Javascript Array Bifurcate

This is a pattern to split the values from an array into two groups based on a condition, if is truthy the corresponding element belongs to the first group, otherwise it belongs to the second group.

This is a pattern to split the values from an array into two groups based on a condition, if is truthy the corresponding element belongs to the first group, otherwise it belongs to the second group, we are going to use two method from the Array, the reduce and the push to add the elements to groups.

I found myself filtering an array of items to get all items that matched a certain condition, then doing that again for a different condition. That meant looping over an array twice but I could have just done it once with this pattern.

const values = [1, 15, 8, 32, 23, 2];

const [truthyValues, falseyValues] = values.reduce((accumulator, value) => {
  if (value > 10) {
    accumulator[0].push(value);
  } else {
    accumulator[1].push(value);
  }
  
  return accumulator;
}, [[], []]);

console.log(truthyValues);
// expected output: [15, 32, 23]

console.log(falseyValues);
// expected output: [1, 8, 2]

We can improve this by implementing a filter function

const bifurcate = (arr, filter) =>
  arr.reduce((accumulator, value) => {
    accumulator[filter(value) ? 0 : 1].push(value);
    
    return accumulator;
  }, [[], []]);

const values = [1, 15, 8, 32, 23, 2];

const [truthyValues, falseyValues] = bifurcate(values, v => v > 10);

console.log(truthyValues);
// expected output: [15, 32, 23]

console.log(falseyValues);
// expected output: [1, 8, 2]