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Return to TypeScript Version History, TypeScript, JavaScript

TypeScript 3.9, released in May 2020, brought several improvements and new features to the TypeScript language, focusing on performance enhancements, bug fixes, and developer productivity. This version aimed to make the language more robust and efficient, especially for large projects. Here, we summarize the key features and fixes introduced in TypeScript 3.9, including code examples and comparisons with similar features in other major languages like JavaScript, C#, Java, and Python. Additionally, links to essential TypeScript resources are provided:

- Language Documentation: [TypeScript Documentation](https://www.typescriptlang.org/docs/) - GitHub Repository: [TypeScript GitHub](https://github.com/microsoft/TypeScript) - Official Website: [TypeScript](https://www.typescriptlang.org/) - Wikipedia: [TypeScript Wikipedia](https://en.wikipedia.org/wiki/TypeScript)

1. Improved Inference and Promise.all

TypeScript 3.9 improved type inference for `Promise.all`, making it easier to work with the results of multiple promises in a type-safe manner.

```typescript let [user, settings] = await Promise.all([fetchUser(), fetchSettings()]); ```

In JavaScript, `Promise.all` allows for parallel execution of promises but lacks static typing. C#'s `Task.WhenAll` offers similar functionality with type safety. Java's `CompletableFuture.allOf` provides parallel promise execution but requires more boilerplate for type safety. Python's `asyncio.gather` provides similar functionality but, like JavaScript, lacks built-in static typing.

1. Speed Improvements in `tsc` and `–incremental`

TypeScript 3.9 introduced speed improvements in the TypeScript compiler (`tsc`), especially when using the `–incremental` flag, reducing build times for large projects.

```bash tsc –incremental ```

While this improvement is specific to TypeScript's compilation process, incremental compilation concepts are also found in compiled languages like C# and Java, which aim to reduce build times by compiling only changed parts of the codebase.

1. `// @ts-expect-error` Comments

The `// @ts-expect-error` comment directive tells TypeScript to expect a compilation error on the next line, useful for testing or dealing with known issues.

```typescript // @ts-expect-error let x: number = “string”; ```

JavaScript does not have a built-in mechanism for this, as it is dynamically typed. C# and Java use annotations or pragmas for similar purposes, but not directly for expecting errors. Python's typing system does not have an equivalent feature.

1. Improvements in Inference and `Promise.all`

TypeScript 3.9 made improvements in the type inference for `Promise.all`, enhancing the handling of tuple types for promises resolved in parallel.

```typescript let [user, settings] = await Promise.all([fetchUser(), fetchSettings()]); ```

This feature is unique to TypeScript's static type system and does not have direct equivalents in dynamically typed languages like JavaScript or Python. C# and Java offer similar promise and future handling mechanisms but with different syntactical approaches to type safety.

1. Editor Performance Improvements

TypeScript 3.9 focused on improving the performance of the language services that power code editors, making the development experience smoother and faster, especially in large codebases.

While not directly related to language features, improvements in editor performance impact the developer experience across many programming environments, including those for C#, Java, and Python, where IDEs also focus on performance optimizations.

1. New Syntax for Importing Types

TypeScript 3.9 introduced a new syntax for importing types, allowing developers to explicitly state when they are importing only types, which can be erased during compilation.

```typescript import type { User } from “./user”; ```

This syntax enhancement has no direct equivalent in JavaScript, as it pertains to TypeScript's static type system. C# and Java have distinct mechanisms for dealing with imports and type usage but nothing that directly corresponds to TypeScript's type-only imports. Python's type hints system, while offering some capabilities for type checking, does not require a separate syntax for type imports.

1. `export * as ns` Syntax

TypeScript 3.9 added support for the `export * as ns` syntax from ECMAScript 2020, enabling more concise and readable module re-exports.

```typescript export * as utilities from “./utilities”; ```

JavaScript ES2020 introduced this syntax to simplify module re-exports. C# and Java handle module and namespace exports differently, typically through explicit class and interface exports. Python's module system uses import statements, but nothing akin to TypeScript's re-export syntax.

1. Uncalled Function Checks

TypeScript 3.9 introduced checks for uncalled functions within conditions, helping to catch common errors where a function reference is used instead of calling the function.

```typescript if (myFunction) { // Error if myFunction is intended to be called

 // ...

JavaScript runs this code without error but might not behave as

expected. C# and Java both have compile-time checks that can catch similar mistakes. Python's dynamic nature means it does not perform such checks at runtime.

1. Improvements to `?.` Operator

TypeScript 3.9 made improvements to the optional chaining operator (`?.`), aligning its behavior more closely with the ECMAScript standard and improving type checks.

