TypeScript Cheatsheet

TypeScript

Install globally
npm install -g typescript

Within your npm project, run the following command to install the compiler:
npm install typescript --save-dev

The compiler is installed in the node_modules directory and can be run with:
npx tsc

Check Version
tsc --version

The compiler can be configured using a tsconfig.json file.

  target: es2016
  module: commonjs
  strict: true
  esModuleInterop: true
  skipLibCheck: true
  forceConsistentCasingInFileNames: true
        

{
  "include": ["src"],
  "compilerOptions": {
    "outDir": "./build"
  }
}
        

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

Primitives in TypeScript
boolean - true or false values
number - whole numbers and floating point values
string - text values like "TypeScript Rocks"
bigint - allows larger numbers
symbol - used to create a globally unique identifier

Explicit Type - writing out the type:
let firstName: string = "Dylan";

Implicit Type - TypeScript will "guess" the type, based on the assigned value:
let firstName = "Dylan";

Type: any
Setting any to the special type any disables type checking:
let v: any = true;
v = "string"; // no error
Math.round(v); // no error

Type: unknown
unknown is best used when you don't know the type of data being typed. To add a type later, you'll need to cast it.
let w: unknown = 1;
w = "string"; // no error
Casting is when we use the "as" keyword to say property or variable is of the casted type.

Type: never
never effectively throws an error whenever it is defined.
let x: never = true;

Type: undefined & null
let y: undefined = undefined;
let z: null = null;

Arrays
const names: string[] = [];
names.push("Dylan"); // no error
// names.push(3); // Error

Readonly
The readonly keyword can prevent arrays from being changed.
const names: readonly string[] = ["Dylan"];
// names.push("Jack"); // Error

Type Inference
TypeScript can infer the type of an array if it has values.
const numbers = [1, 2, 3]; // inferred to type number[]
numbers.push(4); // no error
// numbers.push("2"); // Error
let head: number = numbers[0]; // no error

Typed Arrays (Tuples)
A tuple is a typed array with a pre-defined length and types for each index.
let ourTuple: [number, boolean, string];
ourTuple = [5, false, 'Coding God was here'];

Readonly Tuple Example
const ourReadonlyTuple: readonly [number, boolean, string] = [5, true, 'The Real Coding God'];
// ourReadonlyTuple.push('Coding God took a day off'); // Error