +8,7 @@ var csstree = require('css-tree'),
 /**
  * Flatten a CSS AST to a selectors list.
  *
- * @param {Object} cssAst css-tree AST to flatten
+ * @param {import('css-tree').CssNode} cssAst css-tree AST to flatten
  * @return {Array} selectors
  */
 function flattenToSelectors(cssAst) {
@@ -29,7 +29,7 @@ function flattenToSelectors(cssAst) {
           item: selectorItem,
           atrule: atrule,
           rule: rule,
-          pseudos: [],
+          pseudos: /** @type {{item: any; list: any[]}[]} */ ([]),
         };
 
         selectorNode.children.each(function (
@@ -108,7 +108,7 @@ function filterByPseudos(selectors, usePseudos) {
  * Remove pseudo-elements and/or -classes from the selectors for proper matching.
  *
  * @param {Array} selectors to clean
- * @return {Array} Selectors without pseudo-elements and/or -classes
+ * @return {void}
  */
 function cleanPseudos(selectors) {
   selectors.forEach(function (selector) {
@@ -124,7 +124,7 @@ function cleanPseudos(selectors) {
  *
  * @param {Array} aSpecificity Specificity of selector A
  * @param {Array} bSpecificity Specificity of selector B
- * @return {Number} Score of selector specificity A compared to selector specificity B
+ * @return {number} Score of selector specificity A compared to selector specificity B
  */
 function compareSpecificity(aSpecificity, bSpecificity) {
   for (var i = 0; i < 4; i += 1) {
@@ -143,7 +143,7 @@ function compareSpecificity(aSpecificity, bSpecificity) {
  *
  * @param {Object} aSimpleSelectorNode Simple selector A
  * @param {Object} bSimpleSelectorNode Simple selector B
- * @return {Number} Score of selector A compared to selector B
+ * @return {number} Score of selector A compared to selector B
  */
 function compareSimpleSelectorNode(aSimpleSelectorNode, bSimpleSelectorNode) {
   var aSpecificity = specificity(aSimpleSelectorNode),
@@ -168,7 +168,7 @@ function sortSelectors(selectors) {
 /**
  * Convert a css-tree AST style declaration to CSSStyleDeclaration property.
  *
- * @param {Object} declaration css-tree style declaration
+ * @param {import('css-tree').CssNode} declaration css-tree style declaration
  * @return {Object} CSSStyleDeclaration property
  */
 function csstreeToStyleDeclaration(declaration) {
@@ -185,8 +185,8 @@ function csstreeToStyleDeclaration(declaration) {
 /**
  * Gets the CSS string of a style element
  *
- * @param {Object} element style element
- * @return {String|Array} CSS string or empty array if no styles are set
+ * @param {Object} elem style element
+ * @return {string|Array} CSS string or empty array if no styles are set
  */
 function getCssStr(elem) {
   return elem.content[0].text || elem.content[0].cdata || [];
@@ -195,8 +195,8 @@ function getCssStr(elem) {
 /**
  * Sets the CSS string of a style element
  *
- * @param {Object} element style element
- * @param {String} CSS string to be set
+ * @param {Object} elem style element
+ * @param {string} css string to be set
  * @return {Object} reference to field with CSS
  */
 function setCssStr(elem, css) {
diff --git a/lib/svgo/css-style-declaration.js b/lib/svgo/css-style-declaration.js
index 6d07952d..3bffcf9c 100644
--- a/lib/svgo/css-style-declaration.js
+++ b/lib/svgo/css-style-declaration.js
@@ -139,7 +139,7 @@ CSSStyleDeclaration.prototype._loadCssText = function () {
 /**
  * Get the textual representation of the declaration block (equivalent to .cssText attribute).
  *
- * @return {String} Textual representation of the declaration block (empty string for no properties)
+ * @return {string} Textual representation of the declaration block (empty string for no properties)
  */
 CSSStyleDeclaration.prototype.getCssText = function () {
   var properties = this.getProperties();
@@ -183,8 +183,8 @@ CSSStyleDeclaration.prototype._getProperty = function (propertyName) {
 /**
  * Return the optional priority, "important".
  *
- * @param {String} propertyName representing the property name to be checked.
- * @return {String} priority that represents the priority (e.g. "important") if one exists. If none exists, returns the empty string.
+ * @param {string} propertyName representing the property name to be checked.
+ * @return {string} priority that represents the priority (e.g. "important") if one exists. If none exists, returns the empty string.
