[JavaScript] 자주 쓰이는 함수

var integerNum = parseInt(str);
// string--> integer

var binaryNum = parseInt(str, 2);
//binary number라는 것을 컴퓨터에게 알려줌

num > 0 ? "positive" : num < 0 ? "negative" : "zero";

if(num > 0){
	return "positive";
}
else if(num < 0){
	return "negative";
}
else{
	return "zero";
}

"use strict"

//var와 let 차이
function checkScope(){
    "use strict";

    var i = "function scope";
    if(true){
        i = "block scope";
        console.log("block scope i is : ",i);
    }
    console.log("function scope i is : ",i);
    return i;
}
checkScope();

//block scope
//block scope





function checkScope(){
    "use strict";

    let i = "function scope";
    if(true){
        let i = "block scope";
        console.log("block scope i is : ",i);
    }
    console.log("function scope i is : ",i);
    return i;
}
checkScope();
//block scope
//function scope




function checkScope(){
    "use strict";

    //let i = "function scope";
    if(true){
        var i = "block scope";
        console.log("block scope i is : ",i);
    }
    console.log("function scope i is : ",i); //can still access
    return i;
}
checkScope();
//block scope
//block scope




function checkScope(){
    "use strict";

    //let i = "function scope";
    if(true){
        let i = "block scope";
        console.log("block scope i is : ",i);
    }
    console.log("function scope i is : ",i); //can not access
    return i;
}
checkScope();
//block scope
//not defined


//var는 블록 밖에서도 여전히 접근, 수정 가능하다.
//let은 해당 블록 내에서만 사용 가능하다. ex)if, while, for
//let 대신 var를 사용하는 이유.

const sentence = "hello";
//수정 불가


const s= [5,7,2];
function test(){
    "use strict";
    //s=[2,5,7];
    s[0]=2;
    s[1]=5;
    s[2]=7;
    //배열 주소는 접근이 안 되고
    //인덱스로 접근해야 함
}

test();
console.log(s);

 function freezeObj(){
     "use strict";

     const MATH_CONSTANTS = {
         PI: 3.14
     };

     Object.freeze(MATH_CONSTANTS);

     try{
         MATH_CONSTANTS.PI = 99;
         console.log(MATH_CONSTANTS.PI);
     }catch(ex){
         console.log(ex);
     }

     return MATH_CONSTANTS.PI;
 }

 const PI = freezeObj();
 
 //read only error

//arrow function

var myConcat = function(arr1, arr2){
    return arr1.concat(arr2);
}

var myConcat2 = (arr1, arr2) => arr1.concat(arr2);
//위 아래 같음





 const real = [4, 5.6, -9.8, 3.14, 42, 6, 8.34, -2];
 
 const squareList = (arr) => {
     const squaredIntegers = arr.filter(num => Number.isInteger(num) && num > 0).map(x => x*x);
     return squaredIntegers;
 }

 const squaredIntegers = squareList(real);
 console.log(squaredIntegers);
 
 
 
 
 const increment = (function(){
 	return function increment(number, value =1){
    	return number+value;
    };
})();
console.log(increment(5,2));
console.log(increment(5));
//7
//6
 
 
 
 
 
 const sum = (function(){
    return function sum(...args){
        return args.reduce((a,b)=> a+b, 0);
    };
})();

console.log(sum(1,2,3));
//아래와 같음
const sum = (function(){
    return function sum(x,y,z){
    	const args = [x,y,z];
        return args.reduce((a,b)=> a+b, 0);
    };
})();

console.log(sum(1,2,3));