TypeScript Simple Types

There are three main primitives in JavaScript and TypeScript:

• boolean - true or false values
• number - whole numbers and floating point values
• string - text values like "TypeScript Rocks"

Type Assignment

When creating a variable, there are two main ways TypeScript assigns a type:

• Explicit
• Implicit

Explict Type

Explicit - writing out the type:

let firstName: string = "Dylan";

Implicit Type

Implicit - TypeScript will "guess" the type, based on the assigned value:

let firstName = "Dylan";

Error In Type Assignment

TypeScript will throw an error if data types do not match.

let firstName: string = "Dylan"; // type string
firstName = 33; // attempts to re-assign the value to a different type

let firstName = "Dylan"; // inferred to type string
firstName = 33; // attempts to re-assign the value to a different type

Unable to Infer

TypeScript may not always properly infer what the type of a variable may be. In such cases, it will set the type to 'any' which disables type checking.

// Implicit any as JSON.parse doesn't know what type of data it returns so it can be "any" thing...
const json = JSON.parse("55");
// Most expect json to be an object, but it can be a string or a number like this example
console.log(typeof json);

This behavior can be disabled by enabling noImplicitAny as an option in a TypeScript's project tsconfig.json.