Types

Main Primitives

• Boolean
• Number
• String

Explicit

let firstName: string = 'Matt';

Implicit

let firstName = 'Matt';

In some cases, type can be inferred from the value.

Any

Disables type checking. Unsafe.

let x = true;
x = 'string'; // Error: Type 'string' is not assignable
// to type 'boolean'.
Math.round(x); // Error: Argument of type 'boolean' is
// not assignable to parameter of type 'number'.

// Now with any
let y: any = true;
y = 'string'; // no error; can be "any" type
Math.round(y); // no error; can be "any" type

Unknown

Kind of like any, but not as unsafe. Better to use unknown over any if you don’t know the type.

let z: unknown = 1;
z = 'string'; // no error
z = {
  runANonExistentMethod: () => {
    console.log('I am running a non existent method');
  },
} as { runANonExistentMethod: () => void };
// In the absence of type information, what measures can
// we take to prevent the occurrence of the error in the
// code snippet that has been commented out?
// z.runANonExistentMethod(); // Error: Object is of type
// 'unknown'.
if (typeof z === 'object' && z !== null) {
  (z as { runANonExistentMethod: Function }).runANonExistentMethod();
}
// We cast multiple times, we can check in the if() if our
// type is secure and have a safer casting.

Never

Forces an error.

let a: never = false; // Error: Type 'boolean' is not
// assignable to type 'never'.

Null & Undefined

Primitives in JS, but also types in TS.

let b: undefined = undefined;
let c: null = null;

strictNullChecks must be enabled in tsconfig.json

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