Arrays

Remove Duplicates from array without loop

let nums = [1, 2, 2, 3];
let uniNums = [...new Set(nums)]; // [1,2,3]

Return total number of arguments without loop

let x = function () {
  return [].slice.call(arguments).length;
};

Random

console.log(2 + '2'); // $: 22 // ?: + is a string operator
console.log(2 - '2'); // $: 0 // ?: - is only a number operator

// ----------------------

function a() {
  return 'hello';
}
const sentence = a`hi`; // same as a('hi')
console.log(sentence); // $: "hello"

// ----------------------

let x = function () {
  return; // <-- remove ; in doc // line break after return is interpreted as return;
  {
    message: 'hi';
  }
};

console.log(x()); // $: undefined

// ----------------------

console.log(5 < 6 < 7); // true
console.log(7 > 6 > 5); // false

// ? 5 < 6 < 7 // true
let step1 = 5 < 6 < 7; // 5 < 6 evaluates to true
let step2 = true < 7; // true evaluates to 1
let step3 = 1 < 7; // 1 is less than 7 // $: true

// ? 7 > 6 > 5 // false
let step1 = 7 > 6 > 5; // 7 > 6 evaluates to true
let step2 = true > 5; // true evaluates to 1
let step3 = 1 > 5; // 1 is not greater than 7 // $: false

// ----------------------

function y() {
  console.log(this.length);
}

var x = {
  length: 5,
  method: function (z) {
    // arguments = [y, 1]
    // calling y on arguments returns arguments.length
    // [y, 1].length
    arguments[0]();
  },
};

x.method(y, 1); // $: 2

// ----------------------

const x = 'constructor';
console.log(x[x](01)); // $: "1"
// x is String and has constructor function String()
// String(01) outputs 1

// ----------------------

console.log(Math.max(1, 2, 3)); // $: 3
console.log(Math.max()); // -infinity

/**
 * Lowest possible number is -infinity
 * max() takes input[0] > -infinty
 * then continues on to input[1] > previous max
 * if nothing passed -infinity is still the max
 * **/

// ----------------------

console.log(0.1 + 0.2); // $: 0.30000000000000004
// all numbers in javascript are treated with floating point precision

// ----------------------

console.log('hi'.__proto__); // will be String() // $: ""
console.log('hi'.__proto__.__proto__); // will be Object() // $: Object {}
console.log('hi'.__proto__.__proto__.__proto__); // nothing higher than object // $: null

// ----------------------

console.log([] + []); // $: ""
// empty string because arrays are typecast to strings
// String([])  // $: ""

// ----------------------

What is Undefined?

A variable has been declared, but no value set

var declareMe; // declare variable with no value set
typeof declareMe; // $:  "undefined"

var declareMe = undefined; // can be manually set
typeof declareMe; // $:  "undefined"

var person = { name: 'tiffany' };
typeof person.age; // non-existent object properties // $:  "undefined"

What is Null?

An assigned empty or non-existent value that explicitly means nothing

typeof null; // $:  "object"

// checking for null
console.log(null === 0); // false
console.log(null === 0n); // false
console.log(null === null); // true
console.log(null === undefined); // false
console.log(null === false); // false
console.log(null === NaN); // false
console.log(null === ''); // false

// Generally, it is a good idea to catch both null and undefined values,
// as both represent an absence of a value, thus checking for null is
// one of the few times in Javascript that using == is recommended
// while otherwise === is generally recommended

What is Void operator?

The void operator evaluates the given expression and then returns undefined.

Use Cases

// Immediately Invoked Function Expressions
void (function iife() {
  console.log('Executed!');
})(); // $: "Executed!"

// non-leaking Arrow Functions
button.onclick = () => void doSomething();
// ensures return value of doSomething changing from undefined to true will not change the onclick behavior

<!-- Javascript URIs: Used to prevent page from refreshing -->
<a href="javascript:void(0);">Click here to do nothing</a>

<a href="javascript:void(document.body.style.backgroundColor='green');">
  Click here for green background
</a>

Typeof

// Primitive Type of
typeof 'John'; // $: "string"
typeof 3.14; // $: "number"
typeof true; // $: "boolean"
typeof false; // $: "boolean"
typeof x; // $: "undefined" (if x has no value)

// Complex Type of
typeof { name: 'John', age: 34 }; // $:  "object"
typeof [1, 2, 3, 4]; // $:  "object"
typeof null; // $:  "object"
typeof function myFunc() {}; // $: "function"

What is the difference between .forEach() and .map() ?

Both .forEach() and .map() iterate over items in a list and execute a function for each

• .forEach() doesn't return any value
• .map() returns new array with callback function return values