TS type declarations

third party JS libraries

• for libraries written in javascript, typescript can not tell their types, so you need to tell typescript about them.
• library @types provide type definitions (.d.ts files) for js libraries.

TypeScript third party libraries

• @types: contains ts declarations for js libraries.
• class-transformer: convert json to the corresponding classes.
• class-validator: validate the classes.

declare a variable

• for global variables, that you are sure they will be available at run time, you can use declare var statement.
• with declare, you don’t give the variable any value, instead you just declare its type.

declare var jQuery: any; // jQuery is a global variable, and tsc won't complain about it.
declare var global: any; // global is a global variable, and tsc won't complain about it.