A type-preserving TypeScript helper to partition arrays.

partition.ts

/**
 * Partition an array into two arrays based on a predicate.
 *
 * @param array
 * @param isValid
 */
export function partition<T, V>( array: ( T | V )[], isValid: ( x: T | V ) => x is T ): [ T[], V[] ] {
    return array.reduce( ( [ pass, fail ], elem ) => {
        return isValid( elem ) ? [ [ ...pass, elem ], fail ] : [ pass, [ ...fail, elem ] ];
    }, [ [] as T[], [] as V[] ] );
}