diff --git a/bin/concurrently.spec.ts b/bin/concurrently.spec.ts
index d41c2889..27baf3a7 100644
--- a/bin/concurrently.spec.ts
+++ b/bin/concurrently.spec.ts
@@ -59,12 +59,10 @@ const run = (args: string, ctrlcWrapper?: boolean) => {
 
     const stdout = readline.createInterface({
         input: child.stdout,
-        output: null,
     });
 
     const stderr = readline.createInterface({
         input: child.stderr,
-        output: null,
     });
 
     const log = new Rx.Observable<string>((observer) => {
@@ -82,8 +80,8 @@ const run = (args: string, ctrlcWrapper?: boolean) => {
     });
 
     const exit = Rx.firstValueFrom(
-        Rx.fromEvent(child, 'exit').pipe(
-            map((exit: [number | null, NodeJS.Signals | null]) => {
+        Rx.fromEvent<[number | null, NodeJS.Signals | null]>(child, 'exit').pipe(
+            map((exit) => {
                 return {
                     /** The exit code if the child exited on its own. */
                     code: exit[0],
@@ -180,7 +178,7 @@ describe('exiting conditions', () => {
                     // Instruct the wrapper to send CTRL+C to its child
                     sendCtrlC(child.process);
                 } else {
-                    process.kill(child.pid, 'SIGINT');
+                    process.kill(Number(child.pid), 'SIGINT');
                 }
             }
         });
diff --git a/src/command-parser/expand-npm-shortcut.spec.ts b/src/command-parser/expand-npm-shortcut.spec.ts
index 7d791c1a..286b100f 100644
--- a/src/command-parser/expand-npm-shortcut.spec.ts
+++ b/src/command-parser/expand-npm-shortcut.spec.ts
@@ -3,7 +3,7 @@ import { ExpandNpmShortcut } from './expand-npm-shortcut';
 
 const parser = new ExpandNpmShortcut();
 
-const createCommandInfo = (command: string, name?: string): CommandInfo => ({
+const createCommandInfo = (command: string, name = ''): CommandInfo => ({
     name,
     command,
 });
diff --git a/src/command-parser/expand-npm-wildcard.spec.ts b/src/command-parser/expand-npm-wildcard.spec.ts
index b5ac271e..9c0a7954 100644
--- a/src/command-parser/expand-npm-wildcard.spec.ts
+++ b/src/command-parser/expand-npm-wildcard.spec.ts
@@ -30,7 +30,7 @@ describe('ExpandNpmWildcard#readPackage', () => {
             if (path === 'package.json') {
                 return JSON.stringify(expectedPackage);
             }
-            return null;
+            return '';
         });
 
         const actualReadPackage = ExpandNpmWildcard.readPackage();
diff --git a/src/command.spec.ts b/src/command.spec.ts
index 0a03579c..b6e25172 100644
--- a/src/command.spec.ts
+++ b/src/command.spec.ts
@@ -44,7 +44,7 @@ beforeEach(() => {
     killProcess = jest.fn();
 });
 
-const createCommand = (overrides?: Partial<CommandInfo>, spawnOpts?: SpawnOptions) => {
+const createCommand = (overrides?: Partial<CommandInfo>, spawnOpts: SpawnOptions = {}) => {
     const command = new Command(
         { index: 0, name: '', command: 'echo foo', ...overrides },
         spawnOpts,
@@ -198,7 +198,7 @@ describe('#start()', () => {
         const { command } = createCommand();
         const stdout = Rx.firstValueFrom(command.stdout);
         command.start();
-        process.stdout.emit('data', Buffer.from('hello'));
+        process.stdout?.emit('data', Buffer.from('hello'));
 
         expect((await stdout).toString()).toBe('hello');
     });
@@ -207,7 +207,7 @@ describe('#start()', () => {
         const { command } = createCommand();
         const stderr = Rx.firstValueFrom(command.stderr);
         command.start();
-        process.stderr.emit('data', Buffer.from('dang'));
+        process.stderr?.emit('data', Buffer.from('dang'));
 
         expect((await stderr).toString()).toBe('dang');
     });
@@ -252,3 +252,16 @@ describe('#kill()', () => {
         expect(close).toMatchObject({ exitCode: 1, killed: true });
     });
 });
+
+describe('.canKill()', () => {
+    it('returns whether command has both PID and process', () => {
+        const { command } = createCommand();
+        expect(Command.canKill(command)).toBe(false);
+
+        command.pid = 1;
+        expect(Command.canKill(command)).toBe(false);
+
+        command.process = process;
+        expect(Command.canKill(command)).toBe(true);
+    });
+});
diff --git a/src/command.ts b/src/command.ts
index 77196b9c..17255f8b 100644
--- a/src/command.ts
+++ b/src/command.ts
@@ -92,7 +92,7 @@ export class Command implements CommandInfo {
     readonly command: string;
 
