Infer is easier than you think

TypeScript's infer keyword is MUCH narrower, much less widely used than you think it is. It has ONE use case, and we cover it in a ridiculous amount of depth here.
Become a TypeScript Wizard with Matt's upcoming TypeScript Course:
www.totaltypescript.com/
Follow Matt on Twitter
/ mattpocockuk

[@nyambe]

You really are the Typescript wizard.

[@marsinearth1264]

Indeed he is

[@vikingthedude]

Harry

[@johnyepthomi892]

He filters out BS and get to the point in the clearest way possible. Communication is key.

[@jeralm]

I already considered myself an advanced TypeScript dev, but a good understanding of infer was missing from my arsenal. Thank you so much for this.

[@ivan_dramaliev]

To me, the most intuitive mental model for types (in TypeScript) is that of (mathematical) sets. 'never' is simply the empty set, 'any' is the set of everything, 'string' is the infinite set of all possible strings, while any particular string (e.g. 'abc') when used as a type is simply the set consisting of a single element that is that particular string, and so on. Generics act somewhat like functions at the level of the type system (with the types listed between < and > acting as the arguments), and the 'extends' and 'infer' operators are there for type pattern matching and extraction.

[@user-hd7ju4wu4b]

Came to say this, you are very right

[@Rostgnom]

What's the mathematical anology to the `unknown` type?

[@user-hd7ju4wu4b]

@@Rostgnom i see it as a set that contains all the other sets

[@Alastairtheduke1]

What's the 'never' type? Something you haven't put into a set yet?

[@user-hd7ju4wu4b]

@@Alastairtheduke1 just empty set with no elements

[@dueft4479]

The comparison with the replace-regex was awesome. Helped me alot to understand infer better! Thx Matt!

[@WilderPoo]

I love that this is 13:37 long

[@mattpocockuk]

Gotta go deep

[@hansschenker]

the longer an explanation is the more shallow it gets!@@mattpocockuk

[@jasonstewart7983]

Very cleanly spoken. The mental model for 'infer' is super valuable. The comparison of 'extends' to a regex match and 'infer' to the capture group of the regex was what I was missing. It's so much easier to get my head around this after watching this video. Three cheers!

[@prcodes3479]

I was happy and satisfied with the basic pattern matching mental model of infer, then you completely blew my mind when you went even deeper into infer black magic and completely demystified it. Well done!

[@EpicNicks]

Came here after seeing the infer keyword in the ReturnType generic type definition wondering what this magic was. Thanks for the explanation.

[@edgeeffect]

I'm a recent "convert" to your channel. There's a lot of (let's face it) simply awful programming tutorial channels on KZbin and it's such a rare treat to find one, like this, that's of really good quality. It's got to the stage now where you're knocking "Arjan Codes" of of my personal number 1 spot. Keep up the excellent work.

[@ljuglampa]

You might be a TypeScript wizard, not sure, but I do know you're a educational wizard! Thank you for doing these ❤️

[@MrMcFyfey]

Great explanation, thanks Matt! I've been trying to find some more advanced typescript tutorials for a while now

[@codecleric4972]

Incredible. I grok'd the official docs explanation and it helped me understand some parts of a project codebase that I was struggling with before. But after watching this video, it's absolutely crystal clear and gives me much more context for how it can be used more broadly

[@darrenvong9404]

Even though I have an idea of what infer does, I still learnt something new from this (didn't know it's only applicable to conditional types)! Can't wait to dig into Total TypeScript more as you expand the content in that course!

[@peachesfruitella]

Liked the pairing with ‘never’ explanation, and how to think about its usage, felt like an aha moment 🎉

[@samerkayali4525]

You just made infer so easy! it was a thing that I have struggle to understand until seeing this video! Matt thanks a lot! You are a true hero and TypeScript mage!

[@BlueeyesChineseguy]

The regex analogue really helps paint the mental model, great video!

[@SimonCoulton]

Been working on some typesafe express middleware lately, and infer really did a number on me. This cleared things up heaps, thanks for this!

[@user-mf1ft6iz4l]

He explained everything in a very simple and easy to understand way. thank you

[@birrafondaio]

Underrated Master of Typescript 👌

[@GustavoDiaz93]

Love your style of teaching.

[@samautrey3834]

This is definitely the missing explanation for "infer" that I was needing. Great job, can't wait to see more content.

