extensible-read-only-array ·

Creates a read only array, allowing extension with custom methods

Description

Extensible Read Only Array uses a proxy to create a makeshift API to your array.

The array can be read as normal, with direct requests for specific indexes, the length property and iterator function, however all requests to set an index are blocked.

The array can then be extended by providing an object or class containing your own code.

See...

• Install/Usage
• Syntax
• Examples
• Allowing Array Methods

Install

npm install --save @chriscodesthings/extensible-read-only-array

Usage

import makeReadOnlyArray from '@chriscodesthings/extensible-read-only-array';

const numbers = makeReadOnlyArray([], {
    addSquareNumber(n, arr) {
        return arr.push(n * n);
    }
});

console.log(numbers);
// => []

numbers.addSquareNumber(2);
numbers.addSquareNumber(4);
numbers.addSquareNumber(6);

console.log(numbers);
// => [ 4, 16, 36 ]

Note, the array is always passed by the proxy as the last argument to the function.

Important! Do not return the array since this will allow direct access to it.

Syntax

makeReadOnlyArray(arr, obj, allow, allowDefaults);

Parameters

• arr: the array to make read only
• obj: your object containing methods/properties to redirect to
• allow: an array containing a list of array methods to allow
• allowDefaults: Default true. If false, blocks access to the methods allowed by default.

See also, Allowing Array Methods

Return Value

Returns a proxy attached to the original array.

Examples

Use with a handler object

We can create an array that will only store people's names by using a handler object with a method to add a person to the array.

Note, within your code, you have full access to the array.

import makeReadOnlyArray from '@chriscodesthings/extensible-read-only-array';

const peopleHandler = {
    addPerson(first, last, age, arr) {
        arr.push({
            firstname: first,
            lastname: last,
            age: age
        });
    }
};

const people = makeReadOnlyArray([], peopleHandler);

Use with a class

You can use Extensible Read Only Array directly from your constructor to allow read only access to an array, while keeping it private within your class.

import makeReadOnlyArray from '@chriscodesthings/extensible-read-only-array';

class extendedReadOnlyArrayClass {
    #people = [];

    constructor() {
        return makeReadOnlyArray(this.#arr, this);
    }

    addPerson(first, last, age) {
        this.#people.push({
            firstname: first,
            lastname: last,
            age: age
        });
    }
}

const testArray = new extendedReadOnlyArrayClass();

Remember! Even though it isn't used, the array is still passed as the last argument to the function.

Allowing Array Methods

Routing priority

The proxy will route the request in the following order:

1. A matching method in obj. This allows you to override any of the built in array methods.
2. A matching property in obj. This allows you to override any of the built in array properties.
3. If request is a number >= 0, an index from the array.
4. If the request is in the allow list (see below), the request is passed to the native array.
5. Returns undefined.

Access to the native array methods is divided into 3 categories.

Always allowed

These properties/methods are always allowed since they are required for normal array iteration.

• length property
• [@@iterator] method

Allowed by default

These methods are allowed by default, unless allowDefaults is set to false. They are all considered to be 'read only' methods which do not modify, or allow modification in any way, of the original array.

There is little point in excluding access to these methods since they could easily be replicated by simply first copying the array contents into a new array, element by element.

• at
• concat
• entries
• every
• find
• findIndex
• findLast
• findLastIndex
• forEach
• includes
• indexOf
• join
• keys
• lastIndexOf
• map
• reduce
• reduceRight
• some
• toLocaleString
• toReversed
• toSorted
• toSpliced
• toString
• values
• with

Allow specific methods

To allow access to certain methods, specify these in an array when creating the Extensible Read Only Array.

For example, to allow access to the reverse() and sort() methods:

const readOnlyArray = makeReadOnlyArray([], arrayHandler, ["reverse", "sort"]);