  */
 CSSStyleDeclaration.prototype.getPropertyPriority = function (propertyName) {
   var property = this._getProperty(propertyName);
@@ -194,8 +194,8 @@ CSSStyleDeclaration.prototype.getPropertyPriority = function (propertyName) {
 /**
  * Return the property value given a property name.
  *
- * @param {String} propertyName representing the property name to be checked.
- * @return {String} value containing the value of the property. If not set, returns the empty string.
+ * @param {string} propertyName representing the property name to be checked.
+ * @return {string} value containing the value of the property. If not set, returns the empty string.
  */
 CSSStyleDeclaration.prototype.getPropertyValue = function (propertyName) {
   var property = this._getProperty(propertyName);
@@ -205,8 +205,8 @@ CSSStyleDeclaration.prototype.getPropertyValue = function (propertyName) {
 /**
  * Return a property name.
  *
- * @param {Number} index of the node to be fetched. The index is zero-based.
- * @return {String} propertyName that is the name of the CSS property at the specified index.
+ * @param {number} index of the node to be fetched. The index is zero-based.
+ * @return {string} propertyName that is the name of the CSS property at the specified index.
  */
 CSSStyleDeclaration.prototype.item = function (index) {
   if (typeof index === 'undefined') {
@@ -234,8 +234,8 @@ CSSStyleDeclaration.prototype.getProperties = function () {
 /**
  * Remove a property from the CSS declaration block.
  *
- * @param {String} propertyName representing the property name to be removed.
- * @return {String} oldValue equal to the value of the CSS property before it was removed.
+ * @param {string} propertyName representing the property name to be removed.
+ * @return {string} oldValue equal to the value of the CSS property before it was removed.
  */
 CSSStyleDeclaration.prototype.removeProperty = function (propertyName) {
   if (typeof propertyName === 'undefined') {
@@ -255,10 +255,10 @@ CSSStyleDeclaration.prototype.removeProperty = function (propertyName) {
 /**
  * Modify an existing CSS property or creates a new CSS property in the declaration block.
  *
- * @param {String} propertyName representing the CSS property name to be modified.
- * @param {String} [value] containing the new property value. If not specified, treated as the empty string. value must not contain "!important" -- that should be set using the priority parameter.
- * @param {String} [priority] allowing the "important" CSS priority to be set. If not specified, treated as the empty string.
- * @return {undefined}
+ * @param {string} propertyName representing the CSS property name to be modified.
+ * @param {string} value containing the new property value. If not specified, treated as the empty string. value must not contain "!important" -- that should be set using the priority parameter.
+ * @param {string} priority allowing the "important" CSS priority to be set. If not specified, treated as the empty string.
+ * @return {{value: string, priority: string}}
  */
 CSSStyleDeclaration.prototype.setProperty = function (
   propertyName,
diff --git a/lib/svgo/plugins.js b/lib/svgo/plugins.js
index b4b32e06..f99e7fe9 100644
--- a/lib/svgo/plugins.js
+++ b/lib/svgo/plugins.js
@@ -7,7 +7,7 @@
  *
  * @param {Object} data input data
  * @param {Object} info extra information
- * @param {Object} plugins plugins object from config
+ * @param {Array} plugins plugins object from config
  * @return {Object} output data
  */
 module.exports = function (data, info, plugins) {
@@ -45,7 +45,7 @@ module.exports = function (data, info, plugins) {
  * @param {Object} data input data
  * @param {Object} info extra information
  * @param {Array} plugins plugins list to process
- * @param {Boolean} [reverse] reverse pass?
+ * @param {boolean} [reverse] reverse pass?
  * @return {Object} output data
  */
 function perItem(data, info, plugins, reverse) {
diff --git a/lib/svgo/tools.js b/lib/svgo/tools.js
index ec183209..f52e9c81 100644
--- a/lib/svgo/tools.js
+++ b/lib/svgo/tools.js
@@ -3,9 +3,9 @@
 /**
  * Encode plain SVG data string into Data URI string.
  *
- * @param {String} str input string
- * @param {String} type Data URI type
- * @return {String} output string
+ * @param {string} str input string
+ * @param {string} type Data URI type
+ * @return {string} output string
  */
 exports.encodeSVGDatauri = function (str, type) {
   var prefix = 'data:image/svg+xml';
@@ -27,7 +27,7 @@ exports.encodeSVGDatauri = function (str, type) {
  * Decode SVG Data URI string into plain SVG string.