```typescript let x = obj?.prop; ```

JavaScript ES2020 introduced optional chaining for handling undefined or null values gracefully. C# has the null-conditional operator that serves a similar purpose. Java and Python do not have built-in equivalents to this operator, requiring more verbose null checks.

1. Support for `export * as ns` from ECMAScript 2020

TypeScript 3.9 added full support for the `export * as ns` syntax, a feature from ECMAScript 2020 that allows for more expressive module exports.

```typescript export * as Utils from “./utils”; ```

This syntax is specific to ECMAScript modules and does not have direct equivalents in C#, Java, or Python, which have their own systems for modules and namespaces.

1. Conclusion

TypeScript 3.9 focused on enhancing the language's performance, developer experience, and type safety, with significant improvements in compilation speed, editor responsiveness, and static type checking. Through features like the `// @ts-expect-error` comment, improved `Promise.all` typing, and new import/export syntaxes, TypeScript continues to provide developers with powerful tools for building complex and robust applications. Comparing these features to similar ones in languages like C#, Java, and Python highlights TypeScript's unique position in leveraging JavaScript's dynamic capabilities while offering the benefits of static typing. The version's focus on both language features and tooling improvements ensures that TypeScript remains an indispensable tool for developers working in the diverse ecosystem of JavaScript and beyond.

GPT o1 Pro mode:

TypeScript Programming Language, TypeScript Compiler, TypeScript tsc Command, TypeScript Transpilation, TypeScript Type Annotation, TypeScript Interface, TypeScript Type Alias, TypeScript Enum, TypeScript Class, TypeScript Module, TypeScript Namespace, TypeScript Ambient Declaration, TypeScript d.ts File, TypeScript Declaration File, TypeScript tsconfig.json, TypeScript Strict Mode, TypeScript Strict Null Checks, TypeScript Non-Null Assertion Operator, TypeScript Type Inference, TypeScript Union Type, TypeScript Intersection Type, TypeScript Generics, TypeScript Generic Constraints, TypeScript Generic Default Type, TypeScript Partial Type, TypeScript Readonly Type, TypeScript Pick Type, TypeScript Omit Type, TypeScript Exclude Type, TypeScript Extract Type, TypeScript Record Type, TypeScript ReturnType Type, TypeScript Parameters Type, TypeScript ConstructorParameters Type, TypeScript InstanceType Type, TypeScript Required Type, TypeScript NonNullable Type, TypeScript ThisType, TypeScript Unknown Type, TypeScript Never Type, TypeScript Any Type, TypeScript Void Type, TypeScript Object Type, TypeScript Symbol Type, TypeScript Literal Type, TypeScript Template Literal Type, TypeScript Indexed Access Type, TypeScript Conditional Type, TypeScript Infer Keyword, TypeScript Mapped Type, TypeScript Keyof Keyword, TypeScript In Keyword (Types), TypeScript Extends Keyword (Types), TypeScript Structural Typing, TypeScript Duck Typing, TypeScript Nominal Typing (via branding), TypeScript Interface Merging, TypeScript Declaration Merging, TypeScript Module Augmentation, TypeScript Ambient Module, TypeScript Triple-Slash Directive, TypeScript Reference Path, TypeScript Reference Types, TypeScript Reference Lib, TypeScript Reference NoDefaultLib, TypeScript JSX, TypeScript TSX, TypeScript React JSX, TypeScript JSX.Element, TypeScript JSX.IntrinsicElements, TypeScript JSX.ElementClass, TypeScript Decorator, TypeScript Experimental Decorators, TypeScript Metadata Reflection API, TypeScript EmitDecoratorMetadata, TypeScript Decorator Factory, TypeScript Class Decorator, TypeScript Method Decorator, TypeScript Property Decorator, TypeScript Parameter Decorator, TypeScript Accessor Decorator, TypeScript Abstract Class, TypeScript Public Keyword, TypeScript Private Keyword, TypeScript Protected Keyword, TypeScript Readonly Keyword, TypeScript Static Keyword, TypeScript Implements Keyword, TypeScript Extends Keyword (Classes), TypeScript Super Keyword, TypeScript Constructor Parameter Properties, TypeScript Access Modifiers, TypeScript Index Signature, TypeScript Optional Chaining Operator, TypeScript Nullish Coalescing Operator, TypeScript Assertion Function, TypeScript