diff --git a/lib/argument.js b/lib/argument.js
index c16430250..d7acadfb4 100644
--- a/lib/argument.js
+++ b/lib/argument.js
@@ -99,8 +99,9 @@ class Argument {
   choices(values) {
     this.argChoices = values.slice();
     this.parseArg = (arg, previous) => {
-      if (!this.argChoices.includes(arg)) {
-        throw new InvalidArgumentError(`Allowed choices are ${this.argChoices.join(', ')}.`);
+      const argChoices = /** @type {string[]} */ (this.argChoices);
+      if (!argChoices.includes(arg)) {
+        throw new InvalidArgumentError(`Allowed choices are ${argChoices.join(', ')}.`);
       }
       if (this.variadic) {
         return this._concatValue(arg, previous);
diff --git a/lib/command.js b/lib/command.js
index 590a271dd..edd7e4285 100644
--- a/lib/command.js
+++ b/lib/command.js
@@ -142,7 +142,9 @@ class Command extends EventEmitter {
       desc = null;
     }
     opts = opts || {};
-    const [, name, args] = nameAndArgs.match(/([^ ]+) *(.*)/);
+    const [, name, args] = /** @type {[*, string, string | undefined]} */ (
+      nameAndArgs.match(/([^ ]+) *(.*)/)
+    );
 
     const cmd = this.createCommand(name);
     if (desc) {
@@ -385,7 +387,9 @@ class Command extends EventEmitter {
 
   _hasImplicitHelpCommand() {
     if (this._addImplicitHelpCommand === undefined) {
-      return this.commands.length && !this._actionHandler && !this._findCommand('help');
+      return Boolean(
+        this.commands.length && !this._actionHandler && !this._findCommand('help')
+      );
     }
     return this._addImplicitHelpCommand;
   }
@@ -538,7 +542,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
         try {
           val = option.parseArg(val, oldValue);
         } catch (err) {
-          if (err.code === 'commander.invalidArgument') {
+          if (err instanceof CommanderError && err.code === 'commander.invalidArgument') {
             const message = `${invalidValueMessage} ${err.message}`;
             this.error(message, { exitCode: err.exitCode, code: err.code });
           }
@@ -780,7 +784,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
    */
 
   setOptionValue(key, value) {
-    return this.setOptionValueWithSource(key, value, undefined);
+    return this.setOptionValueWithSource(key, value);
   }
 
   /**
@@ -788,7 +792,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
     *
     * @param {string} key
     * @param {Object} value
-    * @param {string} source - expected values are default/config/env/cli/implied
+    * @param {string} [source] - expected values are default/config/env/cli/implied
     * @return {Command} `this` command for chaining
     */
 
@@ -807,7 +811,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
     * Expected values are default | config | env | cli | implied
     *
     * @param {string} key
-    * @return {string}
+    * @return {string | undefined}
     */
 
   getOptionValueSource(key) {
@@ -819,7 +823,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
     * Expected values are default | config | env | cli | implied
     *
     * @param {string} key
-    * @return {string}
+    * @return {string | undefined}
     */
 
   getOptionValueSourceWithGlobals(key) {
@@ -1070,7 +1074,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
 
   _dispatchSubcommand(commandName, operands, unknown) {
     const subCommand = this._findCommand(commandName);
-    if (!subCommand) this.help({ error: true });
+    if (!subCommand) return this.help({ error: true });
 
     let hookResult;
     hookResult = this._chainOrCallSubCommandHook(hookResult, subCommand, 'preSubcommand');
@@ -1093,11 +1097,11 @@ Expecting one of '${allowedValues.join("', '")}'`);
 
   _dispatchHelpCommand(subcommandName) {
     if (!subcommandName) {
-      this.help();
+      return this.help();
     }
     const subCommand = this._findCommand(subcommandName);
     if (subCommand && !subCommand._executableHandler) {
-      subCommand.help();
+      return subCommand.help();
     }
 
