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TypeScript Syntax

How do you type variables and functions and arrays and all that?

When writing TypeScript code, you type your variables with a colon : followed by the type.

Simple types

Here’s the syntax:

let name: string;

Here, we say the name variable has a string type. Which means, you can only assign values of that type to the variable:

let name: string;

name = "deeecode" // fine
namee = "typescript" // fine

Any attempt to assign something else to this type, you get a TypeScript error:

let name: string; 

/**/name/**/ = true
Type 'boolean' is not assignable to type 'string'.

Typing function parameters

You can also type function parameters:

function sum(num1: number, num2: number) {
  return num1 + num2
}

Now when you pass a different type, you get an error:

function sum(num1: number, num2: number) {
  return num1 + num2
}

sum(10, /**/"50"/**/)
Argument of type 'string' is not assignable to parameter of type 'number'.

Here, "50" is not a number and num2 is expected to be a number.

You can also type an array of items like this:

function sum(nums: number[]) {
  return nums.reduce((a, b) => a + b)
}

sum([2, 10, 100, /**/"hello"/**/])
Type 'string' is not assignable to type 'number'.

Here, we type nums to be “an array of numbers”. When we pass a string in the array, we get an error.

Return type of Function

You can also type the returned value of a function:

function sum(nums: number[]): number {
  return nums.reduce((a, b) => a + b)
}

sum([2, 10, 100])

By having : number after the parentheses, you specify the type of value that this function would return when called.

Also, with an arrow function:

function sum(nums: number[]): number {
  return nums.reduce((a, b) => a + b)
}

sum([2, 10, 100])

There are many more types you can use.