     /** @inheritdoc */
-    readonly prefixColor: string;
+    readonly prefixColor?: string;
 
     /** @inheritdoc */
     readonly env: Record<string, unknown>;
@@ -112,8 +112,9 @@ export class Command implements CommandInfo {
     killed = false;
     exited = false;
 
+    /** @deprecated */
     get killable() {
-        return !!this.process;
+        return Command.canKill(this);
     }
 
     constructor(
@@ -126,7 +127,7 @@ export class Command implements CommandInfo {
         this.name = name;
         this.command = command;
         this.prefixColor = prefixColor;
-        this.env = env;
+        this.env = env || {};
         this.cwd = cwd;
         this.killProcess = killProcess;
         this.spawn = spawn;
@@ -161,7 +162,7 @@ export class Command implements CommandInfo {
                 this.close.next({
                     command: this,
                     index: this.index,
-                    exitCode: exitCode === null ? signal : exitCode,
+                    exitCode: exitCode ?? String(signal),
                     killed: this.killed,
                     timings: {
                         startDate,
@@ -173,18 +174,27 @@ export class Command implements CommandInfo {
         );
         child.stdout && pipeTo(Rx.fromEvent<Buffer>(child.stdout, 'data'), this.stdout);
         child.stderr && pipeTo(Rx.fromEvent<Buffer>(child.stderr, 'data'), this.stderr);
-        this.stdin = child.stdin;
+        this.stdin = child.stdin || undefined;
     }
 
     /**
      * Kills this command, optionally specifying a signal to send to it.
      */
     kill(code?: string) {
-        if (this.killable) {
+        if (Command.canKill(this)) {
             this.killed = true;
             this.killProcess(this.pid, code);
         }
     }
+
+    /**
+     * Detects whether a command can be killed.
+     *
+     * Also works as a type guard on the input `command`.
+     */
+    static canKill(command: Command): command is Command & { pid: number; process: ChildProcess } {
+        return !!command.pid && !!command.process;
+    }
 }
 
 /**
diff --git a/src/completion-listener.ts b/src/completion-listener.ts
index a3aa2deb..26035f43 100644
--- a/src/completion-listener.ts
+++ b/src/completion-listener.ts
@@ -92,11 +92,15 @@ export class CompletionListener {
                 bufferCount(closeStreams.length),
                 switchMap((exitInfos) =>
                     this.isSuccess(exitInfos)
-                        ? Rx.of(exitInfos, this.scheduler)
-                        : Rx.throwError(exitInfos, this.scheduler)
+                        ? this.emitWithScheduler(Rx.of(exitInfos))
+                        : this.emitWithScheduler(Rx.throwError(exitInfos))
                 ),
                 take(1)
             )
         );
     }
+
+    private emitWithScheduler<O>(input: Rx.Observable<O>): Rx.Observable<O> {
+        return this.scheduler ? input.pipe(Rx.observeOn(this.scheduler)) : input;
+    }
 }
diff --git a/src/concurrently.ts b/src/concurrently.ts
index 98863bee..ba269e6f 100644
--- a/src/concurrently.ts
+++ b/src/concurrently.ts
@@ -31,7 +31,7 @@ const defaults: ConcurrentlyOptions = {
  * If value is a string, then that's the command's command line.
  * Fine grained options can be defined by using the object format.
  */
-export type ConcurrentlyCommandInput = string | Partial<CommandInfo>;
+export type ConcurrentlyCommandInput = string | ({ command: string } & Partial<CommandInfo>);
 
 export type ConcurrentlyResult = {
     /**
@@ -175,14 +175,17 @@ export function concurrently(
                 onFinishCallbacks: _.concat(onFinishCallbacks, onFinish ? [onFinish] : []),
             };
         },
-        { commands, onFinishCallbacks: [] }
+        { commands, onFinishCallbacks: [] } as {
+            commands: Command[];
+            onFinishCallbacks: (() => void)[];
+        }
     );
     commands = handleResult.commands;
 
     if (options.logger && options.outputStream) {
         const outputWriter = new OutputWriter({
             outputStream: options.outputStream,
-            group: options.group,
+            group: !!options.group,
             commands,
         });
         options.logger.output.subscribe(({ command, text }) => outputWriter.write(command, text));
@@ -213,14 +216,10 @@ export function concurrently(
 
 function mapToCommandInfo(command: ConcurrentlyCommandInput): CommandInfo {
     if (typeof command === 'string') {
-        return {
-            command,
-            name: '',
-            env: {},
-            cwd: '',
-        };
+        return mapToCommandInfo({ command });
     }
 