Assertion Signature, TypeScript Type Assertion, TypeScript as Keyword, TypeScript Angle Bracket Assertion (Older Syntax), TypeScript Definite Assignment Assertion, TypeScript Discriminated Union, TypeScript Tagged Union, TypeScript Branded Type, TypeScript Intersection Property Distribution, TypeScript Function Type, TypeScript Call Signature, TypeScript Construct Signature, TypeScript Overloaded Function, TypeScript Rest Parameters, TypeScript Default Parameters, TypeScript Arrow Function Type, TypeScript Void Return Type, TypeScript Unknown Return Type, TypeScript Promise Type, TypeScript Async Function Type, TypeScript Awaited Type, TypeScript Generator Type, TypeScript Iterator Type, TypeScript Iterable Type, TypeScript IterableIterator Type, TypeScript AsyncIterable Type, TypeScript AsyncIterator Type, TypeScript AsyncIterableIterator Type, TypeScript Symbol.iterator, TypeScript Symbol.asyncIterator, TypeScript Symbol.hasInstance, TypeScript Symbol.species, TypeScript Symbol.toPrimitive, TypeScript Symbol.toStringTag, TypeScript Interface Inheritance, TypeScript Extending Interface, TypeScript Implementing Interface, TypeScript Hybrid Types, TypeScript Callable Interface, TypeScript Newable Interface, TypeScript Class Expression, TypeScript Abstract Method, TypeScript Abstract Property, TypeScript Abstract Constructor, TypeScript Class Fields, TypeScript Parameter Decorators, TypeScript Reflect Metadata, TypeScript JSDoc Annotations, TypeScript @type JSDoc Tag, TypeScript @typedef JSDoc Tag, TypeScript @template JSDoc Tag, TypeScript @param JSDoc Tag, TypeScript @returns JSDoc Tag, TypeScript StrictBindCallApply, TypeScript StrictFunctionTypes, TypeScript StrictPropertyInitialization, TypeScript StrictNullChecks Flag, TypeScript NoImplicitAny, TypeScript NoImplicitThis, TypeScript AlwaysStrict, TypeScript NoUnusedLocals, TypeScript NoUnusedParameters, TypeScript NoImplicitReturns, TypeScript NoFallthroughCasesInSwitch, TypeScript StrictUnknownChecks, TypeScript StrictBindCallApply, TypeScript NoImplicitOverride, TypeScript UseUnknownInCatchVariables, TypeScript ImportsNotUsedAsValues, TypeScript IsolatedModules, TypeScript PreserveSymlinks, TypeScript BaseUrl, TypeScript Paths Mapping, TypeScript RootDirs, TypeScript Composite Projects, TypeScript 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= require, TypeScript Type-Only Imports, TypeScript Type-Only Exports, TypeScript Export Type, TypeScript Import Type, TypeScript typeof Type Operator, TypeScript keyof Type Operator, TypeScript unique symbol, TypeScript Template Literal Type Inference, TypeScript Variadic Tuple Types, TypeScript Labeled Tuple Elements, TypeScript Template Literal Type, TypeScript Template Literal String, TypeScript Indexed Access Types, TypeScript Conditional Types Extended, TypeScript Distributed Conditional Types, TypeScript Recursive Conditional Types, TypeScript Tail Recursion Elimination, TypeScript Type Guards, TypeScript User-Defined Type Guard, TypeScript in Type Guards, TypeScript instanceof Type Guards, TypeScript typeof Type Guards, TypeScript Assertion Signatures, TypeScript Definite Assignment Assertions, TypeScript Control Flow Analysis, TypeScript Control Flow Based Type Analysis, TypeScript Control Flow Graph, TypeScript Flow Nodes, TypeScript Narrowing, TypeScript Type Predicates, 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resolveTypeReferenceDirective, TypeScript Custom Module Resolution, TypeScript Plugin API, TypeScript Compiler Plugin, TypeScript Language Service Plugin, TypeScript CompletionInfo, TypeScript CompletionEntry, TypeScript SignatureHelp, TypeScript QuickInfo, TypeScript DefinitionInfo, TypeScript ImplementationInfo, TypeScript CodeFixAction, TypeScript CodeFix, TypeScript RefactorInfo, TypeScript RefactorActionInfo, TypeScript NavigationBarItem, TypeScript NavigationTree, TypeScript RenameInfo, TypeScript RenameLocations, TypeScript FormattingOptions, TypeScript TextChange, TypeScript EditorSettings, TypeScript UserPreferences, TypeScript CompilerHost.getSourceFile, TypeScript CompilerHost.getDefaultLibFileName, TypeScript CompilerHost.writeFile, TypeScript resolveModuleNames Hook, TypeScript resolveTypeReferenceDirectives Hook, TypeScript writeFile Callback, TypeScript CustomTransformers, TypeScript