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TypeScript

TypeScript 5.9

TypeScript: 0% done

Module 1 · Your first program

A program is a list of instructions

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A line of words about as long as a slide’s first line, and its second.

Another line, shorter.

1 / 189 · Your first program

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A program is a list of instructions
JavaScript runs in browsers and on servers
, 1 to practiseYour first line: console.log
Three places to run code today
, 1 to practiseInstructions run from top to bottom
The terminal runs programs by name
, 1 to practiseAn error tells you what, and where
Recap

, 1 to practiseEvery value has a kind
A variable is a name for a value
, 1 to practiseconst never changes, let can
, 1 to practiseNumbers do arithmetic
, 1 to practiseJoining text, and template strings
, 1 to practiseTry it: build a sentence
, 1 to practiseComparisons give booleans
Good names make code readable
Recap

if runs code only when a condition is true
, 1 to practiseelse if chooses between several cases
, 1 to practiseCombine conditions with and, or, not
, 1 to practiseAny value can act as a condition
, 1 to practiseswitch compares one value against many
, 1 to practiseA for loop repeats code a set number of times
, 1 to practiseA while loop repeats until a condition is false
Recap

A function is a named, reusable piece of work
, 1 to practiseParameters are the inputs
, 1 to practisereturn hands a value back
, 2 to practiseArrow functions are a shorter way to write one
, 1 to practiseVariables live inside their block
Default values for missing arguments
, 1 to practiseTry it: a late fee
, 1 to practiseFunctions are values too
Recap

, 1 to practiseAn array is an ordered list
Arrays can grow and shrink
, 1 to practisefor...of visits every element
, 1 to practisemap, filter and find take a callback
, 1 to practiseTry it: fees for late books
An object groups named values
, 1 to practiseArrays of objects are everyday data
, 1 to practiseTry it: total a cart
, 1 to practiseundefined and null mean "nothing here"
, 1 to practise?. and ?? deal with missing values
, 1 to practiseObjects are shared, not copied
Recap

A module is a file that shares some of its names
, 1 to practisePackages are modules other people wrote
, 1 to practisethrow signals a problem, catch handles it
, 1 to practiseSome work finishes later
A promise is a value that arrives later
await waits for a promise inside an async function
, 1 to practiseAwaited errors are caught with try...catch
, 1 to practiseTry it: count, or fall back
, 1 to practisePromise.all waits for several at once
Recap

JavaScript finds mistakes only when it runs
An annotation says what a value must be
, 1 to practiseReading a type error
Types are removed before the code runs
Installing and running the checker
, 1 to practiseTypeScript works most types out itself
, 1 to practiseTry it: annotate, then fix what the checker finds
, 1 to practiseYour editor runs the checker as you type
, 1 to practiseAlways turn strict mode on
Recap

The basic types match the basic values
, 1 to practiseArray types: string[] and friends
, 1 to practiseAn object type lists the properties
Name a type with type or interface
, 1 to practiseOptional and readonly properties
, 1 to practiseTyping functions and callbacks
, 1 to practiseSaying a value may be missing
, 1 to practiseTry it: a label for a book
Recap

A literal type is one exact value
, 1 to practiseA union is one of several types
A check narrows the type
, 1 to practiseEvery everyday check narrows
, 1 to practiseDiscriminated unions: a tag says which shape
, 1 to practiseMake the checker prove every case is handled
, 1 to practiseTry it: a fee for every kind of loan
, 1 to practiseWrite your own type guards
Recap

Types are compared by shape, not by name
, 1 to practiseFresh object literals are checked for extras
Classes make objects of the same shape
, 1 to practiseTry it: a shelf that fills up
, 1 to practiseIntersections combine types
any switches the checker off
unknown: anything goes in, nothing comes out unchecked
, 1 to practiseas tells the checker to trust you
Recap

