Built-in types in Typescript

• Built-in Types in Typescript
  • The “any” type
  • Arrays
  • Tuples
  • Enum
  • Functions
    • A void function
    • A function that returns a number
    • A function that takes optional parameter.

Built-in Types in Typescript

The “any” type

If you declare but don’t initialise to anything, it becomes “any” type

let someValue;

This “any” type can be assigned to anything, but it is not recommended to use “any” type because it defeats purpose of Typescript

someValue = 1;
console.log(someValue); // => 1
someValue = "Typescript";
console.log(someValue); // => 'Typescript'

Arrays

let numbersArray: number[] = [1, 2, 3]; // Explicit declaration
let numbersArray1 = [1, 2, 3]; // Implicit declaration

When declaring empty array, you must explicitly declare the type. Otherwise it becomes “any” type array

let emptyArray:number[] = []

// Example function
let numbersArray3: number[] = [1, 2, 3, 4, 5];
let logValue = () => {
  numbersArray3.forEach((n) => {
    if (n < 4) console.log(n);
    else console.log(n.toExponential(2));
  });
};

logValue();

Tuples

A Fixed type array, used for assigning key/value pairs mostly. It is a single variable name, but has multiple types in the array, unlike tuples in other languages (c#) where each value in a tuple has its own variable name

// Declaration
let user: [number, string] = [33, "Arun"];

Typescript is smart enough to understand each value’s type, so we can perform appropriate actions on them. For example, the following is valid.

user[0].toFixed(1);
user[1].toLower();

The following code will throw error since the first value is defined as number, and hence any string specific operations are not possible.

user[0].toLower(); // throws 'Property 'toLower' does not exist on type 'number'.ts(2339)'

Enum

Enums are a set of named constants. enum Variable name should be in PascalCase, and so are its keys.

// Declaration
enum ShirtSize {
  Small,
  Medium,
  Large,
}

• By default, the first key in an enum gets assigned 0 as the value. But it can be changed
• In the declaration below, we assign Small as 1, and the compiler will automatically assign 2 and 3 for Medium / Large

  enum ShirtSize1 {
  Small = 1,
  Medium,
  Large,
  }
  

• If we need to assign any other value other than number, we need to explicitly declare for each key in enum

enum ShirtSizeWithStringValues {
  Small = "s",
  Medium = "m",
  Large = "l",
}

// Usage
console.log(ShirtSize.Small); // => 0
console.log(ShirtSize1.Medium); // => 2
console.log(ShirtSizeWithStringValues.Large); // => 'l'

Functions

Function is a block of code that does performs a.. function. They form the building block of any programming language

A void function

function calculateTaxReturn(income: number) {
  if (income < 10_000) console.log(income * 2.3);
  else console.log(income * 1.5);
}

A function that returns a number

• It is highly recommended to specify the return type in function declaration

function calculateTaxReturn1(income: number): number {
  if (income < 10_000) return income * 2.3;
  else return income * 1.5;
}

A function that takes optional parameter.

• The second param is optional in the following example

function calculateTaxReturnWithOptionalParam(
  income: number,
  taxOffset = 10_000
): number {
  if (taxOffset < 10_000) return income * 2.3;
  else return income * 1.5;
}

let taxReturn = calculateTaxReturnWithOptionalParam(2000); // 2000 for income & default 10_000 for taxOffset
let taxReturn1 = calculateTaxReturnWithOptionalParam(200_000, 3000); // 200_000 for income & 3000 as taxOffset