README.md

eslint-plugin-array-func

Rules for Array functions and methods.

Contents

• Installation
• Rules
  • from-map
    • Examples
  • no-unnecessary-this-arg
    • Checked Functions
    • Checked Methods
    • Examples
  • prefer-array-from
    • Examples
  • avoid-reverse
    • Examples
• array-func/recommended Configuration
  • Using the Configuration
• License

Installation

Install ESLint either locally or globally.

$ npm install -D eslint

If you installed ESLint globally, you have to install the array-func plugin globally too. Otherwise, install it locally.

$ npm install -D eslint-plugin-array-func

Rules

from-map

Prefer using the mapFn callback of Array.from over an immediate .map() call on the Array.from result.

Array.from has a mapFn callback that lets you map the items of the iterable to an array like you would with .map() except that values have not yet been truncated to fit types allowed in an array. Some iterables can't be directly converted to an array and thus have to be iterated either way. In that case using the mapping callback of Array.from avoids an iteration. See also MDN for an explanation of the potential benefits of using the mapping callback of Array.from directly.

This rule is auto fixable. It will produce nested function calls if you use the Array.from map callback and have a .map() call following it.

Examples

Code that triggers this rule:

Array.from(iterable).map((t) => t.id);

Array.from(iterable, (t) => t.id).map((id) => id[0]);

Code that doesn't trigger this rule:

Array.from(iterable, (t) => t.id);

Array.from(iterable, function(t) { this.format(t); }, this);

const arr = Array.from(iterable);
const mappedArray = arr.map((t) => t.id);

no-unnecessary-this-arg

Avoid the this parameter when providing arrow function as callback in array functions.

The this parameter is useless when providing arrow functions, since the this of arrow functions can not be rebound, thus the parameter has no effect.

The fix is usually to omit the parameter. The Array methods can't be auto-fixed, since the detection of array methods is not confident enough to know that the method is being called on an array.

Checked Functions

• from (fixable)

Checked Methods

• every
• filter
• find
• findIndex
• forEach
• map
• some

Examples

Code that triggers this rule:

const array = Array.from("example", (char) => char.charCodeAt(0), this);

const e = array.find((char) => char === 101, this);

const exampleAsArray = array.map((char) => String.fromCharCode(char), this);

const eIndex = array.findIndex((char) => char === 101, this);

const containsE = array.some((char) => char === 101, this);

const isOnlyE = array.every((char) => char === 101, this);

const onlyEs = array.filter((char) => char === 101, this);

array.forEach((char) => console.log(char), this);

Code that doesn't trigger this rule:

const array = Array.from("example", (char) => char.charCodeAt(0));
const alternateArray = Array.from("example", function(char) {
    return char.charCodeAt(this)
}, 0);

const e = array.find((char) => char === 101);

const exampleAsArray = array.map((char) => String.fromCharCode(char));

const eIndex = array.findIndex((char) => char === 101);

const containsE = array.some((char) => char === 101);

const isOnlyE = array.every((char) => char === 101);

const onlyEs = array.filter(function(char) {
    return char === this
}, 101);

array.forEach(function(char) {
    this.log(char);
}, console);

array.filter(this.isGood, this);

prefer-array-from

Use Array.from instead of [...iterable] for performance benefits.

This rule is auto fixable.

Examples

Code that triggers this rule:

const iterable = [..."string"];

const arrayCopy = [...iterable];

Code that doesn't trigger this rule:

const array = [1, 2, 3];

const extendedArray =  [0, ...array];

const arrayCopy = Array.from(array);

const characterArray = Array.from("string");

avoid-reverse

Avoid reversing the array and running a method on it if there is an equivalent of the method operating on the array from the other end.

There are two operations with such equivalents: indexOf with lastIndexOf and reduce with reduceRight.

This rule is auto fixable.

Examples

Code that triggers this rule:

const lastIndex = array.reverse().indexOf(1);

const firstIndex = array.reverse().lastIndexOf(1);

const sum = array.reverse().reduce((p, c) => p + c, 0);

const reverseSum = array.reverse().reduceRight((p, c) => p + c, 0);

Code that doesn't trigger this rule:

const lastIndex = array.lastIndexOf(1);

const firstIndex = array.indexOf(1);

const sum = array.reduce((p, c) => p + c, 0);

const reverseSum = array.reduceRight((p, c) => p + c, 0);

const reverseArray = array.reverse();

const reverseMap = array.reverse().map((r) => r + 1);

array-func/recommended Configuration

The recommended configuration will set your parser ECMA Version to 2015, since that's when the Array functions and methods were added.

Rule	Error level	Fixable
from-map	Error	Yes
no-unnecessary-this-arg	Error	Sometimes
prefer-array-from	Error	Yes
avoid-reverse	Error	Yes

Using the Configuration

To enable this configuration use the extends property in your .eslintrc.json config file (may look different for other config file styles):

{
    "extends": [
        "array-func/recommended"
    ]
}

License

The array-func plugin is licensed under the MIT License.