Is it only possible to get IntelliSense in TypeScript files by referencing .ts files with own coded interfaces?
Is there a solution for existing JavaScript libraries?
You are able to get IntelliSense for other TypeScript files by using an external script reference directive at the top of your script:
///<reference path="someOtherScript.ts" />
As a side note, the TypeScript IntelliSense reference directive doesn't support the tilde operator like the JavaScript reference directive does. For example, if your script is located in "~/Scripts/foo/", in JavaScript you can reference:
///<reference path="~/Scripts/otherScriptFile.js" />
whereas in TypeScript you have to reference relative to the current file:
///<reference path="../otherScriptFile.ts" />
More info about this can be found in section 11.1.1 Source Files Dependencies of the TypeScript Spec.
With regard to JavaScript IntelliSense in a TypeScript file, it currently appears to be not possible to get JavaScript reference IntelliSense.