declare keyword is used to declare variables that may not have originated from a TypeScript file.
For example, lets imagine that we have a library called
myLibrary that doesn’t have a TypeScript declaration file and have a namespace called
myLibrary in the global namespace. If you want to use that library in your TypeScript code, you can use the following code:
declare var myLibrary;
The type that the TypeScript runtime will give to
myLibrary variable is the
any type. The problem here is that you won’t have Intellisense for that variable in design time but you will be able to use the library in your code. Another option to have the same behavior without using the declare keyword is just using a variable with the any type:
var myLibrary: any;
declare example is more readable and expresses an ambient declaration.