path/to/example/node_modules/typescript/lib/lib.scripthost.d.ts This might be useful if you want to ensure no values are being accidentally imported, but still make side-effect imports explicit. This setting ensures consistency between accessing a field via the dot (obj.key) syntax, and indexed (obj["key"]) and the way which the property is declared in the type. Without this flag, using an export from a UMD module requires an import declaration. Print names of generated files part of the compilation to the terminal. All relative paths found in the configuration file will be resolved relative to the configuration file they originated in. base config file, and that circularity between configuration files is not allowed. ECMAScript 6 added several new iteration primitives: the for / of loop (for (el of arr)), Array spread ([a, b]), argument spread (fn(args)), and Symbol.iterator. Library referenced via 'webworker.importscripts' from file 'node_modules/typescript/lib/lib.d.ts' Element implicitly has an 'any' type because expression of type '"foo"' can't be used to index type '{ x: number; }'. This list may be out of date, you can see the full list in the TypeScript source code. Default library for target 'es5' update_deps.ts You can find documentation on TypeScript projects in the handbook. When checkJs is enabled then errors are reported in JavaScript files. For example, imagine you are modeling a music album syncing system: Then when you add support for machine-learning generated playlists, you refactor the Album class to have a setup function instead: In this case, TypeScript has provided no warning that download on SharedAlbum expected to override a function in the base class. ../helpers.js). assumeChangesOnlyAffectDirectDependencies, automatically include language service plugins, Core definitions for all ES3 and ES5 functionality, Additional APIs available in ES2015 (also known as ES6) -, Additional APIs available in ESNext - This changes as the JavaScript specification evolves, Will not add globals to your project (e.g, Will not have exports appear as auto-import recommendations. It is expected that if you use noLib you will be including your own type definitions for these. Specifies an allowlist of files to include in the program. tsconfig.json This option can be used on a per-file basis too similar to Babels /* @jsxFrag h */ directive. Cannot find module './settings.json'. .ts, .tsx, and .d.ts by default, with .js and .jsx if allowJs is set to true). You can find a set of community organized TSConfigs at tsconfig/bases, which has configurations for common platforms and their versions. of your application at runtime by emitting the enum value instead of a reference. In 3.7 the source of truth is now your TypeScript files. For example, that means packages within ./node_modules/@types/, ../node_modules/@types/, ../../node_modules/@types/, and so on. You may mix and match target and lib settings as desired, but you could just set target for convenience. By setting rootDir: "." generating a type for the import based on the static JSON shape. The directory structure of the original source files is preserved; see rootDir if the computed root is not what you intended. src work via those relative paths at runtime. If specified, .js (as well as .d.ts, .js.map, etc.) For example: This config file will include all packages under ./typings and ./vendor/types, and no packages from ./node_modules/@types. With emitDecoratorMetadata not set to true (default) the emitted JavaScript is: With emitDecoratorMetadata set to true the emitted JavaScript is: Enables experimental support for decorators, which is in stage 2 index.ts See this blog post by Jonathan New for a longer explanation. MyProj Skip type checking of default library declaration files. Fragment. The strategy for how individual files are watched. "auto": Treat files with imports, exports, import.meta, jsx (with jsx: react-jsx), or esm format (with module: node16+) as modules. But we hope you decide to come check us out. Some examples of code which does not work when isolatedModules is enabled. This flag controls how import works, there are 3 different options: remove: The default behavior of dropping import statements which only reference types. Whether to keep outdated console output in watch mode instead of clearing the screen every time a change happened. while accurate to the ES6 modules spec, most libraries with CommonJS/AMD/UMD modules didnt conform as strictly as TypeScripts implementation. Note the empty string "" in moduleSuffixes which is necessary for TypeScript to also look-up ./foo.ts. Turning on downlevelIteration and importHelpers is still false: Then turning on both downlevelIteration and importHelpers: You can use noEmitHelpers when you provide your own implementations of these functions. Works in tandem with allowJs. This disables reporting of excess property errors, such as the one shown in the following example: This flag was added to help people migrate to the stricter checking of new object literals in TypeScript 1.6. Requires either sourceMap or inlineSourceMap to be set. Turning suppressImplicitAnyIndexErrors on suppresses reporting the error about implicit anys when indexing into objects, as shown in the following example: Using suppressImplicitAnyIndexErrors is quite a drastic approach. types helpers.ts, src See how TypeScript improves day to day working with JavaScript with minimal additional syntax. This can be useful for projects which include other libraries in testing infrastructure which arent needed in the main application. For certain downleveling operations, TypeScript uses some helper code for operations like extending class, spreading arrays or objects, and async operations. b.ts These files allow debuggers and other tools to display the original TypeScript source code when actually working with the emitted JavaScript files. Specify the end of line sequence to be used when emitting files: CRLF (dos) or LF (unix). The strategy for how entire directory trees are watched under systems that lack recursive file-watching functionality. Other packages under node_modules/@types/* will not be included. tsconfig.json Come inside to our Social Lounge where the Seattle Freeze is just a myth and youll actually want to hang. When set, TypeScript will include the original content of the .ts file as an embedded string in the source map (using the source maps sourcesContent property). This setting disables the prologue. If the module youre importing defines its API using inherited properties, you need to use the default import form (import fs from "fs"), or disable esModuleInterop.

