TypeScript Foundations·Types that pay for themselves

Types and inference

TypeScript checks JavaScript before it runs and infers most types for you.

Intermediate 12 min read Lesson 6 of 300 Challenge included

What you will be able to do

Primitive annotations: string, number, boolean, null and undefined.
let widens a value type; const keeps a literal type.
Enable strict mode so null and implicit any cannot slip through.

In plain English

An annotation is a promise about a value. Inference is the compiler filling in that promise when the answer is obvious. Good TypeScript uses inference for locals and explicit types at public boundaries.

inference.tsTypeScript

let title = 'TypeScript'      // inferred: string
const level = 'beginner'       // inferred: "beginner"

function label(name: string, score: number): string {
  return name + ': ' + score
}

// label('Ada', 'high') // Error: argument of type string is not number

Your turn

Type a score formatter

Easy

Write a function formatScore(name, score) that returns a string and rejects a string score at compile time.

TS · starter

function formatScore(name, score) {
  return name + ': ' + score
}

Check your understanding before moving on.

Knowledge checkQuestion 1

What type does TypeScript infer for a const bound to the string "ready"?

Select an answer to continue

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Next: Interfaces and type aliases

Lesson 6 of 300 · 0% of course