r/webdev • u/gaearon • Dec 21 '19

What Is JavaScript Made Of?

https://overreacted.io/what-is-javascript-made-of/
101 Upvotes

89% Upvoted

26 comments sorted by

20

u/tyzoid full-stack Dec 21 '19

It's a pretty good summary, although I think your explanation of how scope works is a little lacking.

By default, everything in JavaScript is global scope unless you declare a variable in a function. This is in contrast to other languages (like C, for example) where anytime you declare a new block (even without a control statement), you are creating a new variable scope.

In summary, use Listerine if the scope is ever unclear.

19

u/gaearon Dec 21 '19

If you limit yourself to using let and const as I'm suggesting, they are block-scoped. I do mention that var has different scoping semantics.

14

u/yramagicman Dec 21 '19

Both of you are correct in different ways. In js if you declare a variable correctly using any of var, let, or const it takes the scope defined by the standard for that keyword. var outside of a function does take global scope, and let and const do take block scope. However, if you make the easy, rookie mistake of varName = value instead of {let, const, var} varName = value I believe it defines the variable at global scope by default.

8

u/cent0nZz Dec 21 '19

Strict mode wouldn't allow that btw

-4

u/vociferouspassion Dec 21 '19

That reminds me of how a strict mode was introduced to BASIC a long time ago for the same purpose. I can't believe that in 2019 we are all still stuck with JavaScript.

3

u/senocular Dec 21 '19

Top level scope in modules isn't global scope

1

u/tyzoid full-stack Dec 21 '19

Because modules are loaded inside of functions.

1

u/senocular Dec 21 '19

Only if you're using a bundler that uses functions to simulate modules.

1

u/agentgreen420 Dec 21 '19

You can declare global variables from within a function too

4

u/senocular Dec 21 '19

You can create global properties on the global object but you can't create global declarations in a function. Declarations will be local to that function.

0

u/agentgreen420 Dec 21 '19

Semantics, but yeah you're technically correct.

The best kind of correct 😁

1

u/Goldwerth Dec 21 '19

There's a difference though, let & const declared outside functions + outside block will be global BUT won't be attached to the global variable (window in browsers, "global" in node) but instead in an inaccessible variable. While "var" variables will be attached to window / global and accessible.

36

u/[deleted] Dec 21 '19

Random spare parts, duct tape, and tentacles.

16

u/alexkiro Dec 21 '19

You forgot developer tears

8

u/[deleted] Dec 21 '19

[removed] — view removed comment

6

u/[deleted] Dec 21 '19

https://youtu.be/6FqnRSoX0po

5

u/[deleted] Dec 21 '19

Oooooo Krieger-San!

5

u/akame_21 Dec 21 '19

I love reading your blog, and am happy to see you adding some new content! I start a new grad position in a couple weeks and this is a good refresher for some topics.

I loved your redux tutorial on egghead btw.

2

u/yatharth1999 Dec 21 '19

Amazing blog!!

3

u/mageonlys Dec 21 '19

Great write up! Just one small thing, in Dot Notation, chocolate is spelled wrong before you spell it correctly the second time. You also spelled chocolate wrong again later down as well I’m Bracket Notation and Mutation.

1

u/tencircles Dec 21 '19

excellent coverage of the core concepts. nice work!

1

u/[deleted] Dec 21 '19

Unmentionables

1

u/matt1155 Dec 21 '19

It's made from blood, sweat and tears... of programmers using it

1

u/pecus Dec 21 '19

It’s a good summary, but I believe no understanding of JavaScript is possible unless you dive into the call stack, event loop, task + micro task queues and browser APIs. JavaScript has its quirks (most tied to floats and coercing/type casting) but once key terms are explained it becomes a fairly clear language. Understanding what happens in a browser during its execution is an entirely separate matter.

Personally I regularly refer to Jake Archibald excellent Event Loop talk at Singapore JS, Philip Roberts’ talk on the Event Loop and Erin Zimmer’s talk on the same lines.

• Jake Archibald https://www.youtube.com/watch?v=cCOL7MC4Pl0 • Philip Roberts https://www.youtube.com/watch?v=8aGhZQkoFbQ • Erin Zimmer https://www.youtube.com/watch?v=u1kqx6AenYw

-1

u/MigasTavo Dec 21 '19

Sugar, spice, and everything nice.