This section is mainly for handling case where fs.watch and fs.watchFile have additional constraints like on Linux. Type Acquisition is only important for JavaScript projects. Using a tsconfig.json which has explainFiles set to true: Running TypeScript against this folder would have output like this: This option is intended for debugging how a file has become a part of your compile. If you get tired of imports always looking like "../" or "./", or needing index.ts For example, lets say you have some input files: The inferred value for rootDir is the longest common path of all non-declaration input files, which in this case is core/. For example, take the following code: or code using Compiles to HTML languages like Svelte or Vue. All paths are relative to the tsconfig.json.

TypeScript will unify type parameters when comparing two generic functions. preserve: Preserves all import statements whose values or types are never used. The path may use Node.js style resolution. Then enable building it with inlineSourceMap enabled there is a comment at the bottom of the file which includes When strictNullChecks is true, null and undefined have their own distinct types and youll get a type error if you try to use them where a concrete value is expected. This file can be opened in a chromium based browser like Chrome or Edge Developer in the CPU profiler section. TypeScript introduced class fields many years before it was ratified in TC39. Even though it feels like it should. By default (with esModuleInterop false or not set) TypeScript treats CommonJS/AMD/UMD modules similar to ES6 modules. Note if using TypeScript 4.2, prefer explainFiles which offers an explanation of why a file was added too. If module is system or amd, all module files will also be concatenated into this file after all global content. package.json In TypeScript, when you reference a const enum member, the reference is replaced by its actual value in the emitted JavaScript. Matched by include pattern '**/*' in 'tsconfig.json', npm run tsc --generateCpuProfile tsc-output.cpuprofile, $ npm run tsc This makes room for another tool like Babel, or swc to handle converting the TypeScript file to a file which can run inside a JavaScript environment. This member must have an 'override' modifier because it overrides a member in the base class 'Album'. While you can use TypeScript to produce JavaScript code from TypeScript code, its also common to use other transpilers such as Babel to do this. tsconfig.json Note: outFile cannot be used unless module is None, System, or AMD. b.ts Given the above configuration, an import like the following: TypeScript will look for the relative files ./foo.ios.ts, ./foo.native.ts, and finally ./foo.ts. Controls whether TypeScript will emit a byte order mark (BOM) when writing output files. These .d.ts files are type definition files which describe the external API of your module. Changes the function called in .js files when compiling JSX Elements using the classic JSX runtime. This flag switches to the upcoming ECMA runtime behavior. For this reason, rootDir also enforces that all files which need to be emitted are underneath the rootDir path. Type '(x: string) => void' is not assignable to type 'StringOrNumberFunc'. In TypeScript, you can import a type and then subsequently export it: Because theres no value for someType, the emitted export will not try to export it (this would be a runtime error in JavaScript): Single-file transpilers dont know whether someType produces a value or not, so its an error to export a name that only refers to a type. E.g. downlevelIteration allows for these iteration primitives to be used more accurately in ES5 environments if a Symbol.iterator implementation is present. This can be problematic if some developers are working in a case-sensitive file system and others arent. Instead of giving a small timeout to allow for potentially multiple edits to occur on a file. itself, under --build mode) to quickly determine if a project has been built yet. Library referenced via 'es5' from file 'node_modules/typescript/lib/lib.d.ts' This can save time during compilation at the expense of type-system accuracy. a source-map for the file. When this option is set, TypeScript will issue an error if a program tries to include a file by a casing different from the casing on disk. Allows importing modules with a .json extension, which is a common practice in node projects. Do not emit declarations for code that has an @internal annotation in its JSDoc comment. Types of parameters 'x' and 'ns' are incompatible. Without this flag enabled, there are three values which you can set colorThemeOverride to be: dark, light and undefined. Explore how TypeScript extends JavaScript to add more safety and tooling. Specifies an array of filenames or patterns to include in the program. Heres some example output for this file: If you are wondering about the difference between ES2015 (aka ES6) and ES2020, ES2020 adds support for dynamic imports, and import.meta. With React 17 the library supports a new form of JSX transformation via a separate