Typescript functions to share

partition.ts

/**
 * Separate a list into two lists (left and right) using a predicate to detect
 * items that should be in the left list. The rest go into the right list.
 * 
 * @param isLeft - predicate to detect items that should be in the left list
 * 
 * @returns a function that accepts an array and returns a pair of arrays
 *
 * @todo - return type [ L[], R[] ] instead of ambiguous type.
 */
const partition
  = <T>(isLeft: (item: T) => boolean) => (array: T[]): [ T[], T[] ] =>
    array.reduce(
      ([ left, right ], item) =>
        isLeft(item)
          ? [ [ ...left, item ], right ]
          : [ left, [ ...right, item ] ],
      [ [], [] ] as [ T[], T[] ]
    )

// Exaample implementation:
const partitionStringsFromNulls = partition(x => typeof x === 'string')

// Example usage:
const [ strings, nulls ] = partitionStringsFromNulls([ 'foo', null, 'bar', 'baz' ]))
console.log(strings, nulls)
// [ 'foo', 'bar', 'baz' ] [ null ]