NegativeArray.js

class NegativeArray extends Array {
	constructor( ...args ) {
		super( ...args );

		return new Proxy( this, {
			get( target, property, ...args ) {
				const translatedProperty = translateProperty( target, property );

				return Reflect.get( target, translatedProperty, ...args );
			},

			set( target, property, value, ...args ) {
				const translatedProperty = translateProperty( target, property );

				return Reflect.set( target, translatedProperty, value, ...args );
			}
		} );
	}
}

function translateProperty( target, property ) {
	const propertyAsNumber = Number( property );

	if ( Number.isNaN( propertyAsNumber ) || propertyAsNumber >= 0 ) {
		return property;
	}

	const translatedProperty = target.length - Math.abs( propertyAsNumber );

	if ( translatedProperty < 0 ) {
		throw new RangeError( 'Array index out of range' );
	}

	return translatedProperty;
}

const array = new NegativeArray( 1, 2, 3, 4 );

console.log( array[ -1 ] );
array[ -2 ] = 6;
console.log( array );

console.log(0, array[0]); returns "undefined" instead of 1

Thx, I've fixed the code.