It is possible to use declare instead of one of the three declarators let,const, and var. However, declare won't generate any JavaScript code during compilation and must be used in conjunction with let, const or var. The role of declare is to indicate to TypeScript's compiler that the variable exists but is defined somewhere else:

declare let myVariable: string;

The main role of declare is when signatures need to be defined. The use of declare in a definition file makes sense because it is only defining the type and not actually declaring the variable.

declare can be used to declare a module. Declaring a module is used to write a definition file outside the actual implementation of the code, which is written in JavaScript or TypeScript:

declare module "messageformat" {
}