Deprecated in favor of verbatimModuleSyntax.

There are some cases where TypeScript can't detect that you're using an import. For example, take the following code:

import { Animal } from "./animal.js";

eval("console.log(new Animal().isDangerous())");

import { Animal } from "./animal.js";

eval("console.log(new Animal().isDangerous())");

or code using 'Compiles to HTML' languages like Svelte or Vue. preserveValueImports will prevent TypeScript from removing the import, even if it appears unused.

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.