import. For example with this TypeScript code, users.find has no guarantee that it will actually find a user, but you can For example, this is a TypeScript file which has a JSDoc comment: Without setting removeComments or having it as false: This means that your comments will show up in the JavaScript code. Consider using '--resolveJsonModule' to import module with '.json' extension. You can use include to specify which types should be used from DefinitelyTyped: Offers a config for disabling the type-acquisition for a certain module in JavaScript projects. For example, this is incorrect JavaScript according to the parseFloat type definition which comes with TypeScript: You will not get any errors. path/to/example/node_modules/typescript/lib/lib.es5.d.ts Source map files are emitted as .js.map (or .jsx.map) files next to the corresponding .js output file. Allowing for code like: This pattern ensures that error handling code becomes more comprehensive because you cannot guarantee that the object being thrown is a Error subclass ahead of time. Parameter 's' implicitly has an 'any' type. This flag works because you can use import type to explicitly create an import statement which should never be emitted into JavaScript. However, import statements are still checked to see if they resolve to a valid module, so youll need to make sure this is satisfied by some other means. A module file is a file that has imports and/or exports. This condition will always return 'false' since the types 'boolean' and 'number' have no overlap. Controls how JSX constructs are emitted in JavaScript files. If noResolve is set, this process doesnt happen. for this inside the function inside getAreaFunction is not the instance of the Rectangle. Defaults to false. The following example has noImplicitOverride enabled, and you can see the error received when override is missing: When enabled, TypeScript will check all code paths in a function to ensure they return a value. This will allow editors such as VS Code to go to the original .ts file when using features like Go to Definition. This only affects modules; global script files will not attempt to import modules. In TypeScript 4.0, support was added to allow changing the type of the variable in a catch clause from any to unknown. ECMAScript strict mode was introduced in ES5 and provides behavior tweaks to the runtime of the JavaScript engine to improve performance, and makes a set of errors throw instead of silently ignoring them. You are not sure that TypeScript has included a file you expected, as a part of debugging the. This defaults to false, making it easier to work with TypeScript in a watch-like environment where you may want to see results of changes to your code in another environment before making sure all errors are resolved. node_modules/typescript/lib/lib.es5.d.ts However, if you turn on checkJs then you will get error messages from the JavaScript file. Report errors on unused parameters in functions. baseUrl This option does not affect how @types/* are included in your application code, for example if you had the above compilerOptions example with code like: When you have this option set, by not including a module in the types array it: This feature differs from typeRoots in that it is about specifying only the exact types you want included, whereas typeRoots supports saying you want particular folders. Turning this on is equivalent to enabling all of the strict mode family options, which are outlined below. For example, in the following code TitleComponent is a function and TitleComponentProps is a type with isolatedModules and preserveValueImports are enabled: Which can be fixed by prefixing TitleComponentProps with type to mark it as a type-only import: Strips all comments from TypeScript files when converting into JavaScript. Type 'U' is not assignable to type 'T'. You can define a root folder where you can do absolute file resolution. Allow JavaScript files to be imported inside your project, instead of just .ts and .tsx files. Note that the end is different from the example in inlineSourceMap because the source-map now contains the original source code also. This option is retained for backward compatibility only and is deprecated. generated This option offers a way to configure the place where TypeScript keeps track of the files it stores on the disk to This can lead to unexpected errors at runtime. This option can be used on a per-file basis too similar to Babels /** @jsx h */ directive. The goal of this flag is to signal intent in your calling syntax about how certain you are this property exists. Rather than doing a full check of all d.ts files, TypeScript When downlevelIteration is enabled, TypeScript will use a helper function that checks for a Symbol.iterator implementation (either native or polyfill). By default all visible @types packages are included in your compilation. Without this flag, TypeScript will allow you to use the dot syntax to access fields which are not defined: Turning the flag on will raise an error because the unknown field uses dot syntax instead of indexed syntax.

