지식 줍줍 🐿️

var test = [1,2,3,4,5]; 
test.reverse(); //test=[5,4,3,2,1] 

var testString = "1,2,3,4,5";
testString.split(",");// ["1", "2", "3", "4", "5"]

var test = [1,2,3,4,5];
test.shift();//1
test.shift();//2
test.shift();//3
test.shift();//4
test.shift();//5
test.shift();//undefined
console.log(test);//

var testA = [6,7,8,9];
testA.shift();//6
console.log(testA);//(3) [7, 8, 9]

var test = [1,2,3]
test.unshift()
console.log(test);// (3) [1, 2, 3]
test.unshift(4) 
console.log(test); // (4) [4, 1, 2, 3]
test.unshift(6)
console.log(test);// (5) [6, 4, 1, 2, 3]

var test = [1,2,3,4,5,6];
test.push(7); //7
console.log(test); // (7) [1, 2, 3, 4, 5, 6, 7]

var test = [1,2,3,4,5,6];
test.push(); //6
console.log(test); // (5) [1, 2, 3, 4, 5]

var test = [1,2,3,4,5,6,7,8,9];
test.splice(0,2)// (2) [1, 2] : 0번째에서 2개 버림
console.log(test)// (7) [3, 4, 5, 6, 7, 8, 9]
test.splice(2,5)// (5) [5, 6, 7, 8, 9] : 012번째에서5개버림
console.log(test)// (2) [3, 4]

var A = [5,6,7]
var B = [1,2,3]
A.concat(B) //(6) [5, 6, 7, 1, 2, 3]

var test = [1,2,3,4,555,6]
test.join(",") //"1,2,3,4,555,6"
test.join("555")//"15552555355545555555556"
test.join("1")//"1121314155516"
test.join("-")//"1-2-3-4-555-6"

var test = [4,3,6,1,2]
test.sort() //(5) [1, 2, 3, 4, 6]

var test = 'Hello, my world!!';
test.substr(2,3) //"llo"//012번째에서3개
test.substr(0,3)//"Hel"//0번째에서3개
test.substr(0,5) //"Hello" //0번째에서5개
test.substr(5,5)//", my " //012345번째에서 5개