  *
  * @param {string} str input string
- * @return {String} output string
+ * @return {string} output string
  */
 exports.decodeSVGDatauri = function (str) {
   var regexp = /data:image\/svg\+xml(;charset=[^;,]*)?(;base64)?,(.*)/;
@@ -51,6 +51,10 @@ exports.decodeSVGDatauri = function (str) {
   return str;
 };
 
+/**
+ * @param {any[]} a
+ * @param {any[]} b
+ */
 exports.intersectArrays = function (a, b) {
   return a.filter(function (n) {
     return b.indexOf(n) > -1;
@@ -65,7 +69,7 @@ exports.intersectArrays = function (a, b) {
  *
  * @param {number[]} data
  * @param {Object} params
- * @param {string?} command path data instruction
+ * @param {string} [command] path data instruction
  * @return {string}
  */
 exports.cleanupOutData = function (data, params, command) {
@@ -90,22 +94,22 @@ exports.cleanupOutData = function (data, params, command) {
     // remove floating-point numbers leading zeros
     // 0.5 → .5
     // -0.5 → -.5
-    if (params.leadingZero) {
-      item = removeLeadingZero(item);
-    }
+    const itemStr = params.leadingZero
+      ? removeLeadingZero(item)
+      : item.toString();
 
     // no extra space in front of negative number or
     // in front of a floating number if a previous number is floating too
     if (
       params.negativeExtraSpace &&
       delimiter != '' &&
-      (item < 0 || (String(item).charCodeAt(0) == 46 && prev % 1 !== 0))
+      (item < 0 || (itemStr.charCodeAt(0) == 46 && prev % 1 !== 0))
     ) {
       delimiter = '';
     }
     // save prev item value
     prev = item;
-    str += delimiter + item;
+    str += delimiter + itemStr;
   });
   return str;
 };
@@ -119,9 +123,9 @@ exports.cleanupOutData = function (data, params, command) {
  * @example
  * -0.5 → -.5
  *
- * @param {Float} num input number
+ * @param {number} num input number
  *
- * @return {String} output number as string
+ * @return {string} output number as string
  */
 var removeLeadingZero = function (num) {
   var strNum = num.toString();
diff --git a/package-lock.json b/package-lock.json
index cd75a172..16f97769 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -2337,6 +2337,12 @@
       "integrity": "sha512-4dbzIzqvjtgiM5rw1k5rEHtBANKmdudhGyBEajN01fEyhaAIhsoKNy6y7+IN93IfpFtwY9iqi7kD+xwKhQsNJA==",
       "dev": true
     },
+    "typescript": {
+      "version": "4.2.2",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.2.2.tgz",
+      "integrity": "sha512-tbb+NVrLfnsJy3M59lsDgrzWIflR4d4TIUjz+heUnHZwdF7YsrMTKoRERiIvI2lvBG95dfpLxB21WZhys1bgaQ==",
+      "dev": true
+    },
     "uri-js": {
       "version": "4.4.1",
       "resolved": "https://registry.npmjs.org/uri-js/-/uri-js-4.4.1.tgz",
diff --git a/package.json b/package.json
index ce218867..a1bf522d 100644
--- a/package.json
+++ b/package.json
@@ -51,6 +51,7 @@
   "scripts": {
     "test": "c8 --reporter=html --reporter=text mocha \"test/*/_index.js\" \"**/*.test.js\" --ignore=\"node_modules/**\"",
     "lint": "eslint .",
+    "typecheck": "tsc",
     "test-browser": "rollup -c && node ./test/browser.js",
     "prepublishOnly": "rm -rf dist && rollup -c"
   },
@@ -113,7 +114,8 @@
     "mock-stdin": "^1.0.0",
     "playwright": "^1.8.1",
     "prettier": "^2.2.1",
-    "rollup": "^2.39.0"
+    "rollup": "^2.39.0",
+    "typescript": "^4.2.2"
   },
   "engines": {
     "node": ">=10.13.0"
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 00000000..72e9c13c
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,21 @@
+{
+  "compilerOptions": {
+    "noEmit": true,
+    "module": "commonjs",
+    "target": "es2018",
+    "lib": ["es2018"],
+    "allowJs": true,
+    "checkJs": true,
+    "strict": true,
+    "resolveJsonModule": true,
+    "noImplicitAny": false
+  },
+  "include": [
+    "lib/css-tools.js",
+    "lib/svgo/css-class-list.js",
+    "lib/svgo/css-select-adapter.js",
+    "lib/svgo/css-style-declaration.js",
+    "lib/svgo/plugins.js",
+    "lib/svgo/tools.js"
+  ]
+}