Functional Programming Patterns With RamdaJS!/

...

/

Comparators

Comparators

Comparators turn any comparing function into a sorting function, without fretting browser inconsistencies. (5 min. read)

We'll cover the following...

Ascend and Descend

You won’t need these as often as the main sorting functions, but they can be useful in some scenarios.

The functions we just saw, ascend/descend, are comparators. They take a function to apply against each value.

Here’s a sort by height.

Javascript (babel-node)
import { ascend, sort } from 'ramda';
const people = [{
  height: 23
}, {
  height: 230
}, {
  height: 2.3
}];
const getHeight = (x) => x.height;
const byHeight = ascend(getHeight);
const result = sort(byHeight, people);
console.log({ result });

Ramda’s prop function can replace the getHeight function.

Javascript (babel-node)
import { ascend, prop, sort } from 'ramda';
const people = [{
  height: 23
}, {
  height: 230
}, {
  height: 2.3
}];
const byHeight = ascend(prop('height'));
const result = sort(byHeight, people);
console.log({ result });

Descend sorts in the opposite direction, greatest first.

Javascript (babel-node)
import { descend, prop, sort } from 'ramda';
const people = [{
  height: 23
}, {
  height: 230
}, {
  height: 2.3
}];
const byHeight = descend(prop('height'));
const result = sort(byHeight, people);
console.log({ result });

And path lets you easily compare things, no matter how nested their properties are.

Javascript (babel-node)
import { ascend, path, sort } from 'ramda';
const people = [{
  name: 'I am second',
  metadata: {
    attributes: {
      height: {
        value: 23
      }
    }
  }
}, {
  name: 'I am last',
  metadata: {
    attributes: {
      height: {
        value: 230
      }
    }
  }
}, {
  name: 'I am first',
  metadata: {
    attributes: {
      height: {
        value: 2.3
      }
    }
  }
}];
const getHeight = path(['metadata', 'attributes', 'height', 'value']);
const byHeight = ascend(getHeight);
const result = sort(byHeight, people);
console.log(result);

Lower-Level Comparator

Ramda also has a comparator function. It creates comparator functions out of regular functions that compare two elements.

This function, for example…

Javascript (babel-node)
const byHeight = (a, b) => a.height > b.height;

…works with sort in modern browsers.

Javascript (babel-node)
import { sort } from 'ramda';
const people = [{ height: 20 }, { height: 10 }];
const byHeight = (a, b) => a.height > b.height;
const result = sort(byHeight, people);
console.log({ result });

But since byHeight returns a boolean, older browsers like Internet Explorer won’t sort people correctly. Older browsers require your comparator to return a number!

See this StackOverflow answer for more details.

A proper comparator looks like this

Javascript (babel-node)
const byHeight = (a, b) => {
  if (a.height > b.height) {
    return 1;
  }
  if (a.height < b.height) {
    return -1;
  }
  return 0;
};

Allowing this sort to work everywhere.

Javascript (babel-node)
import { sort } from 'ramda';
const people = [{ height: 20 }, { height: 10 }];
const byHeight = (a, b) => {
  if (a.height > b.height) {
    return 1;
  }
  if (a.height < b.height) {
    return -1;
  }
  return 0;
};
const result = sort(byHeight, people);
console.log({ result });

A bit verbose. Fortunately, Ramda turns any comparison function into a proper comparator. All we need is the comparator function.

Take your original byHeight function and flip > to <…

Javascript (babel-node)
const byHeight = (a, b) => a.height < b.height;

And wrap it in comparator. I’d prefer renaming it so the comparator function can be named byHeight.

Javascript (babel-node)
// rename this
const compareHeights = (a, b) => a.height < b.height;
// giving this a proper name
const byHeight = comparator(compareHeights)

Now use it.

Javascript (babel-node)
import { comparator, sort } from 'ramda';
const people = [{ height: 20 }, { height: 10 }];
const compareHeights = (a, b) => a.height < b.height;
const byHeight = comparator(compareHeights);
const result = sort(byHeight, people);
console.log({ result });