[@wiskasIO]

I gotta say that your videos are very intimidating for me because I'm still at an intermediate level in JS Bootcamp but your calm style and clear explanations make me kinda understand much of what you expose in your videos once I watch them a couple of times. I can't wait to reach a level where I'm ready to buy your TS course. Thank you, great work!

[@NedCollyer]

Thanks. Perfect :) Infer was the missing piece of the puzzle for me. I sorta understood what it was doing, but the syntax was just a bit nuts with the all the "extends matching".

[@rostislavzhuravsky3342]

Thank you very much! I am a Ruby developer who is interested in TS and your videos really help me dive deep into TS.

[@adiutama]

Thank you Matt, your explanation really help me understand Typescript better. Now I'm also enjoy doing Type Chalenge because of it.

[@EddyVinck]

This is brilliantly explained. Thank you Matt! I got my employer to buy Total TypeScript for me, can't wait to dig in!

[@Spytie1]

i liked because you are awesome, your explanations, the energy, the pedagogie, a true wizard legend

[@kasperaamodt]

Genuinely thought I was subscribed until the shout out at the end, now I am! Great vid

[@WesleyHandy]

Love your videos! Sharing with my team

[@TDefton]

You are so good at teaching 😍 thanks for sharing your knowledge

[@ponssuarez]

Thanks Matt!!

[@Timjstewart]

Very insightful! Thank you.

[@davidzarandi9287]

This is exactly what I was looking for, thank you

[@filipkud1]

I have to apply this knowledge somehow into my head now. That was great!

[@dasten123]

Really, really awesome explanation!

[@GLawSomnia]

The object examples made it clear to me :D Thanks

[@user-cp7fl1jl5e]

Awesome explanation! Easy to digest

[@denischudinov2379]

A brilliant explanation!

[@dawid_dahl]

Great explanation, thanks!

[@ivaylopetrov9956]

Thank you very much for the channel Matt aka TS Wizard 🙏. Just want to mention that something about the starting explanation what is conditional type and basically constrain in TS. So I understand it as the construction 'A extends B' basically means A subset of B. From SetTheory in Maths. I saw that many people are replaying this part, including me. Happy coding 💪

[@culi7068]

Ahh ok. I don't think I fully grasped the potential of `infer` until you brought in the bit about being able to confer constraints on it with the extends keyword. Really cool stuff

[@elielberra2867]

Amazing video! Thank you so much for this super clear and helpful explanation :)

[@johnpeterson8493]

Loved the video. You're a gem

[@matej2714]

great content man keep it up 💪🏼

[@leotravel85]

Best explanation ever. Thank you

[@vladimirmryscuk8409]

Great content, Matt 😜

[@HichamKazan]

explained brilliantly

[@cristiantorres853]

I totally love it! finally, I understand these crazy extends I was thinking of it as a extending some parent class or something It was driving me crazy

[@rafaeltab]

Now this. This is extremely useful.

[@FunctionGermany]

great explanation

[@grzesieksgs]

Dude You are one of the best web-dev related channels on YT, and You only have a bit more than 20k followers... Hope Your channel will grow because You definitely deserve it! Buying Your paid course! :D

[@mattpocockuk]

Thanks pal! Gained 11k subs in 3 weeks so hoping that changes soon!

[@anush8]

You have my respect kind sir.

[@bartoszgoebiowski1538]

Thank you, you are amazing!

[@HHJoshHH]

Found you from Theo. You’re awesome Matt! ❤ 💻

[@betoreyes3564]

Great video ❤, regards from México

[@bartek...]

Video length is epic!!!

[@andsheterliak]

Unfortunately, despite its popularity, Typescript has very poor documentation. Very often I don't understand what Typescript can or can't do. Thanks Matt for explaining those tricky parts.

[@AhsanKhan-eb2zb]

great explanation

[@henrym5034]

This is gem

[@dawidwraga]

Lovely thanks

[@colorofadog]

wow amazing! thanks!

[@ofira8328]

Really helpful! Thank you! The only complaint is the jumping between examples (going down then up the file), makes it harder to follow

[@aldojordanchoque7512]

Best explanation

[@NoOne-ev3jn]

I just found a gem kind of channels

[@jcollins519]

I liked it. Literally and metaphorically

[@EdersonBerti]

Could you make a video explain peer dependencies like a TypeScript Wizard?

[@_timestamp]

Great video Matt! Is it also possible to infer the type of function parameters? What's the correct way?