If the importHelpers flag is on, these helper functions are instead imported from the tslib module. This restriction also applies to TypeScripts ts.transpileModule API which is used by some build tools. By default, when emitting a module file to a non-ES6 target, TypeScript emits a "use strict"; prologue at the top of the file. Turning on esModuleInterop will fix both of these problems in the code transpiled by TypeScript. You very likely want "CommonJS" for node projects. Print names of files part of the compilation. This feature can be useful for React Native projects where each target platform can use a separate tsconfig.json with differing moduleSuffixes. a.ts Function lacks ending return statement and return type does not include 'undefined'. This is useful when you are not sure that TypeScript has view1.ts (can import "./template1", "./view2`) setting TypeScripts preserveSymlinks to true parallels setting Webpacks resolve.symlinks to false, and vice-versa). If a file attempts to import fileManager.ts by specifying ./FileManager.ts the file will be found in a case-insensitive file system, but not on a case-sensitive file system. When set, TypeScript will check that the built-in methods of functions call, bind, and apply are invoked with correct argument for the underlying function: Otherwise, these functions accept any arguments and will return any: When enabled, this flag causes functions parameters to be checked more correctly. The default const enum behavior is to convert any Album.Something to the corresponding number literal, and to remove a reference Used to output diagnostic information for debugging. The factory chosen will also affect where the JSX namespace is looked up (for type checking information) before falling back to the global one. This does not affect how TypeScript emits JavaScript, it only emulates the assumption that they will be able to Normally, TypeScript would return silently on success. With preserveConstEnums set to true, the enum exists at runtime and the numbers are still emitted. prefix. Then enable building it with inlineSources and inlineSourceMap enabled there is a comment at the bottom of the file which includes in tsconfig.json, TypeScript would write this tree: Importantly, rootDir does not affect which files become part of the compilation. If this constraint is violated, tsc will inform you which files werent specified. When combined with isolatedModules: imported types must be marked as type-only because compilers that process single files at a time have no way of knowing whether imports are values that appear unused, or a type that must be removed in order to avoid a runtime crash. Default: The longest common path of all non-declaration input files. This option gives you the chance to have TypeScript emit a v8 CPU profile during the compiler run. true if isolatedModules,,false otherwise. a.js Type '{ x: number; y: number; m: number; }' is not assignable to type 'Point'. creates a series of .tsbuildinfo files in the same folder as your compilation output. You can use excludeFiles to drastically reduce the number of files which are watched during --watch. views Currently, the only top-level property that is excluded from inheritance is references. Do not emit compiler output files like JavaScript source code, source-maps or declarations. Instead, projects are loaded dynamically as you open files through your editor. Another possibility is when you are migrating between TypeScript releases and the changes cause breakages in node_modules and the JS standard libraries which you do not want to deal with during the TypeScript update. For example ./src/index.ts can import the file ./src/css/main.css and TypeScript will be aware of the bundlers behavior for that filetype via the corresponding generated declaration file. You can use tslib via importHelpers to reduce the amount of inline JavaScript too: Note: enabling downlevelIteration does not improve compliance if Symbol.iterator is not present in the runtime. TypeScript includes a default set of type definitions for built-in JS APIs (like Math), as well as type definitions for things found in browser environments (like document).

These options make up the bulk of TypeScripts configuration and it covers how the language should work. Provides a way to override the default list of file name suffixes to search when resolving a module. index.ts With exactOptionalPropertyTypes enabled, TypeScript applies stricter rules around how it handles properties on type or interfaces which have a ? This flag is something you can use to increase responsiveness in large composite projects.

For example, this JS file: When imported into a TypeScript file will raise an error: This flag can be used as a way to incrementally add TypeScript files into JS projects by allowing the .ts and .tsx files to live along-side existing JavaScript files. the ES6 modules spec states that a namespace import (import * as x) can only be an object, by having TypeScript Object literal may only specify known properties, and 'm' does not exist in type 'Point'. The target setting changes which JS features are downleveled and which are left intact. This means that the implementation version in TypeScript may differ from the implementation in JavaScript when it it decided by TC39. Library referenced via 'scripthost' from file 'node_modules/typescript/lib/lib.d.ts' In this case, you can tell the TypeScript file resolver to support a number of custom prefixes to find code. You will need to ensure that the tslib module is able to be imported at runtime. This is useful when you only have a small number of files and dont need to use a glob to reference many files. 'this' implicitly has type 'any' because it does not have a type annotation.

a.ts The rootDir setting, if not explicitly set, defaults to the directory containing the tsconfig.json file. Do not erase const enum declarations in generated code. TypeScript cannot compile anything without a set of interfaces for key primitives like: Array, Boolean, Function, IArguments, Number, Object, RegExp, and String.

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