, 1 to practiseAn async function returns a Promise<T>
Data from outside has no type you can trust
, 1 to practiseA validating function earns the type
, 1 to practisefetch does not fail on an error status
, 1 to practiseA caught error is unknown
, 1 to practiseReturn failures as values when callers must handle them
, 1 to practiseTry it: parse without throwing
Promise.all keeps each type in its place
Recap

Some code works the same for every type
, 1 to practiseA type parameter is a variable for a type
, 1 to practiseextends says what T must have
, 1 to practiseK extends keyof T: a key of the object
, 1 to practiseTry it: look things up by a key
, 1 to practiseTypes and interfaces can be generic too
A generic class: a typed collection
, 1 to practiseA good type parameter links two places
Recap

Say it once, derive the rest
typeof in a type gives a value’s type
, 1 to practisekeyof and indexed access
, 1 to practiseas const keeps values exact and readonly
, 1 to practiseTry it: one list, a type and a check
, 1 to practiseA const object instead of an enum
, 1 to practisesatisfies checks without widening
ReturnType, Parameters and Awaited
Recap

, 1 to practiseBuilt-in types that transform types
, 1 to practisePick checks its keys, Omit does not
A mapped type loops over keys
, 1 to practiseAdding and removing ? and readonly
, 1 to practiseRenaming and dropping keys with as
, 1 to practiseTry it: the fields a form can edit
Index signatures and Record
, 1 to practiseReading nested types from the inside out
Recap

, 1 to practiseA conditional type is an if/else for types
, 1 to practiseConditional types distribute over unions
, 1 to practiseWrap it in brackets to stop distribution
, 1 to practiseinfer captures part of a type
infer can be constrained
, 1 to practiseTry it: unwrap a type
, 1 to practiseConditional return types, and their limits
Recap

, 1 to practiseTemplate literal types build strings from types
Uppercase, Lowercase, Capitalize, Uncapitalize
, 1 to practiseRenaming keys in a mapped type
, 1 to practiseTry it: the same name, in code and in types
Typed event handlers
, 1 to practiseMatching strings with infer
, 1 to practiseRecursion: the parameters of a path
, 1 to practiseKnow where template types stop
Recap

TypeScript is not sound, on purpose
, 1 to practiseVariance: which way assignability flows
, 1 to practiseMethods are checked loosely
Mutable arrays are covariant, and that's unsound
, 1 to practiseEscape hatches, and keeping them honest
, 1 to practiseBrands: making lookalike types distinct
, 1 to practiseTry it: brand an ISBN
, 1 to practiseFlags beyond strict
, 1 to practiseTesting your types
Recap

Reading a tsconfig
, 1 to practiseWhat strict turns on
, 1 to practisemoduleResolution: bundler or nodenext
, 1 to practiseimport type and verbatimModuleSyntax
Prefer syntax that can simply be erased
.d.ts files describe JavaScript
, 1 to practiseAdding to types you don't own
, 1 to practiseWhen the checker gets slow
What changed in TypeScript 5.x
Recap

, 1 to practiseTypes can refer to themselves
Recursive mapped types reach every level
, 1 to practiseTuples are lists you can compute with
, 1 to practiseRecursion has limits: keep it tail-recursive
All the paths into a nested object
, 1 to practiseTry it: what a path leads to
, 1 to practiseconst type parameters infer literals
, 1 to practiseNoInfer: don't infer from here
Types are read by people
Recap

, 1 to practiseHow this challenge works
, 1 to practiseAsync under pressure
, 1 to practiseModel a to-do item
, 1 to practiseWhich statements are true?
, 1 to practisePredict the types
, 1 to practiseWrite it: keep the properties of one type
, 1 to practiseSpot the unsound line
, 1 to practiseFollow data from outside to a typed value
, 1 to practiseWrite it: move a loan to its next state
, 1 to practiseWrite it: group a list, keeping the types
, 1 to practiseWrite it: links that know their parameters
Capstone: build a typed catalogue
Recap: the course in six lines