TypeScript compiler options

A TypeScript file does not have one fixed meaning. The options in tsconfig.json decide which programs are valid and what JavaScript is written. This page covers the options that change the result for a given file. The TSConfig reference lists all of them. Defaults here are those of TypeScript 7.0.

Strictness

strict switches on a family of checks, and is true by default since 6.0. Each member can also be set separately, and defaults to the value of strict.

Option	Effect
`noImplicitAny`	An inferred `any` is an error
`noImplicitThis`	Using `this` with an unknown type is an error
`strictNullChecks`	`null` and `undefined` are their own types
`strictFunctionTypes`	Parameters of function types are checked contravariantly; methods stay bivariant
`strictBindCallApply`	`bind`, `call` and `apply` are checked against the function
`strictPropertyInitialization`	Class fields must be initialised
`strictBuiltinIteratorReturn`	Built-in iterators have `undefined`, not `any`, as the type of their return value
`useUnknownInCatchVariables`	A `catch` variable is `unknown`, not `any`

alwaysStrict used to control whether files were parsed in strict mode. All code is now strict, and false is an error in 7.0.

Other checks

These are not part of strict, and are off by default.

Option	Effect
`exactOptionalPropertyTypes`	An optional property cannot be assigned `undefined` unless its type says so
`noUncheckedIndexedAccess`	Reading through an index signature or an array adds `undefined` to the type
`noPropertyAccessFromIndexSignature`	Index-signature keys must be read with `o["k"]`
`noImplicitOverride`	An override must use `override`
`noImplicitReturns`	Every code path must return a value
`noFallthroughCasesInSwitch`	A non-empty `case` must end with a `break` or similar
`noUnusedLocals`, `noUnusedParameters`	Unused names are errors
`allowUnreachableCode`, `allowUnusedLabels`	Unset: editor warnings. `false`: errors. `true`: ignored
`noUncheckedSideEffectImports`	A side-effect import must resolve. On by default since 6.0

interface Options { port?: number }
const o: Options = { port: undefined };   // error with exactOptionalPropertyTypes

const list = [1];
const n: number = list[0];                // error with noUncheckedIndexedAccess

Two options that change what a type means

Language and environment

Option	Effect
`target`	The JavaScript version written, from `es2015` to `esnext`. Default `es2025`. Also sets the default `lib`
`lib`	Which standard library files to load
`noLib`	Loads no standard library
`jsx`	How JSX is written, and whether it is allowed
`experimentalDecorators`	Uses legacy decorators
`emitDecoratorMetadata`	Writes type metadata for legacy decorators
`useDefineForClassFields`	Class fields are defined by the ECMAScript rules. On for `target` ES2022 and later
`moduleDetection`	Which files are modules

Modules

Option	Effect
`module`	The output format. Default `esnext`
`moduleResolution`	How specifiers are found
`paths`, `rootDirs`, `customConditions`	Adjust resolution for type checking
`types`, `typeRoots`	Which `@types` packages are global. `types` defaults to `[]`
`allowImportingTsExtensions`, `rewriteRelativeImportExtensions`	Imports ending in `.ts`
`resolveJsonModule`	Allows importing `.json`
`allowArbitraryExtensions`	Allows importing other file types with a `.d.x.ts` beside them
`allowJs`, `checkJs`	Allow and check JavaScript files

Interop constraints

These restrict the code to what can be handled in a particular way. They are off by default.

Option	Effect
`verbatimModuleSyntax`	Keeps every `import` and `export` without `type`, so no elision. A type imported without `type` is an error. Implies `isolatedModules` (5.0)
`isolatedModules`	Rejects code that cannot be compiled one file at a time, such as re-exporting a type without `export type`, or using an ambient `const enum`
`isolatedDeclarations`	Requires annotations on exports (5.5)
`erasableSyntaxOnly`	Rejects constructs that generate code (5.8)
`forceConsistentCasingInFileNames`	Import paths must match the file’s case. On by default

// With verbatimModuleSyntax
import { User } from "./user.js";       // error: User is a type, use import type
import type { User } from "./user.js";  // fine, and removed from the output
import { type User, save } from "./user.js";   // becomes: import { save } from "./user.js";

// With isolatedModules
import { User } from "./user.js";
export { User };                        // error: use export type

Each of these exists so that a tool working on one file at a time can compile it

Emit

Option	Effect
`noEmit`	Checks only
`noEmitOnError`	Writes nothing if there are errors
`noCheck`	Writes output without a full type check (5.6)
`outDir`, `rootDir`	Where output goes. `rootDir` defaults to the folder of `tsconfig.json`
`declaration`, `declarationMap`	Writes declaration files
`sourceMap`, `inlineSourceMap`, `inlineSources`	Writes source maps, separately or in the JavaScript
`removeComments`	Drops comments from the output
`preserveConstEnums`	Writes `const enum` objects as well as inlining them
`importHelpers`	Imports helpers from the `tslib` package, not writing them in each file
`noEmitHelpers`	Writes no helpers at all
`newLine`, `emitBOM`	Line endings and a byte order mark in the output

Changes in 6.0 and 7.0

6.0 changed several defaults and deprecated many older settings, and 7.0, the native compiler written in Go, removed them.

Setting	Before 6.0	Since 6.0
`strict`	`false`	`true`
`module`	Depended on `target`	`esnext`
`target`	An old ECMAScript version	A recent version: `es2025` in 7.0
`types`	Every `@types` package	`[]`
`rootDir`	Computed from the inputs	The folder of `tsconfig.json`
`noUncheckedSideEffectImports`	`false`	`true`
`libReplacement`	`true`	`false`

Errors in 7.0, removed or no longer settable:

target: es5 and downlevelIteration.
moduleResolution: node (also called node10) and classic.
module: amd, umd, system and none.
baseUrl. paths entries are relative to the project.
esModuleInterop and allowSyntheticDefaultImports set to false, and alwaysStrict set to false.
The module keyword on a namespace, and assert on an import.
Files named on the command line in a folder with a tsconfig.json, without --ignoreConfig.

.TS File Format – Compiler options