+    assert.ok(command.command, '[concurrently] command cannot be empty');
     return {
         command: command.command,
         name: command.name || '',
diff --git a/src/flow-control/input-handler.spec.ts b/src/flow-control/input-handler.spec.ts
index dc48f3ee..165e13a0 100644
--- a/src/flow-control/input-handler.spec.ts
+++ b/src/flow-control/input-handler.spec.ts
@@ -37,7 +37,7 @@ it('does nothing if called without input stream', () => {
     }).handle(commands);
     inputStream.write('something');
 
-    expect(commands[0].stdin.write).not.toHaveBeenCalled();
+    expect(commands[0].stdin?.write).not.toHaveBeenCalled();
 });
 
 it('forwards input stream to default target ID', () => {
@@ -45,9 +45,9 @@ it('forwards input stream to default target ID', () => {
 
     inputStream.write('something');
 
-    expect(commands[0].stdin.write).toHaveBeenCalledTimes(1);
-    expect(commands[0].stdin.write).toHaveBeenCalledWith('something');
-    expect(commands[1].stdin.write).not.toHaveBeenCalled();
+    expect(commands[0].stdin?.write).toHaveBeenCalledTimes(1);
+    expect(commands[0].stdin?.write).toHaveBeenCalledWith('something');
+    expect(commands[1].stdin?.write).not.toHaveBeenCalled();
 });
 
 it('forwards input stream to target index specified in input', () => {
@@ -55,9 +55,9 @@ it('forwards input stream to target index specified in input', () => {
 
     inputStream.write('1:something');
 
-    expect(commands[0].stdin.write).not.toHaveBeenCalled();
-    expect(commands[1].stdin.write).toHaveBeenCalledTimes(1);
-    expect(commands[1].stdin.write).toHaveBeenCalledWith('something');
+    expect(commands[0].stdin?.write).not.toHaveBeenCalled();
+    expect(commands[1].stdin?.write).toHaveBeenCalledTimes(1);
+    expect(commands[1].stdin?.write).toHaveBeenCalledWith('something');
 });
 
 it('forwards input stream to target index specified in input when input contains colon', () => {
@@ -66,10 +66,10 @@ it('forwards input stream to target index specified in input when input contains
     inputStream.emit('data', Buffer.from('1::something'));
     inputStream.emit('data', Buffer.from('1:some:thing'));
 
-    expect(commands[0].stdin.write).not.toHaveBeenCalled();
-    expect(commands[1].stdin.write).toHaveBeenCalledTimes(2);
-    expect(commands[1].stdin.write).toHaveBeenCalledWith(':something');
-    expect(commands[1].stdin.write).toHaveBeenCalledWith('some:thing');
+    expect(commands[0].stdin?.write).not.toHaveBeenCalled();
+    expect(commands[1].stdin?.write).toHaveBeenCalledTimes(2);
+    expect(commands[1].stdin?.write).toHaveBeenCalledWith(':something');
+    expect(commands[1].stdin?.write).toHaveBeenCalledWith('some:thing');
 });
 
 it('forwards input stream to target name specified in input', () => {
@@ -77,18 +77,18 @@ it('forwards input stream to target name specified in input', () => {
 
     inputStream.write('bar:something');
 
-    expect(commands[0].stdin.write).not.toHaveBeenCalled();
-    expect(commands[1].stdin.write).toHaveBeenCalledTimes(1);
-    expect(commands[1].stdin.write).toHaveBeenCalledWith('something');
+    expect(commands[0].stdin?.write).not.toHaveBeenCalled();
+    expect(commands[1].stdin?.write).toHaveBeenCalledTimes(1);
+    expect(commands[1].stdin?.write).toHaveBeenCalledWith('something');
 });
 
 it('logs error if command has no stdin open', () => {
-    commands[0].stdin = null;
+    commands[0].stdin = undefined;
     controller.handle(commands);
 
     inputStream.write('something');
 
-    expect(commands[1].stdin.write).not.toHaveBeenCalled();
+    expect(commands[1].stdin?.write).not.toHaveBeenCalled();
     expect(logger.logGlobalEvent).toHaveBeenCalledWith(
         'Unable to find command 0, or it has no stdin open\n'
     );
@@ -99,8 +99,8 @@ it('logs error if command is not found', () => {
 
     inputStream.write('foobar:something');
 
