TypeScript introduces a very simple syntax to check the type of an object at compile time. This syntax has been referred to as syntactic sugar, or more formally, type annotations. Consider the following version of our original JavaScript code, but written in TypeScript:

var test: string = "this is a string"; 
test = 1; 
test = function(a, b) { return a + b; } 

Note that on the first line of this code snippet, we have introduced a colon : and a string keyword between our variable and its assignment. This type annotation syntax means that we are setting the type of our variable to be of type string, and that any code that does not adhere to these rules will generate a compile error. Running the preceding code through the TypeScript compiler will generate two errors:

hello.ts(3,1): error TS2322: Type 'number' is not assignable to type 'string'.
hello.ts(4,1): error TS2322: Type '(a: any, b: any) => any' is not assignable to type 'string'.

The first error is fairly obvious. We have specified that the variable test is a string, and therefore attempting to assign a number to it will generate a compile error. The second error is similar to the first, and is in essence saying that we cannot assign a function to a string.

In this way, the TypeScript compiler introduces strong, or static typing to our JavaScript code, giving us all of the benefits of a strongly typed language. TypeScript is therefore described as a superset of JavaScript. We will explore this in more detail in Chapter 2, Types, Variables, and Function Techniques.