Note 3

Type Annotation in TypeScript

• TypeScript uses type annotations to explicitly specify types for identifiers such as variables, functions, and objects.
• TypeScript uses the syntax : type after an identifier as the type annotation, where type can be any valid type.
• Once an identifier is annotated with a type, it can be used as that type only. If the identifier is used as a different type, the TypeScript compiler will issue an error.

let variableName: type;
let variableName: type = value;
const constantName: type = value;

Also consider the following:

let counter: number;
counter = 1; // valid
counter = 'Hello'; // compilation error 

For Arrays:

let arrayName: type[];
let names: string[] = ['John', 'Jane', 'Peter', 'David', 'Mary'];
// specify the array is one of strings 

For objects:

let person: {
   name: string;
   age: number
};
 
person = {
   name: 'John',
   age: 25
}; // valid

This specifies the type of each parameter of the object.

Function Arguments and & Return Types

We can declare functions that set up the return and input type beforehand:

let greeting = (name: string) => string;
greeting = function(name: string) {
	return 'Hi ${name}';
};
 
 
// Something like the below would result in compilation error
greeting = function() {
	consolg.log('Hello');
};
 
// as it both does not match the input parameters, nor the return type

Summary

• Use type annotations with the syntax : [type] to explicitly specify a type for a variable, function, function return value, etc.