TypeScript

1. 1

   Hello, TypeScript

   Here is a small JavaScript program that is completely broken, and that JavaScript will happily run:

   What TypeScript isTsc compile stepRunning a .ts fileThe play button
   20 min
2. 2

   Type Basics

   You can already run a .ts file. Now let's meet the basic building blocks: the primitive types TypeScript ships with, plus the two escape hatches (any...

   Primitive typesStringNumberBooleanNullUndefined+3
   25 min
3. 3

   Arrays and Tuples

   A single value is rarely enough. You have a list of temperatures, a list of names, a list of prices. And the moment you have a list, a new kind of...

   Array typesReadonly arraysTuple typesFixed-length tuplesNamed tuple elements
   25 min
4. 4

   Object Types and Interfaces

   Primitives and arrays carry you a long way, but most real data has a shape: a user has a name and an age, a request has a method and a path, a result...

   Object literal typesInterface keywordRequired vs optional propertiesStructural typingExcess property checks
   25 min
5. 5

   Functions

   Functions are where most of the bugs live and where types pay off the most. A function's signature (its parameters and its return type) is a contract...

   Parameter typesReturn typesVoid returnDefault parametersRest parametersFunction type expressions+1
   25 min
6. 6

   Union and Literal Types

   Some values are not one thing. A function that looks up a user either finds one or does not. A form field holds either a number the user typed or...

   Union typesString literal typesNumeric literal typesLiteral narrowing with as constUnion of object shapes
   25 min
7. 7

   Type Narrowing

   A union says "this value is one of several types". Narrowing is the compiler's process of figuring out which one, based on the runtime checks you...

   Typeof narrowingTruthiness narrowingEquality narrowingIn operator narrowingInstanceof narrowingDiscriminated unions+1
   30 min
8. 8

   Type Aliases vs Interfaces

   You have been writing object shapes inline, and it has been fine because the shapes were small. Now here is a function signature from a real codebase:

   Type aliasesInterface declarationsExtending interfacesIntersection typesDeclaration mergingWhen to pick which
   20 min
9. 9

   Optional, Readonly, and Nullable

   The single most common runtime error in JavaScript reads like this:

   Optional propertiesReadonly propertiesNon-null assertion operatorOptional chainingNullish coalescingStrictNullChecks
   25 min
10. 10

    Enums

    Some values come from a short, fixed list. An order is pending, shipped or delivered. A log line is debug, info, warn or error. A direction is up,...

    Numeric enumsString enumsConst enumsEnums vs string literal unionsReverse mapping caveats
    20 min
11. 11

    Generics

    You need a function that takes an array and returns its first element. Easy:

    Generic functionsGeneric interfacesType parametersType inference for genericsBounded generics with extendsDefault type parameters
    30 min
12. 12

    Classes

    Here is a bank account written as a plain object:

    Class declarationsConstructorsInstance propertiesAccess modifiers (public, private, protected)Readonly fieldsParameter properties+1
    25 min
13. 13

    Inheritance and Abstract Classes

    You have written three classes for a payments system: CardPayment, BankTransfer and Voucher. All three have an amount, a currency and a createdAt....

    Extends keywordSuper callsMethod overrideAbstract classesAbstract methodsImplements with interface
    25 min
14. 14

    Modules

    Every TypeScript file is its own module. The moment a file has an import or export, its top-level names stop being globals and become module-scoped....

    ES module syntaxNamed exportsDefault exportType-only importsRe-exportsModule resolution basics
    25 min
15. 15

    Utility Types

    TypeScript ships with a small library of generic types that take a type as input and return a transformed type as output. They have unassuming names...

    Partial<T>Required<T>Readonly<T>Pick<T, K>Omit<T, K>Record<K, V>+2
    30 min
16. 16

    Mapped and Conditional Types

    The utility types you just used (Partial, Pick, Record, ReturnType) are all built from two language features: mapped types for transforming the keys...

    Keyof operatorMapped types syntaxKey remapping with asConditional typesInfer keywordDistributive conditional types
    30 min
17. 17

    Declaration Files and Libraries

    You install a library, import it, and your editor lights up with autocomplete, parameter hints and the exact shape of every return value. You install...

    .d.ts filesDefinitelyTyped (@types packages)Ambient declarationsDeclare moduleTypeRoots and types option
    25 min
18. 18

    tsconfig and the Compiler

    tsconfig.json is the file the TypeScript compiler reads to figure out what to compile and how. There are over a hundred options, but a real project...

    Tsconfig.jsonStrict modeTarget and libModule and moduleResolutionOutDir and rootDirIncremental and watch+1
    30 min

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