Nicholas Brown - TypeScript: JavaScript Development Guide

As you must have noticed, the type annotations are passed to the parameters of the function, and they are used for indicating the kind of parameters which are supported by the function. In the above example, the parameter shape is a string, while the parameters length and width are numbers. You have to know that both Type annotations and other TypeScript features are enforced during compile time. In case one passes other types of values to these parameters, a compile-time error will be displayed by the compiler. This is a very good idea when it comes to working with large applications. We need to demonstrate how errors will be detected in our case.

Consider the code given below: function area(shape: string, width: number, height: number) { var area = width * height; return "The shape is " + shape + " having an area of " + area + " cm squared."; } document.body.innerHTML = area("rectangle", "width", 15); // wrong type of width In the above example, the width has been passed in as a string, which is wrong. The compiler expects that the width should be a number. After compiling the program given above, you will get the following as the error: $ tsc type.ts type.ts(6,26): Supplied parameters do not match any signature of call target That is the kind of error you get. However, you must have noticed that although there was a warning due to type mismatch, this did not stop the TypeScript file from compiling and creating a JavaScript file. As shown in the above examples, it is very clear that annotations are used for the purpose of recording the function which a function or a variable is intended to perform. Suppose that you have a function which expects that only a single parameter should be passed to it, and the parameter should be of the string type.