With the --noImplicitThis flag set, then the first line will fail to compile, with an error: 'this' implicitly has type 'any' because it does not have a ty

With the --noImplicitThis flag set, then the first line will fail to compile, with an error: 'this' implicitly has type 'any' because it does not have a type annotation. That error can be fixed by specifying an explicit type annotation for this: function example(this: { v: string }) { console.log(this.v); }; However, the type checking for apply is still too weak to produce any useful errors, as even with the above type annotation, this will still compile: example.apply({ b: 3 }) The type of apply is currently: Function.apply(this: Function, thisArg: any, argArray?: any): any It accepts basically any arguments. In future perhaps they will make this stricter, more discussion here: https://github.com/Microsoft/TypeScript/issues/212 https://github.com/Microsoft/TypeScript/issues/212