Modern Front-End Development for Rails

Adding Type Annotations to Variables

That last section was a little on the abstract side, so let’s talk more concretely.

TypeScript allows you, but does not require you, to annotate your JavaScript code with information about the types of various pieces of data in the code. TypeScript uses this information as a constraint on the values that those pieces of data can contain. If you declare a variable to be a number and then assign it the string value "3", the TypeScript compiler will complain and will not allow the code to be compiled.

If you don’t add any explicit type information, however, you don’t get the full benefit of using TypeScript. It will still catch some errors, such as assigning two different types of values to the same variable ...