Skip to content

Learn

All CoursesBrowse the full catalog of expert-led courses
WorkshopsDeep-dive workshops from industry experts

AI-Powered

Learning PathsNewA personal path to your goal, built by AI
AI AgentLearn with an AI agent on every lesson
MCP ServerBring Dometrain into your AI coding tools
Mock InterviewsNewPractice real interviews with an AI interviewer

Trending now

Dometrain ProAll courses. One subscription.Every course and workshop replay, plus the AI-powered tools.Explore Pro

Community

LeaderboardSee how you rank against fellow learners
ReviewsWhat developers say about our courses
Customer StoriesHow real teams level up with Dometrain
ChangelogSee what's new and improved at Dometrain

Content

BlogsArticles and deep dives from our authors
AuthorsMeet the engineers behind the courses

Partners

AffiliatesEarn by sharing Dometrain with your audience
Dometrain for BusinessTraining and team management for companies

SkillScoreNew Dometrain Pro

Shopping Basket

Your basket is empty

Business Portal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

TypeScript

6h 54m

Deep Dive

Progress0%

0 / 227 lessons6h 53m left

Course overview(0/3)

WelcomeFree Preview30s What will you learn in this course?Free Preview2m 5s Who is the course for and prerequisitesFree Preview26s

Narrowing Types Effectively(0/44)

Intro to Type Narrowing1m 14s Narrowing via typeof Type Guard57s Exercise: typeof40s Answers: typeof39s Narrowing via Truthiness1m 49s Exercise: Narrowing via Truthiness26s Answers: Narrowing via Truthiness48s Narrowing via Equality Checking1m 10s Exercise: Narrowing via Equality Checking10s Answers: Narrowing via Equality Checking17s Narrowing via the in Operator1m 28s Exercise: Narrowing via the in Operator24s Answer: Narrowing via the in Operator39s Narrowing via instanceOf38s Exercise: Narrowing via instanceOf20s Answers: Narrowing via instanceOf23s Narrowing via Control Flow33s Exercise: Narrowing via Control Flow19s Answers: Narrowing via Control Flow24s Narrowing via a Type Predicate3m 12s Exercise: Narrowing via a Type Predicate53s Answers: Narrowing via a Type Predicate1m 48s Narrowing via a Discriminated Union1m 47s Exercise: Narrowing via a Discriminated Union32s Answers: Narrowing via a Discriminated Union33s Narrowing via an Assertion Function1m 22s Exercise: Narrowing via an Assertion Function36s Answers: Narrowing via an Assertion Function51s Narrowing via Exhaustiveness Checking2m 50s Exercise: Narrowing via Exhaustiveness Checking28s Answers: Narrowing via Exhaustiveness Checking46s Narrowing via satisfies4m 30s Exercise: Narrowing via satisfies56s Answer: Narrowing via satisfies1m 5s Narrowing via a Type Assertion3m 45s Exercise: Narrowing via a Type Assertion47s Answer: Narrowing via a Type Assertion1m 39s Narrowing via a Non-null Assertion2m 54s Exercise: Narrowing via a Non-null Assertion49s Answers: Narrowing via a Non-null Assertion49s Typing this6m 24s Exercise: Typing this34s Answers: Typing this59s Summary3m 58s

Using Built-in Utility Types(0/25)

Intro to Utility Types47s Pick and Omit2m 21s Exercise: Pick and Omit31s Answers: Pick and Omit29s Partial2m 31s Exercise: Partial40s Answers: Partial1m 10s Required27s Exercise: Required8s Answers: Required7s Readonly and ReadonlyArray1m 15s Exercise: Readonly and ReadonlyArray38s Answers: Readonly and ReadonlyArray1m 0s Exclude33s Exercise: Exclude26s Answers: Exclude28s Extract1m 1s Exercise: Extract11s Answers: Extract16s String Manipulation Utilities2m 3s Exercise: String Manipulation Utilities16s Answers: String Manipulation Utilities11s Other Built-in Utility Types7m 2s Built-in Utility Types Summary1m 25s Third-Party Utility Types51s

Implementing Generics(0/27)

Intro to Implementing Generics11m 6s Exercise: Implementing Generics49s Answers: Implementing Generics3m 1s Generic Function Types2m 6s Exercise: Generic Function Types18s Answers: Generic Function Types51s Generic Interfaces4m 8s Exercise: Generic Interfaces34s Answers: Generic Interfaces58s Generic Classes1m 8s Exercise: Generic Classes22s Answers: Generic Classes1m 10s Generic Constraints via Extends8m 19s Exercise: Generic Constraints via Extends1m 20s Answers: Generic Constraints via Extends3m 37s Const Modifier4m 29s Exercise: Const Modifier42s Answers: Const Modifier59s Default Type Arguments5m 51s Exercise: Default Type Arguments1m 14s Answers: Default Type Arguments2m 26s Pushing Type Params Down3m 22s Exercise: Pushing Type Params Down6s Answers: Pushing Type Params Down21s Generics Best Practices3m 29s Exercise: Eliminating Needless Type Arguments13s Answers: Eliminating Needless Type Arguments10s

Transforming Types via Mapped Types(0/31)

Intro to Mapped Types4m 13s Exercise: Implement a Mapped Type27s Answers: Implement a Mapped Type10s Template Literal Types5m 8s Exercise: Template Literal Types29s Answers: Template Literal Types1m 6s keyof4m 53s Exercise: keyof43s Answers: keyof50s typeof3m 19s Exericse: typeof57s Answers: typeof1m 42s Indexed Access9m 10s Exercise: Indexed Access56s Answers: Indexed Access2m 40s Conditional Types7m 6s Exercise: Conditional Types55s Answers: Conditional Types2m 21s infer10m 10s Exercise: infer47s Answers: infer1m 37s Looping via the in Operator7m 59s Exercise: Looping via the in Operator18s Answers: Looping via the in Operator48s Remapping Properties via as6m 10s Exercise: Remapping Properties via as32s Answers: Remapping Properties via as1m 11s Recursion in Mapped Types1m 59s Exercise: Recursion in Mapped Types43s Answers: Recursion in Mapped Types1m 22s JS vs TS Syntax Comparision4m 30s

