A Guide to Discord Bots

What's The Difference Between...

"a", 'a' and `a`

There's no difference between them: they are all strings.
However, you can use variables more conveniently with ``.

let variable = "World";

let strA = "Hello, " + variable + "!";
let strB = 'Hello, ' + variable + '!';
let strC = `Hello, ${variable}!`;

But, depending on which one you choose, you might need to escape some characters using \.
Escaping a character will print as it is, without considering it as the start/end of a string for example.

let strA = "Let's see... What's a \"variable\"?";
let strB = 'Let\'s see... What\'s a "variable"?';
let strC = `Let's see... What's a "variable"?`;

Note: in JSON, name: 'John' is invalid while name: "John" is valid.

let, var, const

• Const declares a variable globally, and its content cannot be changed.
• Var declares a variable globally
• Let declares a variable in its block scope and sub-blocks (function, try, switch, if, etc.)

A 'block' is defined by braces/curly brackets { }.

const a=1; // accessible everywhere

var b=2; // accessible everywhere
let c=3; // accessible in all sub-blocks, which is everywhere

function calc() => {
    var d=4; // accessible everywhere
    let e=5; // accessible only inside calc()

    console.log(a, b, c, d, e, f); // 1, 2, 3, 4, 5, undefined
}

if (a==1) {
    let f=6; // accessible only inside 'if'
    console.log(a, b, c, d, e, f); // 1, 2, 3, 4, undefined, 6
}

console.log(a, b, c, d, e, f); // 1, 2, 3, 4, undefined, undefined

for, .forEach(), for x in y

Technically, you can sometimes use any of them, but:

• for can make use of variables and conditions
• You can't use break nor continue with forEach()
• for x in y can be used to go through objects

let array = ['one', 'two', 'three', 'four', 'five'];

// This loop...
array.forEach(string => console.log(string));

// ... and this one
for (let string in array)
    console.log(string);

// ... and this one
for (i = 0; i < array.length; i++)
    console.log(array[i]);

// ... are all equal

str.includes() and str.indexOf() > -1

• The includes method finds NaN and undefined whereas the indexOf() method doesn't.
• The includes() method does not distinguish between -0 and +0.

Source.