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The vanilla Javascript basics to know before learning React JS

Jane Tracy 👩🏽‍💻 on September 20, 2020

REACT JS Background Story If you just finished JavaScript online courses or other self-taught resources going to your first JS framewo...

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maxdevjs • Sep 22 '20 • Edited

@tracycss thank you for the great article :)

This snippet

//const object properties or array values can be updated
const friends = [{
    name: 'Bob 🥽',
    age: 22,
    hobby: 'golf🏌',
    music: 'rock 🎸'
  }

];

const result = friends.age = 23;
console.log(result);
//output => 23

does not update the original age (I guess the code was intended to update age from 22 to 23, right?). Instead it creates a new named property of the array object:

console.log(friends);
[
  { name: 'Bob 🥽', age: 22, hobby: 'golf🏌', music: 'rock 🎸' },
  age: 23
]

This can be checked:

// new (named) age property
console.log(friends.age);
23
// also
console.log(friends["age"]) ;
23

// original age
console.log(friends[0].age) ;
22

To update the original age:

friends[0].age = 23;
console.log(friends[0].age) ;
23
// also
console.log(friends[0]["age"]) ;
23

Jane Tracy 👩🏽‍💻 • Sep 22 '20 • Edited

Great input Max, I might the values or properties can be changed. Let me update that part.
Thank you. 💯🌟🙂

peerreynders • Sep 20 '20 • Edited

We can also use the arrow functions as class methods.

Despite of what you may see in framework tutorials in terms of "Vanilla JS" that is considered bad form.

The issue is that in arrow functions this refers to the lexical scope that created the function.

As a consequence each object instance will create a new, separate function.

For a regular method a single function exists on the prototype object of all the object instances that share it - i.e. one single function is reused across all object instances. For traditional methods (including the short hand syntax) this (the function context) refers to the calling context.

So when using arrow functions for "regular methods" you are creating more functions than you need to.

Arrow functions in class declarations are justified when that function is intended to be used as a callback (e.g. event listener), i.e. it needs to be bound to (this of) that particular object instance.

Please have a look at:

Also ES6 is the legacy term for ES2015. There is no ES7. Since ES2015 TC39 has annually published updates to the spec, e.g. ES2016, ES2017, etc.

The additions (finished proposals) for each publication year are identified here.

Some people use "ES.Next" when Babel offers implementations of proposals before they reach stage 4 (finished).

var variables are globally scoped or accessible.

Inside a function they are scoped to the function - i.e. not globally accessible - that is what the "immediately invoked function expressions" IIFE are about. However var declarations are effectively hoisted to the top of the function - independent of where they appear inside the function.

A class is another type of function

There is a class declaration and class expression.

The MDN's statement:

Classes are in fact "special functions"

is a bit misleading. It's more accurate to say that classes are syntax sugar for constructor functions.

we use this keyword to refer to the current class.

"we use this keyword to refer to the current object (an instance of the class)."

And as already mentioned this has a different meaning in arrow and traditional methods:

arrow function - the object the function was created on
traditional method - the object the method is called on

The super method

super is a keyword. super([arguments]) is only valid in the constructor. super.name or super[name]can be used to access properties (and functions/methods) on the parent.

The spread operator is used for splitting up the values of an array

The spread syntax also works on object literals and can also be used to convert an iterable to arguments for a function call.

JSX supports spread attributes:

function App1() {
  return <Greeting firstName="Ben" lastName="Hector" />;
}

function App2() {
  const props = {firstName: 'Ben', lastName: 'Hector'};
  return <Greeting {...props} />;
}

This operator is used to represent an infinite amount to arguments in a function.

Some people think of the rest syntax as "gather" because it collects the remaining arguments.

The rest syntax can also be used for Array destructuring and Object destructuring.

You can use "rest in object destructuring" to omit object properties.

let {b, c, ...redacted} = {a: 4, b: 3, c: 2, d: 1};
console.log(redacted); // {a: 4, d: 1}

Array functions

One issue is that these functions are limited to arrays rather than general iterables. While at times Array.from, Object.entries, Object.keys, and Object.values can be helpful often it's more performant to stick to using for..of.

Jacob Evans • Sep 21 '20

ES7 => ES2016

peerreynders • Sep 21 '20 • Edited

ECMA-262 7ᵗʰ Edition / June 2016 => ECMAScript® 2016 Language Specification

Edit:
When the "Standard ECMA-262 6th Edition" was being drafted it was referred to as ES6.

However before it was finalized the committee decided to go with the ES2015 moniker to reflect the fact that they intended to publish yearly updates to the specification - which they have done so far.

Publications before June 2015 can be excused for using the ES6 moniker, past that ES2015 should have been used.

It's been 5 years - it's time to let go of the edition based moniker unless one is referring to ES5.1 and below.

Jacob Evans • Sep 22 '20

Cool...Ill keep this in mind while writing configs that use ES5-11 but hey you do you.

Jane Tracy 👩🏽‍💻 • Sep 21 '20

True, I agree Jacob.

