Recently, I started writing some TypeScript definition files from my team. These definition files addressed the problem that some old common packages writing in JavaScript are missing TypeScript support. Meanwhile, our team is migrating projects from JavaScript to TypeScript incrementally.<\/p>\n\n\n\n
TypeScript definition file (.d.ts) is like an add-on for JavaScript files that can provide TypeScript definitions without changing the original file. We call declarations that don\u2019t define an implementation \u201cambient\u201d[1]<\/sup>. To write an ambient module from an existing module, use When writing an ambient module, there are serveral points you need to take care of.<\/p>\n\n\n\n It’s strange but I found VSCode and WebStorm sometimes have different behaviors treating the same definition file. Test from both IDEs to make sure your definition works.<\/p>\n\n\n\n You can’t use declare modul<\/code>e keyword. For example.<\/p>\n\n\n\ndeclare module \"lodash\" {\n export function uniq<T>(data: T[]) => T[];\n}<\/code><\/pre>\n\n\n\nimport<\/code> outside the module<\/li><\/ul>\n\n\n\nimport<\/code> outside of the declare module<\/code> block. This will cause TypeScript treating the file as a normal module rather than an ambient module[2]<\/sup>. The right way to reference types from other modules is<\/p>\n\n\n\ndeclare module \"lodash\" {\n import { Request, Response } from \"express\";\n export interface CustomRequest extends Request {\n \/\/ ....\n }\n}<\/code><\/pre>\n\n\n\n