[@mattpocockuk]

Yes! export type Parameters = T extends (...args: infer TParameters) => any ? TParameters : never;

[@nabinsaud4688]

I am your fan typescript wizard ❤️

[@adtc]

What happens when you have a union of infers and the object being inferred from matches more than one pattern? Does it follow the first pattern, last one or a union of all?

[@justMeJustBri]

Any tips on how to use infer to get the type of an array item? I have types being generated from swagger This comes from the generator: type Thing = { org_key: string; role: "viewer" | "editor" | "admin"; }[] | undefined Trying to do type Blah = Thing[0] doesn't work because ofthe | undefined. Any help would be greatly appreciated!

[@mattpocockuk]

type Blah = NonNullable;

[@KevinCourbet]

Great pedagogy

[@ajgubi19]

Request: Please plan your examples ahead of time. After making each change, take some time to explain it. No need to rush. Thanks for your videos. They are super helpful.

[@elmalleable]

I might be wrong but is infer basically acting like a tag that says tells ts to pay attention to something. Which we can reuse for more things?

[@aleksander5298]

Hi Matt, when will the course be available for purchase? What bonuses do I lose if I use the regional price?

[@mattpocockuk]

Hey pal, we'll put it back on sale in January once we've got the second module ready. We are planning some bonus content, but we haven't released info about it yet. If you buy it via PPP we can always upgrade you if needed.

[@aleksander5298]

@@mattpocockuk sounds good, thanks for the answer

[@btiwari-games5279]

I'll like to tell my friends about you as that typescript guy

[@mauriciopereyra3339]

Que bien explicado

[@KoenVerheyen]

Why not use regex backreferences ($1) here as an analogy?

[@mateussarmento7692]

I believe never should be nothing and unknown should be something in typescript.

[@clarkdnro]

love how he said boolean with french style :)) funny teacher

[@hugodsa89]

My man

[@BlackxesWasTaken]

Can you tell me the font-family you used here ?

[@novailoveyou]

Just another quite not so easy thing made simple. infer is really useful and super fun!

[@CompanionCube]

manager: when a user clicks this button, please increase the counter in the db web devs af:

[@lukaszklejszta2736]

Combine composition with infer keyword and you will get unimaginable solutions.

[@culi7068]

what I learned was, do type AntiArray = T extends (infer U)[] ? U : never; not type AntiArray = T extends unknown[] ? T[0] : never;

[@flydexo]

Hi, my request may be peculiar but, could you make a video where you show how typescript utility types are used in real life situations because the only time I need to use them is within type-challenges (kinda like a bubble). So is it only me or many people feel this ?

[@lukaszklejszta2736]

Which utility types?

[@frontend3409]

my mind exploded somehere near 5 minute, but i survived until the end and remember one crucial thing: about the only use case of infer. Did i hate the video? I made it until the end so i give a thumb up.

[@alvesvaren]

I've seen infer used in a really cool type that replaces parts of strings in a template literal within a type. I still can't really understand how it works there even after watching this.

[@alvesvaren]

The type looks like this: export type Replace = T extends `${infer L}${S}${infer R}` ? Replace : `${A}${T}`

[@ya4dang1]

Hello TS Wizard!

[@TheSaintsVEVO]

Why did you reverse the order of type Result in the beginning? And then never used that Result type? I don’t get it

[@EthanStandel]

Good God, I've been declaring so many different generic arguments with default values when I should have been using infer

[@vkiperman]

This went by a little fast. But very interesting!!!

[@frandrumming]

well.. subscribed :D

[@sidisting1381]

I didn't understand anything. 😅😅 what are the prerequisites to understand this?

[@HappyCheeryChap]

Video duration is leet!

[@Atmos41]

Infer is like extracting a type as a variable. As soon as I understand that, Infer was easy.

[@aprilmintacpineda2713]

So infer keyword allows you to grab the type of the thing that was passed and use it later as if it’s a variable.

[@Palundrium]

Great video, thank you. Your way of building a mental model similar to `String.replace` is a great way to think and remember it. One interesting bit about GetFromDeepObject that I played around with and found out is that if you have this: type GetFromDeepObject = T extends { a: { b: { c: infer C; } } } | { c: infer C } ? C : never You could even do something like this: type Test = GetFromDeepObject and Test will end up as string | boolean. Cool!

[@arturamorfati]

i should like it and i do like it🖤

[@kilo.ironblossom]

What are you??!! A wiza.... Ah okay!! Right!!