diff --git a/day-01/Exercises/arr_level1.js b/day-01/Exercises/arr_level1.js
new file mode 100644
index 0000000..b98092f
--- /dev/null
+++ b/day-01/Exercises/arr_level1.js
@@ -0,0 +1,135 @@
+// Declare an empty array;
+const myarr1 = []
+// Declare an array with more than 5 number of elements
+const myarr2 = [1, 2, 3, 4, 5]
+// Find the length of your array
+console.log(myarr2.length)
+
+// Get the first item, the middle item and the last item of the array
+let myarr2_length = myarr2.length
+let myarr2_middle = 0
+if (myarr2_length%2 === 0) {
+  myarr2_middle = myarr2_length/2
+  console.log("myarr2_middle: ", myarr2_middle)
+} else {
+  myarr2_middle = (myarr2_length - 1)/2
+  console.log("myarr2_middle: ", myarr2_middle)
+}
+console.log("first item:\t", myarr2[0], " / middle item:\t", myarr2[myarr2_middle], " / last item:\t", myarr2[myarr2_length - 1])
+
+// Declare an array called mixedDataTypes, put different data types in the array and find the length of the array. The array size should be greater than 5
+const mixedDataTypes = [
+  1234,
+  "myName",
+  {class: 'math', numberOfStudents: 32 },
+  {classes: ['math', 'science', 'literature']},
+  true,
+  "1234",
+  ['a', 'b', 'c']
+]
+console.log(mixedDataTypes)
+
+// Declare an array variable name itCompanies and assign initial values Facebook, Google, Microsoft, Apple, IBM, Oracle and Amazon
+const Companies = ['Facebook', 'Google', 'Microsoft', 'Apple', 'IBM', 'Oracle', 'Amazon']
+// Print the array using console.log()
+console.log(Companies)
+// Print the number of companies in the array
+console.log(Companies.length)
+// Print the first company, middle and last company
+let companyLength = Companies.length
+let companyMiddle = 0
+if (companyLength%2 === 0) {
+  companyMiddle = companyLength/2
+} else {
+  companyMiddle = (companyLength - 1)/2
+}
+console.log("first item:\t", Companies[0], " / middle item:\t", Companies[companyMiddle], " / last item:\t", Companies[companyLength - 1])
+
+// Print out each company
+Companies.forEach(company => {
+  console.log(company)
+});
+
+// Change each company name to uppercase one by one and print them out
+Companies.forEach(company => {
+  console.log(company.toUpperCase())
+})
+
+// Print the array like as a sentence: Facebook, Google, Microsoft, Apple, IBM,Oracle and Amazon are big IT companies.
+let temp = Companies.pop()
+// console.log("poped Companies",Companies)
+console.log(Companies.join(), "and", temp, "are big IT companies.")
+Companies.push(temp)
+// console.log("rejoin",Companies)
+
+// Check if a certain company exists in the itCompanies array. If it exist return the company else return a company is not found
+function isCompany () {
+  const company = document.getElementById("iscompany").value
+  if (Companies.includes(company)) {
+    document.getElementById("companyresult").innerText = company
+  } else {
+    document.getElementById("companyresult").innerText = (company + ' is not found')
+  }
+}
+
+// Filter out companies which have more than one 'o' without the filter method
+function myfilter (array) {
+  const result = Array()
+
+  array.forEach(string => {
+    let o = 0;
+    let temp = string.split('')
+    temp.forEach(char => {
+      if (char === 'o') {
+        o++
+      }
+    })
+    if (o <= 1) {
+      result.push(string)
+    }
+  })
+  return result
+}
+const new_company = myfilter(Companies)
+console.log(new_company)
+
+// Sort the array using sort() method
+Companies.sort()
+console.log(Companies)
+
+// Reverse the array using reverse() method
+Companies.reverse()
+console.log(Companies)
+
+// Slice out the first 3 companies from the array
+const slice1 = Companies.slice(3)
+console.log(slice1)
+
+// Slice out the last 3 companies from the array
+const slice2 = Companies.slice(0, companyLength - 3)
+console.log(slice2)
+
+// Slice out the middle IT company or companies from the array
+const slice3 = Companies.slice(0, 3).concat(Companies.slice(4))
+console.log(slice3)
+
+// Remove the first IT company from the array
+const notOracle = Companies.splice(0, 1)
+console.log(notOracle)
+console.log(Companies)
+
+Companies.push(notOracle.toString())
+Companies.sort()
+
+// Remove the middle IT company or companies from the array
+Companies.splice(3, 1)
+console.log(Companies)
+
+// Remove the last IT company from the array
+Companies.pop();
+console.log(Companies)
+console.log("after pop last item")
+
+// Remove all IT companies
+const afterSplice = Companies.splice()
+console.log(afterSplice)
\ No newline at end of file
diff --git a/day-01/Exercises/index.html b/day-01/Exercises/index.html
new file mode 100644
index 0000000..76a774f
--- /dev/null
+++ b/day-01/Exercises/index.html
@@ -0,0 +1,17 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <meta charset="UTF-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1.0">
+  <title>Document</title>
+
+  <script src="arr_level1.js"></script>
+</head>
+<body>
+  <h1>Exercises</h1>
+  <h2>arr_level1</h2>
+  <input id="iscompany" value="some company">
+  <button id='companybutton' onclick='isCompany()'>눌러</button>
+  <div id='companyresult'></div>
+</body>
+</html>
\ No newline at end of file
diff --git a/day-01/level1.js b/day-01/level1.js
deleted file mode 100644
index e69de29..0000000
diff --git a/mydocs/simpleJS.md b/mydocs/simpleJS.md
index 7c3342f..d147185 100644
--- a/mydocs/simpleJS.md
+++ b/mydocs/simpleJS.md
@@ -1,4 +1,7 @@
-자바스크립트를 적용하는 방법들
+# 간단한 자바스크립트 내용
+
+#### *자바스크립트를 적용하는 방법들*
+
 1. Inline script
 2. Internal script
 3. External script
@@ -14,5 +17,44 @@ External의 경우
 - JS파일을 따로 만들어서 넣는다
 - <script src="자바스크립트 파일 경로"></script>
 
----
+## 1. 변수
+
+const 와 let (~~var은 죽었어! 더이상 없어!~~)
+
+const는 한번 선언하면 값이 안바뀜(constant == 상수)
+
+```js
+let firstDay = "첫째 날"
+firstDay = "둘째 날"
+
+const first_day = "첫째 날" (위에서 firstDay가있어 중복 변수 안됨)
+first_day = "둘째 날"
+> Uncaught TypeError: Assignment to constant variable. (값 변경 불가능)
+```
+
+## 2. Data types
+
+## 3. Arrays
+
+```js
+const arr1 = Array()	// 보통 const로 배열을 선언한다고 한다
+let arr2 = new Array()
+const arr3 = []
+
+arr1
+> []
+arr2
+> []
+```
+
+```js
+const perfectArr2 = [
+    '승민',
+    25,
+    true,
+    {country: 'Korea', city: 'Seoul'},
+    {classes: ['a','b','c','d']}
+    ]
+// 여러 데이터 타입도 넣을 수 있음
+```