This Is How I Mastered TypeScript Like I’m 5 (Type Inference!)(7)

This content originally appeared on DEV Community and was authored by Karandeep Singh

Today! We’re going to continue TypeScript learning like you’re a smart 5-year-old who loves to build things and asks “why?” (which is the best thing ever).

& yes “why?” is my way of learning.

I’ve divided this into 20 Chapters. and will go one by one and each will be of 2 – 3 min. of read.
This is a Continuation. if you have not read the Previous chapter – Chapter 6

Chapter 7: Type Inference – When TypeScript Thinks for You

Imagine This:

You say:

“Let’s play a game!”

And your friend immediately knows:

• It’s probably a board game
• Probably indoors
• Probably with snacks

You never told them all that, but they guessed from experience.

TypeScript does that too, it infers (guesses) the type when you don’t write it explicitly!

Example 1 – Simple variable

let age = 10;

You didn’t say : number, but TypeScript knows age is a number.

So later:

age = "ten"; // ❌ ERROR! You can’t put a string here!

Example 2 – From functions

function double(x: number) {
  return x * 2;
}

let result = double(5); // TypeScript infers result is number

Even though you didn’t write : number for result, TypeScript figured it out.

Example 3 – Arrays

let colors = ["red", "green", "blue"];
// Inferred as: string[]

colors.push("yellow"); // ✅
colors.push(123);      // ❌ Nope!

Example 4 – Object Inference

let user = {
  name: "Chatu",
  active: true,
};
// Inferred as: { name: string; active: boolean }

But Sometimes You Gotta Be Explicit

If you say:

let guess; // 🤔 What is this?
guess = 5;
guess = "hello";

TypeScript says: “Okay, I give up. I’ll treat it as any”

To avoid this, always assign immediately or give a type:

let guess: number;

Rule of Thumb:

Summary

• TS can guess types for you (this is inference)
• It works best when you assign a value right away
• For tricky or empty variables, tell TS yourself

If you enjoyed this and want to master TypeScript and other technologies, follow the series and drop a like!

I’m a passionate software developer sharing the cool things I discover, hoping they help you level up on your coding journey.

How i created my own State Management Library : Rethinking State Management in React — From a Dev, for Developers.

This content originally appeared on DEV Community and was authored by Karandeep Singh