Implicit and explicit data type assignment

Now, with TypeScript we have the ability to explicitly set data type for a value like this:

//script.ts 

let str: string = "hello";
console.log(str);

However, when we assign a value, even without explicitly declaring data type, TypeScript can implicitly set data type. This technique is called inferred typing. The following code is still valid:

//script.ts 

let str = "hello"; // type is "string"
console.log(str);

TypeScript will still show an error if we try to assign a new value that is not of the previous data type.

//script.ts 

let str = "hello";
str = 1; // Type 'number' is not assignable to type 'string'.
console.log(str);

So, in what cases would we need to explicitly set data type?

• if we not immediately assign a value to a variable when we declare it:

let str: string;
str = "hello";
console.log(str);

• for function input and output:

function f(s: string): string {

}

• inside object types (interfaces);
• inside classes.

Duck inferred typing

In more complex variable types if they are not explicitely set, TypeScript uses so-called "duck typing" method:

"If it looks like a duck, and quacks like a duck, then it probably is a duck"

const obj1 = {
  id: 1,
  name: "Mary",
  greet() {
    console.log(`Hello, ${this.name}!`);
  },
};

obj1.name = "Ann"; // Ok

obj1.age = 45; // Property 'age' does not exist on type '{ id: number; name: string; greet(): void; }'

TypeScript defined implicityle the type as { id: number; name: string; greet(): void; } and doesn't allow to assign new properties.

Primitive JavaScript types

Each primitive type in JavaScript has a corresponding type in TypeScript:

• number - floating-point numbers:

let a: number;

a = 123;
a = 123.0;
a = 0x7b; //hexadecimal notation
a = 0b1111011; //binary notation
a = 0.123e3; // exponential notation
a = Infinity;

• bigint - used for large integers (usually higher than 2^53-1 or lower than -(2^53-1)):

let a: bigint;

a = 9007199254740992n;
a = BigInt("9007199254740992");

• string - sequence of characters:

let a: string;

a = 'red';
a = "apple";
a = `${1+2} bears`;

• boolean - true of false:

let a: boolean;

a = true;
a = Boolean(0); //false

• symbol - special type that makes value unique:

let a: symbol;

a = Symbol('key');

Null and undefined

• null - indicates value, that's empty or non-existent and we don't expect it to change.
• undefined - works as a placeholder for a value of a variable, that was declared but has not yet been assigned to other value.

These two values in TypeScript have any data type. It means that we can change the value of the variable to any value.

let a = undefined; // data type is any

a = 1;

a = 'str';

If we explicitly set data type as null or undefined, the variable won't have the ability to change its value:

let a: undefined;

a = 1; // Type '1' is not assignable to type 'undefined'