JavaScript Cheatsheet

한국어

QUICK VIEW

array manipulation

// let arr1 = [1,3,4,5,7,9];
// let arr2 = [2,5,6,7];

// intersection
let inter = arr1.filter(x => arr2.includes(x));

// difference
let diff = arr1.filter(x => !arr2.includes(x));
let diffTwo = arr1.filter(x => arr2.indexOf(x) < 0);

// union
let union = [...arr1, ...arr2]; // [key=combine/union]

convert number to string

see also String() & toString()

const val = 21 + ""; // -> "21"; typeof val -> "string";

convert string to number

var int = "421";
console.log(+int); // -> 421

NOTE

var number = "1e5";
console.log(+number); // -> 100000

filter unique values

const arr = [1, 2, 2, 4, 7, 2, 1];
const uni = [...new Set(arr)]; // -> uni = [1, 2, 4, 7];
const unique = arr.filter((v, i, a) => a.indexOf(v) === i); // -> unique = [1, 2, 4, 7];

// reduce consecutive to single value => remove duplicates
const removeDuplicate = arr.filter((c,i) => c !== arr[i+1]); // -> removeDuplicate = [1, 2, 4, 7];

function within a function

function power(base) {
  return function(exp) {
    return Math.pow(base, exp);
  }
}
const powerOf = power(4);
console.log(powerOf(3)) // -> 64

methods

methods	Description	Returns	Argument
every()	EVERY item has to pass the test	TRUE/FALSE
some()	SOME item has to pass the test	TRUE/FALSE	function
includes()	checks if the array has certain string	TRUE/FALSE	string
filter()	check for elements that pass the test	new array
map()	similar to filter(), but modifies items	new array

reverse without reverse()

let ans = "";
for (let i=0; i<str.length;i++) {
 const char = str[i];
 ans = char + ans;
}
// or //
return str.split("").reduce((output, char) => {
 output = char + output;
 return output;
}, "");

let str = "bbbaaabaaaoijeroajgalerifjaliohgailwuegha";

function letCnter(str) {
  let counter = {};
  for (let i=0; i<str.length; i++) {
    counter[str[i]] = counter[str[i]] + 1 || 1;
  }
  for (let i in counter) {
    console.log("key -> " + i, "value -> " + counter[i]);
  }
  return counter;
}

###

---

B_

1. Bubble Sort

const bubbleSort = arr => {
 for (var i=0; i<arr.length; i++) {
  for (var j=0; j<arr.length; j++) {
   if (arr[j] > arr[j+1]) {
    let tmp = arr[j];
    arr[j] = arr[j+1];
    arr[j+1] = tmp;
   }
  }
 }
 return arr;
}

const bubbleSort = arr => {
 var swapped;
  for (var i=arr.length; i>0; i--) {
   swapped = true;
   for(var j=0; j<i; j++) {
    if (arr[j] > arr[j+1]) {
     var temp = arr[j];
     arr[j] = arr[j+1];
     arr[j+1] = temp;
     swapped = false;
    }
   }
  if(swapped) break;
 }
 return arr;
}

---

C_

1. Consecutive Numbers

// solution 1
let solution1 = [];

for (let i=0; i < 11; i++) {
  solution1.push(i);
}
// [0,1,2,3,4,5,6,7,8,9,10]

// solution 2
let solution2 = [...Array(10).keys()];
// [0,1,2,3,4,5,6,7,8,9,10]

---

D_

1. Destructuring

const myStock = {
  stockName: "JavaScript",
  amount: 4,
  worth: "$3,000"
}

const { stockName, amount, worth } = myStock;

console.log(`You are holding ${amount} of ${stockName} and is worth ${worth}`);
// => You are holding 4 of JavaScript and is worth $3,000

---

E_

1. every()

// every() method goes through an array and returns TRUE if ALL items pass the test or FALSE if not.
// EVERY ITEM has to pass the test
  // ex. var IQ = [99, 105, 138, 89]; or const IQ = [99, 105, 138, 89];
IQ.every(function(intelligence) {
  return intelligence >= 100;
})
// false
// ES6
IQ.every(intelligence => intelligence >= 75);
// true

---

F_

1. filter()

// filter() returns a new array with elements that pass the certain condition
  // ex. return a new array with integers only
filter_list function(l) {
  l.filter(function(c) {
    return (typeof c == "number");
  })
}
// ES6
const filter_list = l => l.filter(c => (typeof c == "number"));

---

N_

###### (2) Null & Undefined

null indicates a declared variable with “blank/ empty” value

undefined means that the variable is declared, but a value is not assigned.

let car; // => undefined
let truck = null; // => null

---

O_

###### (1) Object!

What is object?

