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:

``js
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:

`js
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

$3

Install using [npm][npm] or [yarn][yarn]:

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

or

yarn add @humanwhocodes/array-with-default
`

Import into your Node.js project:

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

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

$3

Import into your Deno project:

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

$3

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

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

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

`js
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:

`js
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

[npm]: https://npmjs.com/
[yarn]: https://yarnpkg.com/