     // Fallback to parsing the help flag to invoke the help.
@@ -1140,7 +1144,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
         try {
           parsedValue = argument.parseArg(value, previous);
         } catch (err) {
-          if (err.code === 'commander.invalidArgument') {
+          if (err instanceof CommanderError && err.code === 'commander.invalidArgument') {
             const message = `error: command-argument value '${value}' is invalid for argument '${argument.name()}'. ${err.message}`;
             this.error(message, { exitCode: err.exitCode, code: err.code });
           }
@@ -1297,7 +1301,9 @@ Expecting one of '${allowedValues.join("', '")}'`);
 
       let actionResult;
       actionResult = this._chainOrCallHooks(actionResult, 'preAction');
-      actionResult = this._chainOrCall(actionResult, () => this._actionHandler(this.processedArgs));
+      actionResult = this._chainOrCall(actionResult, () => (
+        /** @type {Function} */ (this._actionHandler)(this.processedArgs)
+      ));
       if (this.parent) {
         actionResult = this._chainOrCall(actionResult, () => {
           this.parent.emit(commandEvent, operands, unknown); // legacy
@@ -1348,7 +1354,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
    * Return an option matching `arg` if any.
    *
    * @param {string} arg
-   * @return {Option}
+   * @return {Option | undefined}
    * @api private
    */
 
@@ -1446,7 +1452,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
     // parse options
     let activeVariadicOption = null;
     while (args.length) {
-      const arg = args.shift();
+      const arg = /** @type {string} */ (args.shift());
 
       // literal
       if (arg === '--') {
@@ -1619,7 +1625,11 @@ Expecting one of '${allowedValues.join("', '")}'`);
       if (option.envVar && option.envVar in process.env) {
         const optionKey = option.attributeName();
         // Priority check. Do not overwrite cli or options from unknown source (client-code).
-        if (this.getOptionValue(optionKey) === undefined || ['default', 'config', 'env'].includes(this.getOptionValueSource(optionKey))) {
+        if (this.getOptionValue(optionKey) === undefined || (
+          /** @type {Array} */ (['default', 'config', 'env']).includes(
+            this.getOptionValueSource(optionKey)
+          )
+        )) {
           if (option.required || option.optional) { // option can take a value
             // keep very simple, optional always takes value
             this.emit(`optionEnv:${option.name()}`, process.env[option.envVar]);
@@ -1640,7 +1650,11 @@ Expecting one of '${allowedValues.join("', '")}'`);
   _parseOptionsImplied() {
     const dualHelper = new DualOptions(this.options);
     const hasCustomOptionValue = (optionKey) => {
-      return this.getOptionValue(optionKey) !== undefined && !['default', 'implied'].includes(this.getOptionValueSource(optionKey));
+      return this.getOptionValue(optionKey) !== undefined && !(
+        /** @type {Array} */ (['default', 'implied']).includes(
+          this.getOptionValueSource(optionKey)
+        )
+      );
     };
     this.options
       .filter(option => (option.implied !== undefined) &&
@@ -1810,7 +1824,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
    * @param {string} str
    * @param {string} [flags]
    * @param {string} [description]
-   * @return {this | string} `this` command for chaining, or version string if no arguments
+   * @return {this | string | undefined} `this` command for chaining, or version string if no arguments
    */
 
   version(str, flags, description) {
@@ -1837,7 +1851,9 @@ Expecting one of '${allowedValues.join("', '")}'`);
    */
   description(str, argsDescription) {
     if (str === undefined && argsDescription === undefined) return this._description;
-    this._description = str;
+    if (str) {
+      this._description = str;
+    }
     if (argsDescription) {
       this._argsDescription = argsDescription;
     }
@@ -1912,11 +1928,11 @@ Expecting one of '${allowedValues.join("', '")}'`);
       const args = this._args.map((arg) => {
         return humanReadableArgName(arg);
       });
-      return [].concat(
-        (this.options.length || this._hasHelpOption ? '[options]' : []),
-        (this.commands.length ? '[command]' : []),
-        (this._args.length ? args : [])
-      ).join(' ');
+      return [
+        ...(this.options.length || this._hasHelpOption ? ['[options]'] : []),
+        ...(this.commands.length ? ['[command]'] : []),
+        ...(this._args.length ? args : [])
+      ].join(' ');
     }
 
     this._usage = str;
@@ -1964,7 +1980,7 @@ Expecting one of '${allowedValues.join("', '")}'`);
    * program.executableDir('subcommands');
    *
    * @param {string} [path]
-   * @return {string|Command}
+   * @return {string|null|Command}
    */
 