Recreating Utility Types(0/21)

Intro to Utility Types49s Exercise: MyPartial29s Answers: MyPartial30s Exercise: MyRequired30s Answers: MyRequired39s Exercise: MyReadonly28s Answers: MyReadonly42s Exercise: MyPick51s Answers: MyPick40s Exercise: MyRecord23s Answers: MyRecord1m 31s Exercise: MyExclude45s Answers: MyExclude1m 3s Exercise: MyExtract36s Answers: MyExtract28s Exercise: MyOmit48s Answers: MyOmit1m 4s Exercise: MyNonNullable28s Answers: MyNonNullable1m 34s Exercise: RecursivePartial1m 5s Answers: RecursivePartial1m 11s

Validating at Runtime(0/15)

Intro to Runtime Validation3m 34s Declaring Zod Schemas5m 23s Exercise: Declaring Zod Schemas31s Answers: Declaring Zod Schemas50s Nullable and Optional42s Zod Utility Types58s Implementing Custom Validation via Refine42s Validating a File via Zod3m 1s Validating at Runtime in Web Apps44s Validating URL Params via Zod2m 59s Validating Forms via Zod1m 24s Validating localStorage via Zod1m 39s Validating API Responses via Zod2m 35s Validating Environment Variables via Zod2m 9s Suggested Next Steps39s

Typing Decorators(0/24)

Understanding Decorator Implementations1m 40s Intro to Decorators5m 4s Context Parameter1m 58s Typing Decorators and Invocation Order2m 21s Decorating Methods5m 26s Exercise: Decorating Methods14s Answers: Decorating Methods43s Decorating Getters and Setters4m 53s Exercise: Decorating Getters and Setters16s Answers: Decorating Getters and Setters38s Decorating Fields59s Exercise: Decorating Fields27s Answers: Decorating Fields1m 15s Decorating Classes1m 38s Exercise: Decorating Classes25s Answers: Decorating Classes35s Implementing Universal Decorators2m 28s Accepting Arguments via a Decorator1m 48s Exercise: Accepting Arguments via a Decorator24s Answers: Accepting Arguments via a Decorator1m 6s Composing Multiple Decorators41s Exercise: Composing Multiple Decorators41s Answers: Composing Multiple Decorators1m 34s Decorator Tradeoffs1m 32s

Advanced Features and Techniques(0/25)

Intro26s Accepting Random Strings via Loose Autocomplete2m 2s Exercise: Accepting Random Strings via Loose Autocomplete9s Answer: Accepting Random Strings via Loose Autocomplete18s Type-only Imports and Exports6m 36s Named Tuples1m 29s Exercise: Named Tuples11s Answer: Named Tuples12s Typing Object.keys1m 45s Understanding Optional Keys vs Optional Values1m 11s Working with Evolving anys2m 42s Handling Excess Properties2m 58s Naming Values and Types the Same2m 44s Working with JS Array Methods1m 57s Disabling TypeScript via Comments2m 31s Typing Symbols4m 31s Simulating Nominal Types via Branded Types11m 19s Exercise: Simulating Nominal Types via Branded Types42s Answers: Simulating Nominal Types via Branded Types1m 11s Testing Types6m 32s Exercise: Testing Types21s Answers: Testing Types33s Using JSDocs to Type JS Files7m 50s Exercise: Using JSDocs to Type JS Files12s Answers: Using JSDocs to Type JS Files1m 12s

Choosing tsconfig Settings(0/6)

Intro to tsconfig Settings22s Avoiding Configuring From Scratch1m 45s Migrating from JavaScript to TypeScript3m 3s Configuring TypeScript for the Strictest Settings10m 54s Debugging via Sourcemaps2m 46s Optimizing for Performance4m 37s

Final Tips and Next Steps(0/5)

Intro15s Linting TypeScript1m 12s Typing Errors Effectively5m 15s Generating Types1m 24s Wrap up and Next Steps40s

Course conclusion(0/1)

Rate this course

0 matches found

Loading transcript

No transcript available for this lesson.

Sign in to take notes

Timestamped notes are saved to your account and grouped by lesson here, so you can jump back to any moment of the course.

Get Your Certificate

Exercise: Narrowing via Equality Checking

Narrowing Types Effectively

Watch 50% of the video to earn XP

Sign in to watch this course

Already a Dometrain Member?

or

New to Dometrain?

Next Lesson

Answers: Narrowing via Equality Checking

Complete to earn+100 XP

+0 XP

+20 to next Gem

1 day streak!

Level up!

Streak saved!

Lesson Complete!

+0 XP

+20 to next Gem

Streak started!

Level 1!

Freeze earned!

Up next:Answers: Narrowing via Equality Checking

Lesson Notes

Read this lesson

Loading content...

Unable to load content. Please try again.

Upgrade to Dometrain Pro

Get unlimited access to this video and all other courses on Dometrain with a Pro subscription.

Unlimited access to all courses
New courses added regularly
Certificates of completion

Get Dometrain Pro

Where today's experts teach tomorrow's

Learn

Courses Learning Paths Workshops

Community

Blog Authors Reviews Customer Stories Changelog

Business

Dometrain Pro Business Portal Affiliates

Support

Contact Terms of Use Privacy Policy

© 2026 Dometrain. All rights reserved.

What's New