When initializing a variable, if the TypeScript compiler can correctly infer the type of the assigned value, it is not necessary to explicitly define the type. For example, there is no need to specify the type when using const num = 5. However, in some cases, specifying the type is essential, such as when declaring a variable that will be assigned a value later:
let someData: MyData;
If the type is not specified in the above declaration, someData will default to being of type any, which compromises type safety.
Another scenario where explicit typing would be beneficial is when creating an array that will have elements added to it at a later point:
const arr: number[] = [];
// later:
arr.push(5);
Since the compiler cannot determine the possible values in the array at its declaration, it is important to include number[] for arr. Omitting this would result in the array being typed as any[], which is not type-safe and should be avoided.
In cases where TypeScript can accurately infer the variable type automatically - which is often the case - omitting the explicit type annotation is preferred as it does not provide extra value.
It's worth noting that using the keyword declare serves a distinct purpose - it informs TypeScript that a variable with the specified name exists in the current scope due to external code, like a library. Variables declared with declare will not be present in the compiled output. Avoid using declare unless you are referencing variables defined outside the TypeScript context.