   executableDir(path) {
diff --git a/lib/error.js b/lib/error.js
index e7cde9cc7..7f166a3bd 100644
--- a/lib/error.js
+++ b/lib/error.js
@@ -9,7 +9,7 @@ class CommanderError extends Error {
    * Constructs the CommanderError class
    * @param {number} exitCode suggested exit code which could be used with process.exit
    * @param {string} code an id string representing the error
-   * @param {string} message human-readable description of the error
+   * @param {string} [message] human-readable description of the error
    * @constructor
    */
   constructor(exitCode, code, message) {
diff --git a/lib/help.js b/lib/help.js
index 14e0fb9f3..e98c90993 100644
--- a/lib/help.js
+++ b/lib/help.js
@@ -30,7 +30,9 @@ class Help {
     const visibleCommands = cmd.commands.filter(cmd => !cmd._hidden);
     if (cmd._hasImplicitHelpCommand()) {
       // Create a command matching the implicit help command.
-      const [, helpName, helpArgs] = cmd._helpCommandnameAndArgs.match(/([^ ]+) *(.*)/);
+      const [, helpName, helpArgs] = /** @type {[*, string, string | undefined]} */ (
+        cmd._helpCommandnameAndArgs.match(/([^ ]+) *(.*)/)
+      );
       const helpCommand = cmd.createCommand(helpName)
         .helpOption(false);
       helpCommand.description(cmd._helpCommandDescription);
diff --git a/lib/option.js b/lib/option.js
index d61fc5f2f..b48a3404d 100644
--- a/lib/option.js
+++ b/lib/option.js
@@ -181,8 +181,9 @@ class Option {
   choices(values) {
     this.argChoices = values.slice();
     this.parseArg = (arg, previous) => {
-      if (!this.argChoices.includes(arg)) {
-        throw new InvalidArgumentError(`Allowed choices are ${this.argChoices.join(', ')}.`);
+      const argChoices = /** @type {string[]} */ (this.argChoices);
+      if (!argChoices.includes(arg)) {
+        throw new InvalidArgumentError(`Allowed choices are ${argChoices.join(', ')}.`);
       }
       if (this.variadic) {
         return this._concatValue(arg, previous);
diff --git a/typings/index.d.ts b/typings/index.d.ts
index 695c3bd25..2bdbe0cca 100644
--- a/typings/index.d.ts
+++ b/typings/index.d.ts
@@ -595,17 +595,17 @@ export class Command {
   /**
    * Store option value and where the value came from.
    */
-  setOptionValueWithSource(key: string, value: unknown, source: OptionValueSource): this;
+  setOptionValueWithSource(key: string, value: unknown, source?: OptionValueSource | string | undefined): this;
 
   /**
    * Get source of option value.
    */
-  getOptionValueSource(key: string): OptionValueSource | undefined;
+  getOptionValueSource(key: string): OptionValueSource | string | undefined;
 
   /**
     * Get source of option value. See also .optsWithGlobals().
    */
-  getOptionValueSourceWithGlobals(key: string): OptionValueSource | undefined;
+  getOptionValueSourceWithGlobals(key: string): OptionValueSource | string | undefined;
 
   /**
    * Alter parsing of short flags with optional values.
@@ -819,7 +819,7 @@ export class Command {
   /**
    * Get the executable search directory.
    */
-  executableDir(): string;
+  executableDir(): string | null;
 
   /**
    * Output help information for this command.
diff --git a/typings/index.test-d.ts b/typings/index.test-d.ts
index 734036fad..13cdbfe24 100644
--- a/typings/index.test-d.ts
+++ b/typings/index.test-d.ts
@@ -173,10 +173,10 @@ expectType<commander.Command>(program.setOptionValue('example', true));
 expectType<commander.Command>(program.setOptionValueWithSource('example', [], 'cli'));
 
 // getOptionValueSource
-expectType<commander.OptionValueSource | undefined>(program.getOptionValueSource('example'));
+expectType<string | undefined>(program.getOptionValueSource('example'));
 
 // getOptionValueSourceWithGlobals
-expectType<commander.OptionValueSource | undefined>(program.getOptionValueSourceWithGlobals('example'));
+expectType<string | undefined>(program.getOptionValueSourceWithGlobals('example'));
 
 // combineFlagAndOptionalValue
 expectType<commander.Command>(program.combineFlagAndOptionalValue());
@@ -275,7 +275,7 @@ expectType<commander.Command>(program.nameFromFilename(__filename));
 
 // executableDir
 expectType<commander.Command>(program.executableDir(__dirname));
-expectType<string>(program.executableDir());
+expectType<string | null>(program.executableDir());
 
 // outputHelp
 // eslint-disable-next-line @typescript-eslint/no-invalid-void-type, @typescript-eslint/no-confusing-void-expression