TypeScript can inject the core library of ECMAScript into the produced code. By default, some libraries are automatically added. For example, if you specify a target of ES5, TypeScript adds the library: DOM, ES5, and ScriptHost. You can manually add an additional library. For example, if you would like to use iterable you can add the string ES2015.Iterable in the lib array. You can use a feature that is beyond your main target as well. For example, you can have a target of ES2015 and uses an ES2018 feature. See "target" as a main set of features to be included in the compilation and lib as a subset of additional features that you can add to the compilation.