thinkingInTypescript.md

Thinking in TypeScript

by Godfrey Chan

Introduction

TypeScript can help us annotate and document our code signatures. Instead of trying to document everything in code comments, we can implement strict typing. For example:

let i: number;

i = 0; // Ok
i = "Hello"; // Error

// Define our own interface for what we expect
interface Options {
  margin: boolean;
}

let options: Options;

options = { margin: true }; // Ok
options = {}; // Error

// The `:` is used everywhere to denote types
function greetingsFor(name: string): string {
  return `Hello, ${name}!`;
}

undefined

In TypeScript 1, undefined and null were allowed to be assigned everywhere! In TypeScript 2, we now have strict null checks to prevent this.

// TypeScript 1
let i: number;

i = null; // Ok

// TypeScript 2
let i: number;

i = 1; // Ok
i = undefined; // Error

let k: number | undefined;

k = undefined; // Ok
k = 3; // Ok
k = null; // Error

Warning Signs: undefined and unexpected

Bare return's

Using return without a value - does the caller expect undefined?

Property Access

Accessing nonexistent properties on objects

Missing arguments

Not passing enough arguments to a function, will fill them with undefined

Void functions

Functions that don't return anything - does the caller expect a value?