[Ramda] Handle Branching Logic with Ramda's Conditional Functions

When you want to build your logic with small, composable functions you need a functional way to handle conditional logic. You could wrap ternary expressions and if/else statements in functions, handling all of the concerns around data mutation yourself, or you could leverage the conditional functions supplied by Ramda. In this lesson, we'll cover several of Ramda's conditional functions: ifElse, unless, when and cond

const products = [
  {name: 'Jeans', price:80, category: 'clothes'},
  {name: 'Cards', price: 5, category: 'games'},
  {name: 'iPhone', price: 649, category: 'electronics'},
  {name: 'Freakonomics', price: 30, category: 'books'}];


/*
  LOGICS
*/
const pLens = R.lensProp('price');
const addDiscount = R.curry( (prec, amount) => {
  return amount - (amount * (prec/100))
});

/*
  EFFECTS
*/

// Apply discount to all the products --> ifElse
const applyDiscountForAllProduct = () => {
  const adjustPrice = R.over(pLens, addDiscount(50));
  return R.map(adjustPrice, products);
}

// Apply discount with condition to all predicates
const applyDiscountWithCondition = () => {
  const prediction = R.propEq('category', 'clothes');
  const conditionTrue = R.over(pLens, addDiscount(50));
  const conditionFalse = R.over(pLens, addDiscount(10));
  
  const adjustPrice = R.ifElse(
    prediction,
    conditionTrue,
    conditionFalse
  );
  
  return R.map(adjustPrice, products);
}

// Apply disocunt when meet the prediciton --> when
const applyDiscountOnlyToPart = () => {
  const prediction = R.propEq('category', 'clothes');
  const conditionTrue = R.over(pLens, addDiscount(50));
  const conditionFalse = R.identity; // return the original value

  /*const adjustPrice = R.ifElse(
    prediction,
    conditionTrue,
    conditionFalse
  );*/
  
  // or
  const adjustPrice = R.when(
    prediction,
    conditionTrue
  );
  
  return R.map(adjustPrice, products);
  
}

// Apply discount when prediction return false --> unless
const applyDiscountOnlyToPart2 = () => {
  const prediction = R.propEq('category', 'clothes');
  const conditionTrue = R.over(pLens, addDiscount(50));

  const adjustPrice = R.unless(
    prediction,
    conditionTrue
  );
  
  return R.map(adjustPrice, products);
}

// Apply discount for multi conditions --> cond
const applyDiscountForMultiConds = () => {
  const cond1 = [
    R.propEq('category', 'clothes'),
    R.over(pLens, addDiscount(50))
  ];
  const cond2 = [
    R.propEq('category', 'electronics'),
    R.over(pLens, addDiscount(10))
  ];
  const cond3 = [
    R.propEq('category', 'books'),
    R.over(pLens, addDiscount(70))
  ];
  const restCond = [
    R.T,
    R.identity
  ];
  
  const adjustPrice = R.cond([
    cond1,
    cond2,
    cond3,
    restCond
  ]);
  
  return R.map(adjustPrice, products);
};


const result = applyDiscountForMultiConds();
console.clear();
console.log(result);