PureArray.js

// Array proxy
const PureArray = (...args) =>
  new Proxy([...args], // Create internal array

    // Handler with traps
    {
      // Intercept assignment to array using index
      set(target, property, value, receiver) {

        // Allow built in properties like length
        if (Reflect.has(target, property))
          return Reflect.get(target, property, receiver);

        // Check if property is positive integer
        const index = Number.parseInt(property);
        if (Number.isNaN(index) || index < 0)
          throw new Error("Index must be >= 0");

        // Otherwise call the internal array
        return Reflect.set(target, property, value, receiver);
      },

      // Intercept access to the array via index
      get(target, property, receiver) {
        // Check if its one of the built in properties
        if (Reflect.has(target, property))
          return Reflect.get(target, property, receiver);
        
        // Check if property is positive integer
        const index = Number.parseInt(property);
        if (Number.isNaN(index) || index < 0)
          throw new Error("Index must be >= 0");

        // Otherwise call the internal array
        return Reflect.get(target, property, receiver);

      }
    }
  );


// let's create our "Pure" array
const myArray = PureArray(1, 2, 3);
console.log(myArray);

// Normal operations work
myArray[3] = 4;
console.log(myArray);
console.log(myArray.slice(2, 3));
console.log(myArray.length);

// These don't work anymore
myArray[-1] = 5;
console.log(myArray[-3]);