Object consists of keys and values. Below is an example.

let sampleObject = {
  key1: "value1",
  key2: "value2",
    .
    .
  key6: "value6"
}

How to access/get values?

sampleObject["key1"] // => "value1"
sampleObject.key2 // => "value2"

How to create a new property?

sampleObject["key5"] = "value5"
sampleObject.key6 = "value6"
sampleObject // => {key1: "value1", key2: "value2", . . ., key5: "value5", key6: "value6"}

###### (2) Object Constructor // a way to construct a new object

 function Person(name, age) {
   this.name = name;
   this.age = age;
   this.info = function() {
     console.log(this.name + "'s age is " + this.age);
   }
 }
 const p = new Person('David', 29);
 const a = new Person('Cat', 35);
 console.log(p); // Person {name: "David", age: 29}
 console.log(a); // Person {name: "Cat", age: 35}

 p.info(); // => "David's age is 29"
 a.info(); // => "Cat's age is 35"

NOTE:: arrow function cannot have this in it

NOTE:: Constructor’s name starts with a capital letter

---

P_

1. power a number

Math.pow(base, exponent)

---

R_

1. replace()

// remove commas
replace(/,/g, "")

2. reverse()

// this method reverses the order of elements in the array - original array affected
const IQ = [99, 105, 138, 89];
const.reverse(); // IQ = [89, 138, 105, 99];

3. remove last digit(s) + remove last letter(s)/item(s)

const int = 4210 / 10 | 0; // -> 421
const str = ("Hello World!").slice(0,-1); // -> "Hello World"
const arr = [0, 2, 5, 4, 21, 7];
arr.slice(-1); // -> [7];

---

S_

1. some()

// checks if some elements are in the array and returns TRUE/FALSE - similar to `incldues()`
  // ex. var IQ = [99, 105, 138, 89]; or const IQ = [99, 105, 138, 89];
const IQ = [99, 105, 138, 89];
IQ.some(pass => pass > 100);
// true

2. sort()

// sorts items in the array -> can be alphabetical or numerical + ascending/descending - original array affected
const IQ = [99, 105, 138, 89];
IQ.sort((a,b) => a-b); // 89, 99, 105, 138
IQ.sort((a,b) => b-a); // 138, 105, 99, 89

3. spread operator (…)

 // is used for copying/combining arrays, using math functions, etc. [key=combine/union]
 let str = "This is a string!!";
 let arr1 = ["David", "Doe", "Hoon", "LEE"];
 let arr2 = ["Rabbit", "King"];
 let num = [-1, 2, 4, 21, 7, 3];
 
 console.log([...str]); // -> ["T", "h", "i", "s", " ", "i", "s", " ", "a", " ", "s", "t", "r", "i", "n", "g", "!", "!"];
 console.log([...arr1, ...arr2]); // -> ["David", "Doe", "Hoon", "LEE", "Rabbit", "King"];
 console.log(Math.max(...num)); // -> 21;

Note :: Math.max() & Math.min() cannot be used with an array **

4. String() & toString() methods

// String()
String(undefined);
//  => "undefined"

// toString()
(16).toString();
// => "16"
(true).toString();
// => "true"

method	input	return
String()	anything	string
toString()	number or boolean	string

Note :: String() takes any input and returns it as a string while toString() can only take number/boolean as inputs.

---

T_

1. Template literals

const stockName = "JavaScript";
const amount = 4;
const worth = "$3,000";

console.log(`You are holding ${amount} of ${stockName} and is worth ${worth}`);
// => You are holding 4 of JavaScript and is worth $3,000