TypeScript in 50 Lessons — Chapter 2a (8, 9, 10)

Join us this week for lessons 8, 9, and 10 of TypeScript in 50 Lessons. Lessons include:

• Lesson 8: Compiling TypeScript
• Lesson 9: any, Are You OK?
• Lesson 10: Control Flow

Come with questions concerns, and curiosity!

This is a short-term community group for owners of the book TypeScript in 50 Lessons. The only requirement is that you own the book.

• Amazaon — Kindle-only (affiliate link)
• Smashing Magazine — all formats

Notes

Terms

• type annotation, p71
• top type, p73
• emit, p76
• any, p76
• left-hand typing, p78
• right-hand typing, p78
• fixed-point, p83
• type guards, p84
• control flow analysis, p84
• narrowing down, p84
• subtypes, p86
• supertypes, p86
• unknown, p87

High-level takeaways

• any is regular JavaScript
  • You’ll see inference-based warnings
  • But these warnings are based exclusively on context, not intent
• unkown will require type narrowing depending on types used (return type, or methods used on variable)

// example of unkown, function parameter types, return types, nad type narrowing with type guards
function selectDeliveryAddress(addressOrIndex: unkown): string {
  if(typeof addressOrIndex === 'number' && addressOrIndex < deliveryAddresses.length) {
    return deliveryAddrress[addressOrIndex];
  }
  
  if(typeof addressOrIndex === 'string') {
    return addressOrIndex;
  }
  
  return '';
}

Discussion

• MC: Love the author’s transition here. TS is JS w/implicit any (top type). Then from implicit any to unkown, which actually helps you write proper guards
• DG: This is a mental model shift — seeing TS as progressive JS-tooling over a strongly typed language
• MC: Everything in JS is any because JS doesn’t restrict the calling of methods ever. unkown is the unlock for real-time type checking (type narrowing)
• TWM: It’s nice that the TS types don’t attempt to modify JS. But types like any and unkown accept JS for what it is
• DG: Right-hand typing is interesting, coming left-hand typing.
• FT: If you don’t have any types — and TS does not have to perform type inference — does tsc compile faster?
• MC:
• FT: any consistently shows up in tutorials a but rarely unknown. Why?
• MC: unknown is like a pair programmer in defensive programming
• TWM: What is unknown for? Feels like a good fit for external code, maybe second and third-party integration, etc.
• MC/FT: TypeScript is empathetic
• TWM: I dismissed TS for so long because of the “TS makes JS is a real language now” sentiment
• DG: The real world is messy. there will always be exceptions
• MC: I learned early on in my experience as a frontend architect that there will always be exceptions and unmigrated code. So many people think they have a solution that requires only that everyone else change. The most useful tools create a bridge from the present to the future.
• FT: Who didn’t we like TS earlier?
• MC: Progressive typing wasn’t a reality until Flow emerged
• TWM: We use types presence to evaluate library quality. Types can signify intent and/or community commitment

Speaker

• Michael Chan

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