Functional Programming 函式編程

// 非 Functional Programming
let name = `KJ`;
let greeting = `Hi, I'm`;

// Hi, I'm KJ
console.log(`${greeting} ${name}`);

// 非 Functional Programming
let name = `KJ`;
let greet = () => {
    let greeting = `Hi, I'm`;
    return `${greeting} ${name}`;
}

// Hi, I'm KJ
console.log(greet());

// 是 Functional Programming
let greet = (name) => {
    let greeting = `Hi, I'm`;
    return `${greeting} ${name}`;
}

let name = `KJ`;

// Hi, I'm KJ
console.log(greet(name));

// 是 Functional Programming
// 形容詞函式產生器
let makeAdjectifier = (adjective) => {
    return (adjectiveTarget) => {
        return `${adjective} ${adjectiveTarget}!!`;
    }
}

let Coolifier = makeAdjectifier('Cool');
let Niceifier = makeAdjectifier('Nice');

let name = 'KJ';
// Cool KJ!!
console.log(Coolifier(name));
// Nice KJ!!
console.log(Niceifier(name));

// 非 Functional Programming
let Employee = ['Kay', 'Jay', 'KJ'];
Employee[1] = 'Apple';
// [ 'Kay', 'Apple', 'KJ' ]
console.log(Employee);

let Employee = ['Kay', 'Jay', 'KJ'];
let NewEmployee = Employee.map((currentValue) => {
    if (currentValue == 'Jay') {
        return 'Apple';
    } else {
        return currentValue;
    }
});

// [ 'Kay', 'Jay', 'KJ' ]
console.log(Employee);
// [ 'Kay', 'Apple', 'KJ' ]
console.log(NewEmployee);

// ...
// doSomethineChangeEmployee(Employee);
// doSomethineChangeNewEmployee(Employee);

let calculationBMI = (bmi) => {
    if(bmi < 0) {
        return '資料錯誤'
    } else if(bmi < 18.5) {
        return '過輕'
    } else if(bmi >= 18.5 && bmi < 24) {
        return '正常'
    } else if(bmi >= 24 && bmi < 27) {
        return '過重'
    } else if(bmi >= 27 && bmi < 30) {
        return '輕度肥胖'
    } else if(bmi >= 30 && bmi < 35) {
        return '中度肥胖'
    } else{
        return '重度肥胖'
    }
}

// 正常
console.log(calculationBMI(20));

// match 函式先省略
const lessThan = x => bmi => bmi< x ;

const calculationBMI = BMI =>
match(BMI)
  .on(lessThan(0), () => '資料錯誤')
  .on(lessThan(18.5), () => '體重過輕')
  .on(lessThan(24), () => '正常')
  .on(lessThan(27), () => '過重')
  .on(lessThan(30), () => '輕度肥胖')
  .on(lessThan(35), () => '中度肥胖')
  .otherwise(() => "重度肥胖")

// 正常
console.log(calculationBMI(20));