Union types

What a session covers

The tutor works through these with you, in conversation and code. A session ends when you have shown them, not when a video does.

1. Explain what a union type means and give a case where it models a real choice
2. Write a function that takes string | number and narrows before using each member
3. Spot the error from calling a method only one member of the union has

In practice

A few lines that show Union types doing its job.

type Id = string | number;

function normalise(id: Id): string {
  return typeof id === "number" ? String(id) : id.trim();
}

type Loaded = { data: string[] } | { error: string };
function show(state: Loaded) {
  if ("error" in state) return state.error;
  return state.data.length + " items";
}

The common mistake

What to know first

Your roadmap puts these before Union types, unless you already know them.

• Interfaces and type aliases

How it stays learned

Once a session shows you understand Union types, it comes back for review. Each review takes a minute and tells the memory model how well you still know it.

1. Day 3First recall, while it is fresh
2. Day 7Second, a week on
3. Day 18Last, when it has settled

Where it leads

Topics that build on Union types.

• Overload signatures
• Literal types
• typeof and in guards