Jane Tracy 👩🏽‍💻 • Sep 21 '20

Great input, I will take a look at the links.

peerreynders • Sep 21 '20 • Edited

Good coverage for three months 👍

Edit:
Method vs "arrow function on public class field" weirdness:

class A {
  myArrow = () => {
    console.log('A.myArrow()');
  }

  myMethod() {
    console.log('A.myMethod()');
  }
}

class B extends A {
  myArrow = () => {
    super.myArrow(); // arrow function exists on 
                     // public class field of A but
                     // but is not available on 
                     // the prototype chain via `super`
    console.log('B.myArrow()');
  }

  myMix() {
    super.myArrow(); // same problem
    console.log('B.myMix()');
  }

  myMethod() {
    super.myMethod(); // just works
    console.log('B.myMethod()');
  }
} 

let myB = new B();

myB.myMethod(); // 'A.myMethod()'
                // 'B.myMethod()'
myB.myMix();    // Uncaught TypeError: (intermediate value).myArrow is not a function at B.myMix
myB.myArrow();  // Uncaught TypeError: (intermediate value).myArrow is not a function at B.myArrow

Jane Tracy 👩🏽‍💻 • Sep 21 '20

Awesome, explanation. 🙂🌟

Robert Myers • Sep 21 '20

const can neither be re-assigned nor updated, it is constant(doesn’t change).

I think that is a little misleading. The reference is constant, the object is not; fields can be reassigned and added.

const x = { y: 'x is const' };
x.y = 'y is not';
x.a = 'here is a new property';
console.log(x.y); // y is not
console.log(x.a); // here is a new property
delete x.y;
console.log(x.y);  // undefined

Colloquially, those are all updates.

Jane Tracy 👩🏽‍💻 • Sep 22 '20

You can not reassign an object or an array but you can still change the values or properties.
Let me change the text a little.
Thanks for pointing this out Robert, yeahhhh :)

Junxiao Shi • Sep 23 '20

In Netscape era, I could read the entire JavaScript specification.
In ES5 times, the paper book for ECMA-262 standard is still reasonable thickness. I have the book from 2011 and it's 245 pages. (the book is free and they'll mail it to you if you request)
Nowadays, JavaScript is so complicated. I read all of MDN but there must be more.

P.S. I prefer chocolate to vanilla.

Jane Tracy 👩🏽‍💻 • Sep 23 '20

I would be interested to have a look at the book. You can email me, I appreciate it :)

Junxiao Shi • Sep 23 '20 • Edited

ecma-international.org/publication...

It's now 860 pages.

Jane Tracy 👩🏽‍💻 • Sep 24 '20

It's okay. My email is janetracy00@gmail.com.
I will go through it slowly taking my time. No rush :)

ljc-dev • Sep 21 '20

Very useful guide 😀! A lot of people are curious about when to start learning react. Thx for sharing 🤗

Jane Tracy 👩🏽‍💻 • Sep 21 '20

Thank you. Glad you enjoyed it. 😁🙌
I hope it helps in any way, don't forget to share to help some else out there.✨

ljc-dev • Sep 21 '20

I will 😁👍!!

Jane Tracy 👩🏽‍💻 • Sep 21 '20

Awesome 🙌🔥

Chris Campbell • Oct 6 '20

This was really helpful, thanks 👍

Jane Tracy 👩🏽‍💻 • Oct 6 '20

Glad it was helpful in any way, Chris. ✨

Christopher Wray • Sep 20 '20

Thank you for this! I especially like your description of variables (let/const) and arrow functions. I needed that.

Jane Tracy 👩🏽‍💻 • Sep 20 '20

I am glad the information was useful. 🌟💯
Thank you for reading it, Christopher. 🙌🏽

CodenameCookie • Dec 6 '20

This is really useful. Good job. :)

Jane Tracy 👩🏽‍💻 • Dec 7 '20

Thank you so much. Hope it helped in anyway.

Cierra Nease • Oct 18 '20

Hi Jane, great article. I think you really nailed some of the key JS concepts that should be learned before React! Thanks for the article!

Jane Tracy 👩🏽‍💻 • Oct 18 '20

Thank you, Cierra. Glad you found it helpful. ✔✨

derva • Sep 24 '20

Wow, really amazing post - thank you

How much time you need for one like this post, there is so much valuable information and very clearly to understand it!

Jane Tracy 👩🏽‍💻 • Sep 24 '20 • Edited

I a lot of time. But I take breaks and code in between as well. I can't clearly know how much time I use continuously to write it. But if I would make a guess, it would be a min of 6 hours.

Thank you. Hope it helped in any way, Derva :)

Muhammad Awais • Sep 24 '20

good one. I appreciate it.

Jane Tracy 👩🏽‍💻 • Sep 24 '20

Thank you, I appreciate it :)

Yuriy Rymarchuk • Sep 23 '20

Really nice, polished and concrete article. I really enjoyed reading it 😄🤓

Jane Tracy 👩🏽‍💻 • Sep 23 '20

Thank you so much, Yuriy. I really appreciate it. ❤✨

Andrew Baisden • Oct 13 '20

Good article best to know javascript better before jumping into a framework like React. Too many beginners make that mistake.

Harshil Parmar • Sep 23 '20

Please add Computed Property ,if you can
ref : ui.dev/computed-property-names/
And also add shallow copy & deep copy concept, Reference type vs value type.
Thanks!!