The target setting changes which JS features are downleveled and which are left intact.įor example, an arrow function () => this will be turned into an equivalent function expression if target is ES5 or lower.Ĭhanging target also changes the default value of lib. You might choose to set a lower target if your code is deployed to older environments, or a higher target if your code is guaranteed to run in newer environments. Modern browsers support all ES6 features, so ES6 is a good choice. This list may be out of date, you can see the full list in the TypeScript source code. High Level libraries NameĬore definitions for all ES3 and ES5 functionalityĪdditional APIs available in ES2015 (also known as ES6) - array.find, Promise, Proxy, Symbol, Map, Set, Reflect, etc.Īdditional APIs available in ES2016 - array.include, etc.Īdditional APIs available in ES2017 - Object.entries, Object.values, Atomics, SharedArrayBuffer, date.formatToParts, typed arrays, etc.Īdditional APIs available in ES2018 - async iterables, promise.finally, Intl.PluralRules, oups, etc.Īdditional APIs available in ES2019 - array.flat, array.flatMap, omEntries, imStart, imEnd, etc.Īdditional APIs available in ES2020 - string.matchAll, etc.Īdditional APIs available in ES2021 - promise.any, string.replaceAll etc.Īdditional APIs available in ES2022 - array.at, RegExp.hasIndices, etc.Īdditional APIs available in ESNext - This changes as the JavaScript specification evolvesĪPIs for the Windows Script Hosting System In TypeScript 4.5, lib files can be overridden by npm modules, find out more in the blog. You have polyfills or native implementations for some, but not all, of a higher level ECMAScript version.Your runtime platform provides certain JavaScript API objects (maybe through polyfills), but doesn’t yet support the full syntax of a given ECMAScript version.Your program doesn’t run in a browser, so you don’t want the "dom" type definitions.You may want to change these for a few reasons: TypeScript also includes APIs for newer JS features matching the target you specify for example the definition for Map is available if target is ES6 or newer. 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). The first changes the behavior in the compiler, the second is fixed by two new helper functions which provide a shim to ensure compatibility in the emitted JavaScript: Turning on esModuleInterop will fix both of these problems in the code transpiled by TypeScript. While accurate to the ES6 modules spec, most libraries with CommonJS/AMD/UMD modules didn’t conform as strictly as TypeScript’s implementation. Treating it the same as = require("x") then TypeScript allowed for the import to be treated as a function and be callable. The ES6 modules spec states that a namespace import ( import * as x) can only be an object, by having TypeScript In doing this, there are two parts in particular which turned out to be flawed assumptions:Ī namespace import like import * as moment from "moment" acts the same as const moment = require("moment")Ī default import like import moment from "moment" acts the same as const moment = require("moment").default # Allow Importing TS Extensions - allowImportingTsExtensionsīy default (with esModuleInterop false or not set) TypeScript treats CommonJS/AMD/UMD modules similar to ES6 modules. Strictly speaking, the former is interpreted as a declaration file for a JavaScript file named .īecause relative files imports need to include extensions in Node’s ESM support, TypeScript would error on our example in an ESM file under -moduleResolution node16 or nodenext.įor more information, read up the proposal for this feature and its corresponding pull request. Note that historically, a similar effect has often been achievable by adding a declaration file named .ts instead of app.d.css.ts - however, this just worked through Node’s require resolution rules for CommonJS. By default, this import will raise an error to let you know that TypeScript doesn’t understand this file type and your runtime might not support importing it.īut if you’ve configured your runtime or bundler to handle it, you can suppress the error with the new -allowArbitraryExtensions compiler option.
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