-    expect(commands[0].stdin.write).not.toHaveBeenCalled();
-    expect(commands[1].stdin.write).not.toHaveBeenCalled();
+    expect(commands[0].stdin?.write).not.toHaveBeenCalled();
+    expect(commands[1].stdin?.write).not.toHaveBeenCalled();
     expect(logger.logGlobalEvent).toHaveBeenCalledWith(
         'Unable to find command foobar, or it has no stdin open\n'
     );
@@ -112,7 +112,8 @@ it('pauses input stream when finished', () => {
     const { onFinish } = controller.handle(commands);
     expect(inputStream.readableFlowing).toBe(true);
 
-    onFinish();
+    expect(onFinish).toBeDefined();
+    onFinish?.();
     expect(inputStream.readableFlowing).toBe(false);
 });
 
@@ -124,6 +125,7 @@ it('does not pause input stream when pauseInputStreamOnFinish is set to false',
     const { onFinish } = controller.handle(commands);
     expect(inputStream.readableFlowing).toBe(true);
 
-    onFinish();
+    expect(onFinish).toBeDefined();
+    onFinish?.();
     expect(inputStream.readableFlowing).toBe(true);
 });
diff --git a/src/flow-control/input-handler.ts b/src/flow-control/input-handler.ts
index 18fb8d82..f537c8fc 100644
--- a/src/flow-control/input-handler.ts
+++ b/src/flow-control/input-handler.ts
@@ -19,7 +19,7 @@ import { FlowController } from './flow-controller';
 export class InputHandler implements FlowController {
     private readonly logger: Logger;
     private readonly defaultInputTarget: CommandIdentifier;
-    private readonly inputStream: Readable;
+    private readonly inputStream?: Readable;
     private readonly pauseInputStreamOnFinish: boolean;
 
     constructor({
@@ -28,7 +28,7 @@ export class InputHandler implements FlowController {
         pauseInputStreamOnFinish,
         logger,
     }: {
-        inputStream: Readable;
+        inputStream?: Readable;
         logger: Logger;
         defaultInputTarget?: CommandIdentifier;
         pauseInputStreamOnFinish?: boolean;
@@ -43,12 +43,13 @@ export class InputHandler implements FlowController {
         commands: Command[];
         onFinish?: () => void | undefined;
     } {
-        if (!this.inputStream) {
+        const { inputStream } = this;
+        if (!inputStream) {
             return { commands };
         }
 
-        Rx.fromEvent(this.inputStream, 'data')
-            .pipe(map((data) => data.toString()))
+        Rx.fromEvent(inputStream, 'data')
+            .pipe(map((data) => String(data)))
             .subscribe((data) => {
                 const dataParts = data.split(/:(.+)/);
                 const targetId = dataParts.length > 1 ? dataParts[0] : this.defaultInputTarget;
@@ -74,7 +75,7 @@ export class InputHandler implements FlowController {
             onFinish: () => {
                 if (this.pauseInputStreamOnFinish) {
                     // https://github.com/kimmobrunfeldt/concurrently/issues/252
-                    this.inputStream.pause();
+                    inputStream.pause();
                 }
             },
         };
diff --git a/src/flow-control/kill-others.ts b/src/flow-control/kill-others.ts
index 7178c48f..39f9d095 100644
--- a/src/flow-control/kill-others.ts
+++ b/src/flow-control/kill-others.ts
@@ -8,7 +8,7 @@ import { FlowController } from './flow-controller';
 export type ProcessCloseCondition = 'failure' | 'success';
 
 /**
- * Sends a SIGTERM signal to all commands when one of the exits with a matching condition.
+ * Sends a SIGTERM signal to all commands when one of the commands exits with a matching condition.
  */
 export class KillOthers implements FlowController {
     private readonly logger: Logger;
diff --git a/src/flow-control/log-timings.ts b/src/flow-control/log-timings.ts
index 244d149f..8bcd4277 100644
--- a/src/flow-control/log-timings.ts
+++ b/src/flow-control/log-timings.ts
@@ -1,3 +1,4 @@
+import * as assert from 'assert';
 import formatDate from 'date-fns/format';
 import _ from 'lodash';
 import * as Rx from 'rxjs';
@@ -53,6 +54,8 @@ export class LogTimings implements FlowController {
     }
 
     private printExitInfoTimingTable(exitInfos: CloseEvent[]) {
+        assert.ok(this.logger);
+
         const exitInfoTable = _(exitInfos)
             .sortBy(({ timings }) => timings.durationSeconds)
             .reverse()
@@ -65,7 +68,8 @@ export class LogTimings implements FlowController {
     }
 
     handle(commands: Command[]) {
-        if (!this.logger) {
+        const { logger } = this;
+        if (!logger) {
             return { commands };
         }
 
