when/unless

Using when/unless in functional pipelines (4 min. read)

Sometimes you only need the if statement, and the else simply returns the value unchanged.

const isEven = (num) => num % 2 === 0;
const doubleIfEven = (num) => {
  if (isEven(num)) {
    return num * 2;
  }
  
  return num;
};
console.log(
  doubleIfEven(100),
  doubleIfEven(101)
);

Again, ternaries can work well here.

const isEven = (num) => num % 2 === 0;
const doubleIfEven = (num) => isEven(num) ? num * 2 : num;
console.log(
  doubleIfEven(100),
  doubleIfEven(101)
);

But we can also use the when function. It takes three arguments

  1. Predicate (function that returns true or false)
  2. Function to run if predicate returns true
  3. The value to use
import { when } from 'ramda';
const isEven = (num) => num % 2 === 0;
const doubleIfEven = when(
  isEven,
  (num) => num * 2
);
console.log(
  doubleIfEven(100),
  doubleIfEven(101)
);

We can make it point-free.

import { multiply, when } from 'ramda';
const isEven = (num) => num % 2 === 0;
const doubleIfEven = when(
  isEven,
  multiply(2)
);
console.log(
  doubleIfEven(100),
  doubleIfEven(101)
);

Conveniently enough, Ramda lets you express the opposite logic using unless.

This runs your function if the predicate returns false.

import { multiply, unless } from 'ramda';
const isEven = (num) => num % 2 === 0;
const doubleIfOdd = unless(
  isEven,
  multiply(2)
);
console.log(
  doubleIfOdd(100),
  doubleIfOdd(101)
);

Now this function only doubles odd numbers. If we wanted doubleIfEven, our predicate must flip as well.

import { multiply, unless } from 'ramda';
const isOdd = (num) => num % 2 !== 0;
const doubleIfEven = unless(
  isOdd,
  multiply(2)
);
console.log(
  doubleIfEven(100),
  doubleIfEven(101)
);