https://stackoverflow.com/questions/35019987/what-does-declare-do-in-export-declare-class-actions
var creates a new variable. declare is used to tell TypeScript that the variable has been created elsewhere. If you use declare, nothing is added to the JavaScript that is generated - it is simply a hint to the compiler.
The TypeScript declare keyword is used to declare variables that may not have originated from a TypeScript file.
The class’s real implementation is probably in somewhere else—maybe a .js file.
