A proxy that acts like an array

arrayproxy.js

// target is the backing object
let target = { length: 0 },
    proxy = new Proxy(target, {
        set(trapTarget, key, value) {

            let numericKey = Number(key),
                keyIsInteger = Number.isInteger(numericKey);

            // special case for length property - only need to worry if length is
            // shorter than number of array items
            if (key === "length" && value < trapTarget.length) {
                
                // delete everything after the new length
                for (let index = trapTarget.length; index >= value; index--) {
                    Reflect.deleteProperty(trapTarget, index);
                }
            } else if (keyIsInteger && numericKey >= trapTarget.length) {
                // update length if the key is numerically greater than the array length
                Reflect.set(trapTarget, "length", numericKey + 1);
            }

            // assign the property as normal
            Reflect.set(trapTarget, key, value);
        }
    });

console.log(proxy.length);      // 0

proxy[0] = "proxy1";

console.log(proxy.length);      // 1

proxy[1] = "proxy2";

console.log(proxy.length);      // 2

proxy.length = 0;

console.log(proxy.length);      // 0
console.log(proxy[0]);          // undefined
console.log(0 in proxy);        // false