@@ -74,14 +78,14 @@ export class LogTimings implements FlowController {
             command.timer.subscribe(({ startDate, endDate }) => {
                 if (!endDate) {
                     const formattedStartDate = formatDate(startDate, this.timestampFormat);
-                    this.logger.logCommandEvent(
+                    logger.logCommandEvent(
                         `${command.command} started at ${formattedStartDate}`,
                         command
                     );
                 } else {
                     const durationMs = endDate.getTime() - startDate.getTime();
                     const formattedEndDate = formatDate(endDate, this.timestampFormat);
-                    this.logger.logCommandEvent(
+                    logger.logCommandEvent(
                         `${
                             command.command
                         } stopped at ${formattedEndDate} after ${durationMs.toLocaleString()}ms`,
diff --git a/src/index.ts b/src/index.ts
index 4773e701..43c22224 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -118,7 +118,8 @@ export default (
             new InputHandler({
                 logger,
                 defaultInputTarget: options.defaultInputTarget,
-                inputStream: options.inputStream || (options.handleInput && process.stdin),
+                inputStream:
+                    options.inputStream || (options.handleInput ? process.stdin : undefined),
                 pauseInputStreamOnFinish: options.pauseInputStreamOnFinish,
             }),
             new KillOnSignal({ process }),
@@ -129,10 +130,10 @@ export default (
             }),
             new KillOthers({
                 logger,
-                conditions: options.killOthers,
+                conditions: options.killOthers || [],
             }),
             new LogTimings({
-                logger: options.timings ? logger : null,
+                logger: options.timings ? logger : undefined,
                 timestampFormat: options.timestampFormat,
             }),
         ],
diff --git a/src/logger.spec.ts b/src/logger.spec.ts
index b9156c9e..4eab2d68 100644
--- a/src/logger.spec.ts
+++ b/src/logger.spec.ts
@@ -261,7 +261,7 @@ describe('#logTable()', () => {
     it('does not log anything if value is not an array', () => {
         const logger = createLogger({});
         logger.logTable({} as never);
-        logger.logTable(null);
+        logger.logTable(null as never);
         logger.logTable(0 as never);
         logger.logTable('' as never);
 
diff --git a/src/logger.ts b/src/logger.ts
index 8bd800ed..1bdd5e87 100644
--- a/src/logger.ts
+++ b/src/logger.ts
@@ -124,7 +124,7 @@ export class Logger {
             color = chalk.hex(command.prefixColor);
         } else {
             const defaultColor = _.get(chalk, defaults.prefixColors, chalk.reset);
-            color = _.get(chalk, command.prefixColor, defaultColor);
+            color = _.get(chalk, command.prefixColor ?? '', defaultColor);
         }
         return color(text);
     }
diff --git a/src/prefix-color-selector.ts b/src/prefix-color-selector.ts
index 94338ec5..95c17580 100644
--- a/src/prefix-color-selector.ts
+++ b/src/prefix-color-selector.ts
@@ -13,7 +13,7 @@ function getConsoleColorsWithoutCustomColors(customColors: string[]): string[] {
 function* createColorGenerator(customColors: string[]): Generator<string, string> {
     // Custom colors should be used as is, except for "auto"
     const nextAutoColors: string[] = getConsoleColorsWithoutCustomColors(customColors);
-    let lastColor: string;
+    let lastColor: string | undefined;
     for (const customColor of customColors) {
         let currentColor = customColor;
         if (currentColor !== 'auto') {
@@ -25,7 +25,7 @@ function* createColorGenerator(customColors: string[]): Generator<string, string
                     // There could be more "auto" values than auto colors so this needs to be able to refill
                     nextAutoColors.push(...PrefixColorSelector.ACCEPTABLE_CONSOLE_COLORS);
                 }
-                currentColor = nextAutoColors.shift();
+                currentColor = String(nextAutoColors.shift());
             }
             yield currentColor; // Auto color
         }
diff --git a/tsconfig.json b/tsconfig.json
index 79241db7..50fff256 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -8,6 +8,7 @@
     "noImplicitAny": true,
     "esModuleInterop": true,
     "allowSyntheticDefaultImports": true,
+    "strict": true,
     "allowJs": false
   },
   "include": ["./bin", "./declarations", "./src"]