ts declare

TypeScript `declare` keyword is used to tell the compiler that a variable, function, or class is defined externally and will be available at runtime. It is typically used when you want to use code written in JavaScript or other libraries that do not have type definitions in TypeScript. By using `declare`, you can inform the compiler about the shape and type of the external code without actually implementing it in TypeScript.

For example, if you have a JavaScript library called "myLibrary" that defines a global variable called "myGlobalVar", you can use `declare` to inform TypeScript about its existence:

declare var myGlobalVar: string;

This tells TypeScript that `myGlobalVar` will be available at runtime and has the type `string`. You can then use `myGlobalVar` in your TypeScript code without any compilation errors.

Similarly, you can use `declare` with functions, classes, modules, or namespaces to provide type information for external code.

Note that `declare` only provides type information and does not emit any JavaScript code during the compilation process. It is mainly used for static type checking purposes.