TransformerFactory, TypeScript TransformerContext, TypeScript Visitor Function, TypeScript updateSourceFileNode, TypeScript updateNode, TypeScript Printer API, TypeScript createPrinter, TypeScript PrintHandlers, TypeScript PrintFile, TypeScript Type Format Flags, TypeScript Symbol Display Builder, TypeScript TypeToString, TypeScript SymbolToString, TypeScript Diagnostic Message, TypeScript Diagnostic Category, TypeScript Diagnostic Code, TypeScript DiagnosticRelatedInformation, TypeScript DiagnosticReporter, TypeScript EmitResult, TypeScript PreEmitDiagnostics, TypeScript SyntacticDiagnostics, TypeScript SemanticDiagnostics, TypeScript DeclarationDiagnostics, TypeScript getParsedCommandLineOfConfigFile, TypeScript parseJsonConfigFileContent, TypeScript convertCompilerOptionsFromJson, TypeScript resolveProjectReferencePath, TypeScript getAllProjectReferences, TypeScript LanguageServer Protocol Integration, TypeScript tsserver, TypeScript tsserverlibrary, TypeScript tsserver API, TypeScript tsserverplugin, TypeScript Editor Integration, TypeScript Sublime Text 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TypeScript Lib ES2018, TypeScript Lib ES2019, TypeScript Lib ES2020, TypeScript Lib ES2021, TypeScript Lib ES2022, TypeScript Lib DOM, TypeScript Lib Webworker, TypeScript Lib Webworker.Iterable, TypeScript Lib ScriptHost, TypeScript Lib DOM.Iterable, TypeScript Lib ESNext, TypeScript Target Option, TypeScript ModuleResolutionOption, TypeScript BaseUrl Option, TypeScript RootDirs Option, TypeScript Paths Option, TypeScript TypeRoots Option, TypeScript Types Option, TypeScript TraceResolution Option, TypeScript PreserveSymlinks Option, TypeScript AllowJs, TypeScript CheckJs, TypeScript Declaration, TypeScript DeclarationMap, TypeScript RemoveComments, TypeScript Incremental, TypeScript Composite, TypeScript InlineSourceMap, TypeScript InlineSources, TypeScript SourceMap, TypeScript IsolatedModules, TypeScript SkipLibCheck, TypeScript SkipDefaultLibCheck, TypeScript ForceConsistentCasingInFileNames, TypeScript RootDir, TypeScript OutDir, TypeScript OutFile, TypeScript AllowSyntheticDefaultImports, TypeScript ExperimentalDecorators, TypeScript EmitDecoratorMetadata, TypeScript DownlevelIteration, TypeScript StrictBindCallApply Option, TypeScript StrictPropertyInitialization Option, TypeScript UseUnknownInCatchVariables Option, TypeScript ImportsNotUsedAsValues Option, TypeScript PreserveConstEnums, TypeScript NoEmit, TypeScript NoEmitOnError, TypeScript ImportHelpers, TypeScript IsolatedModules Option, TypeScript Babel Plugin Transform TypeScript, TypeScript TypeDoc Integration, TypeScript API Extractor Integration, TypeScript Source Maps Support, TypeScript Decorators Metadata, TypeScript Experimental Flags, TypeScript Constraint Satisfaction, TypeScript Indexed Access Update, TypeScript Watch Mode Iteration, TypeScript watchFile Strategy, TypeScript watchDirectory Strategy, TypeScript incremental compilation cache, TypeScript composite build mode, TypeScript build reference, TypeScript output chunking, TypeScript declaration bundling, TypeScript path mapping resolution, TypeScript module specifier resolution, TypeScript import elision, TypeScript JSX factory, TypeScript JSX fragment factory, TypeScript JSX runtime, TypeScript preserve JSX, TypeScript react-jsx mode, TypeScript react-jsxdev mode, TypeScript isolated JSX namespace, TypeScript intrinsic element attribute types, TypeScript union type narrowing, TypeScript control flow narrowing, TypeScript discriminant property checks, TypeScript exhaustive switch checks, TypeScript unreachable code detection, TypeScript definite assignment checks, TypeScript optional property checks, TypeScript template literal type inference, TypeScript partial inference in generic, TypeScript tuple type inference, TypeScript tuple variance, TypeScript mapped type modifiers, TypeScript template literal type patterns, TypeScript conditional type inference patterns, TypeScript recursive type references, TypeScript tail-call optimization in types, TypeScript big integer literal types, TypeScript symbol literal types, TypeScript unique symbol types, TypeScript indexed access constraints, TypeScript type compatibility rules, TypeScript subtype and assignability, TypeScript assignability checks, TypeScript