Array with Default Utility

by Nicholas C. Zakas

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Description

A class that represents an array where missing elements return a specified default value.

The built-in JavaScript Array class automatically returns undefined when you attempt to access a missing element. Here's an example:

const items = [1, , 3];
console.log(items[1]);      // undefined

In most cases that is okay, but in some cases you may want an alternate default value returned. The ArrayWithDefault class does that:

const items = new ArrayWithDefault({
    default: 0
    elements: [1, , 3]
});

console.log(items[1]);      // 0

The primary use case for this class is when an array is an optional argument for a function or when an array may have holes.

Usage

Node.js

Install using npm or yarn:

npm install @humanwhocodes/array-with-default --save

# or

yarn add @humanwhocodes/array-with-default

Import into your Node.js project:

// CommonJS
const { ArrayWithDefault } = require("@humanwhocodes/array-with-default");

// ESM
import { ArrayWithDefault } from "@humanwhocodes/array-with-default";

Deno

Import into your Deno project:

import { ArrayWithDefault } from "https://cdn.skypack.dev/@humanwhocodes/array-with-default?dts";

Browser

It's recommended to import the minified version to save bandwidth:

import { ArrayWithDefault } from "https://cdn.skypack.dev/@humanwhocodes/array-with-default?min";

However, you can also import the unminified version for debugging purposes:

import { ArrayWithDefault } from "https://cdn.skypack.dev/@humanwhocodes/array-with-default";

API

After importing, create a new instance of ArrayWithDefault. The constructor expects one object argument with the following properties:

• default (required) - the default value to return for the missing items.
• elements - an optional iterable object used to populate the array.
• length - an optional value to set the array's length property to.
• outOfRange - an optional value that, when set to true, indicates that numeric indices after the end of the array should also return the default value.

Here are some examples:

const items = new ArrayWithDefault({
    default: 0,
    elements: [1, , 3]
});

// missing elements return the default
console.log(items[1]);      // 0

// elements after the end of the array return undefined
console.log(items[10])      // undefined

// elements whose value are undefined return the default
items.push(undefined);
console.log(items[3]);      // 0

const emptyItems = new ArrayWithDefault({
    default: 0,
    length: 5
});

// all elements return 0
console.log(emptyItems[0]);     // 0
console.log(emptyItems[1]);     // 0
console.log(emptyItems[2]);     // 0
console.log(emptyItems[3]);     // 0
console.log(emptyItems[4]);     // 0

// items past the end still return undefined
console.log(emptyItems[5]);     // undefined

const numbers = new ArrayWithDefault({
    default: 0,
    elements: [1, 2, 3],
    outOfRange: true
});

// all elements return 0
console.log(emptyItems[0]);     // 1
console.log(emptyItems[1]);     // 2
console.log(emptyItems[2]);     // 3
console.log(emptyItems[3]);     // 0
console.log(emptyItems[4]);     // 0
console.log(emptyItems[5]);     // 0

Developer Setup

1. Fork the repository
2. Clone your fork
3. Run npm install to setup dependencies
4. Run npm test to run tests

License

Apache 2.0