lazy-map.js

/**
 * Map a function over an array, but only return a new array object if the
 * application of the function to some value isn't identical to the value.
 *
 * @example
 * xs !== xs.map(x => x)
 * xs === lazyMap(xs, x => x)
 *
 * @param {Array}    xs Array to be mapped over.
 * @param {Function} f  Function to map over array.
 * @return {Array}      Th input array if possible, or the map as a new object.
 */

function lazyMap( xs, f ) {
	let result = xs;
	for ( const i in xs ) {
		const x = xs[ i ];
		const v = f( x );
		if ( v !== x ) {
			if ( result === xs ) result = xs.slice();
			result[ i ] = v;
		}
	}
	return result;
}

export default lazyMap;