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02-variables-and-values.md

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Variables and Values

In JavaScript we declare variables all the time with let and const like this:

let age = 7; // let age: number;

As we can see, TypeScript is able to infer that age is a number, because we have initialized it with a value of 7 (number)

If we try to give age a value that is incompatible with number, we get an error.

let age = 6;
age = "not a number"; // ERROR:
  • We got an error in the code above: "Type string is not assignable to type number".

Moreover, in TypeScript, variables are "born" with their types.

Const

Let's try with const:

const age = 6; // const age: 6

In this case type of age is not number it is 6. TypeScript is able to make a more specific assumption here, because:

  • We cannot reassign const variable declaration
  • the initial value assigned to age is a number, which is an immutable value type To sum up, value of age is 6 and will always be 6 in this program.

Literal Types

The 6 is called literal type.

Take a look at this below:

let age: 6;
age = 6; // Valid
age = 10; // Invalid - only the literal value 6 is allowed

In this example above, the variable age is declared with a type of 6. This means that age can only be assigned the literal value 6 and no other values are allowed. If you try to assign any other value, TypeScript will raise an error.

Btw

It means that when we are using let we can reassign a variable only what type it is!

let age = 10;
age = 11;
age = 12;
age = 13;

As we can see, we are reassigning age with only type number

Implicit any and type annotation

Any - In TypeScript, the any type is a special type that represents a dynamic or untyped value. Variables of type any can hold values of any type, and TypeScript does not perform type-checking or provide type inference for any-typed variables.

let message;

message = "Hello"; // Valid - assigned a string
message = 42; // Valid - assigned a number
message = true; // Valid - assigned a boolean

// You can perform any operation on a variable of type `any`
console.log(message.foo()); // No type checking error, even if `foo` doesn't exist

// Variables with inferred `any` type can be used in type-incompatible ways
let length: number = message; // No type error, even though `message` can hold any type

Function arguments and return values

function add(a, b) {
  return a + b; // strings? numbers? a mix?
}

Here what our in-editor tooltip would look like if we were using this function:

const result = add(3, "4")
                    // function add(a: any, b: any): any

console.log(result) // const result: any

Addition

function add(a: number, b: number) {
  return a + b; 
}

const result = add(3, "4")

ERROR: Argument of type "string" is not assignable to parameter of type "number"