structural typing rules, TypeScript type alias circular references, TypeScript limit on instantiation depth, TypeScript error message hints, TypeScript suggestion diagnostics, TypeScript code actions, TypeScript refactorings, TypeScript organize imports, TypeScript rename symbol, TypeScript code fix all, TypeScript diagnostic tags, TypeScript unreachable code is error, TypeScript deprecated API warnings, TypeScript quick fixes for missed imports, TypeScript auto import suggestions, TypeScript auto fix on save, TypeScript incremental build watch, TypeScript composite project references, TypeScript solution style tsconfig, TypeScript project graph, TypeScript project dependency, TypeScript build order, TypeScript outDir redirect, TypeScript preserve watch output order, TypeScript advanced module resolution tracing, TypeScript plugin config in tsconfig, TypeScript plugin diagnostics, TypeScript plugin extra file extensions, TypeScript plugin language service extension, TypeScript server high memory usage, TypeScript server performance tracing, TypeScript server logVerbosity, TypeScript server cancellation tokens, TypeScript language server protocol features, TypeScript auto import from inferred project, TypeScript auto @ts-check in JS, TypeScript checkJs mode, TypeScript allowJs mode, TypeScript declaration and allowJs, TypeScript composite and allowJs, TypeScript incremental and composite flags

GPT 4o:

TypeScript: TypeScript Glossary, TypeScript Best Practices, Web Development Best Practices, JavaScript Best Practices, TypeScript Fundamentals, TypeScript Inventor - TypeScript Language Designer: Anders Hejlsberg of Microsoft on October 1, 2012; TypeScript Keywords, TypeScript Built-In Data Types, TypeScript Data Structures - TypeScript Algorithms, TypeScript Syntax, TypeScript on Linux, TypeScript on macOS, TypeScript on Windows, TypeScript on Android, TypeScript on iOS, TypeScript Installation, TypeScript Containerization (TypeScript with Docker, TypeScript with Podman, TypeScript and Kubernetes), TypeScript OOP - TypeScript Design Patterns, Clean TypeScript - TypeScript Style Guide, TypeScript Best Practices - TypeScript BDD, Web Browser, Web Development, HTML-CSS, TypeScript Frameworks (Angular), JavaScript Libraries (React.js with TypeScript, Vue.js with TypeScript, jQuery with TypeScript), TypeScript on the Server (TypeScript with Node.js, TypeScript with Deno, TypeScript with Express.js), TypeScript Compiler (tsc, tsconfig.json), TypeScript Transpiler (Transpile TypeScript into JavaScript), Babel and TypeScript, TypeScript Package Management, NPM and TypeScript, NVM and TypeScript, Yarn Package Manager and TypeScript, TypeScript IDEs (Visual Studio Code, Visual Studio, JetBrains WebStorm), TypeScript Development Tools, TypeScript Linter, TypeScript Debugging (Chrome DevTools, JavaScript Source Maps), TypeScript Testing (TypeScript TDD, Selenium, Jest, Mocha.js, Jasmine, Tape Testing (tap-producing test harness for Node.js and browsers), Supertest, React Testing Library, Enzyme.js React Testing, Angular TestBed), TypeScript DevOps - TypeScript SRE, TypeScript Data Science - TypeScript DataOps, TypeScript Machine Learning, TypeScript Deep Learning, Functional TypeScript, TypeScript Concurrency (WebAssembly - WASM) - TypeScript Async (TypeScript Await, TypeScript Promises, TypeScript Workers - Web Workers, Service Workers, Browser Main Thread), TypeScript Concurrency, TypeScript History, TypeScript Bibliography, Manning TypeScript Series, TypeScript Glossary - Glossaire de TypeScript - French, TypeScript T, TypeScript Courses, TypeScript Standard Library, TypeScript Libraries, TypeScript Frameworks (Angular), TypeScript Research, JavaScript, TypeScript GitHub, Written in TypeScript, TypeScript Popularity, TypeScript Awesome, TypeScript Versions. (navbar_typescript - see also navbar_javascript, navbar_typescript_networking, navbar_javascript_libraries, navbar_typescript_libraries, navbar_typescript_versions, navbar_typescript_standard_library, navbar_typescript_libraries, navbar_typescript_reserved_words, navbar_typescript_functional, navbar_typescript_concurrency, navbar_typescript_async, navbar_javascript_standard_library, navbar_react.js, navbar_angular, navbar_vue, navbar_javascript_standard_library, navbar_web_development)