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Simple concepts of Modern JavaScript

Pachi 🥑 on September 16, 2019

Before I start, please note that this post is me trying to explain these concepts to myself so make sure I fully understood them and have these not...

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Ben Calder • Sep 16 '19

Just to clarify: the intention of arrow functions is not simply to be a shorthand for a normal function; though in practice they do often get used that way. The main intent is to solve a common scoping problem.

When you use a normal function the keyword this is set to that function's scope; but in an arrow function this is set to the parent scope. To illustrate the difference try running the following and looking at the console output:

const PseudoClass = function() {
  this.message = 'this is a pseudo-class property';

  return {
    normalFunction : function() {
      console.log(`normalFunction says: ${this.message}`);
    },
    arrowFunction : () => {
      console.log(`arrowFunction says: ${this.message}`);
    }
  }
};

const instance = new PseudoClass();
// right now you just need to know that you can call
// the functions returned by the PseudoClass function like so:
instance.normalFunction(); // normalFunction says: undefined
instance.arrowFunction(); // arrowFunction says: this is a pseudo-class property

In normalFunction this is scoped to the function where there is no message property.

In arrowFunction this is scoped to the parent where a message property is defined.

Strictly speaking arrow functions are usually intended to be defined in the context of a 'class'.

If you want further detail I always recommend the You don't know JS books for an in-depth and clear explanation of this; scope and why JS doesn't actually have classes :D

One other useful feature of template literals is how they handle white space:

console.log(`there is a line break here
but here \
not`);

Useful to know if you deliberately want to output line breaks; or if you want to break source code onto multiple lines.

Pachi 🥑 • Sep 16 '19

Thank you Ben! As I mentioned, I am currently studying those concepts so your input is VERY helpful!

Ben Calder • Sep 17 '19

Happy to contribute. I have often posted my findings to forums precisely to help solidify concepts in my head and in case I had misunderstood anything. It's a really good learning process and has the benefit of helping others :)

One thing that I realised I didn't mention: a common use of arrow functions is in callbacks where you want to maintain the this context:

window.setTimeout(arrowFunction, 100);

Before arrow functions you had to use bind - which felt a little arcane:

window.setTimeout(normalFunction.bind(this), 100);

Connor Tangney • Sep 17 '19

Akin to the ternary operator, if you just want to quickly evaluate something, and don't really care about the inverse situation, check out guards!

They're contextually similar to the ternary operator and I honestly have found more use-case for this in the past than ternaries:

//a variable to test with
let a = 'a';

//condition && result
a === 'a' && console.log('yes it does!');

Keep in mind, like the ternary operator, you should avoid complex logic in the block that is evaluated when your condition is met:

//a variable to test with
let a = 'a';

//condition && result
// !! THIS WON'T WORK !!
a === 'a' && (
  console.log('did this work?')
  console.log('no :(')
);

Pachi 🥑 • Sep 17 '19

Thank you !!!

Allen Porter • Sep 16 '19

You'll need to be careful with the order you use the spread operator. If the first object contained c it would not have been replaced. Change the order and you'll get your expected result. Also the spread operator is a shallow copy.

Alexandro Disla • Sep 16 '19

thank you

but

const  hello = () => {console.log('Hello')}

morefox • Sep 18 '19 • Edited

const obj1 = {a: 'a', b: 'b'};
const obj2 = {c: 'c', ...obj1};
console.log(obj2);

//The console will log: Object {  a: "a",  b: "b",  c: "c"}

In this case the output is

// Object {c: "c", a: "a",  b: "b"}

I guess you meant to write something like

const obj2 = {...obj1, c: 'c'};

Casi Imin • Sep 16 '19

Thank you P.

Pachi 🥑 • Sep 16 '19

I thank you for reading!

Pachi 🥑 • Sep 16 '19

Thank you Michele!!

Teo Khai Kiong • Sep 17 '19

I guess there is a typo at Cons?
Should be const instead of Cons.

Pachi 🥑 • Sep 18 '19

Thanks, fixed