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README.md

eslint-plugin-array-func

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Rules for Array functions and methods.

Contents

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: reduce with reduceRight.

This rule is auto fixable.

Examples

Code that triggers this rule:

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 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);

prefer-flat-map

Use .flatMap() to flatten an array and map the values instead of using .flat().map().

This rule is auto fixable.

Examples

Code that triggers this rule:

const flattenedAndMapped = array.map((p) => p).flat();

Code that doesn't trigger this rule:

const oneAction = array.flatMap((m) => m);

const flattened = array.flat();

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

const mappedThenFlattened = array.flat().map((r) => r + 1);

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

prefer-flat

Use .flat() to flatten an array of arrays. This rule currently recognizes two patterns and can replace them with a .flat() call:

  • [].concat(...array)
  • array.reduce((p, n) => p.concat(n), [])

This rule is auto fixable.

Examples

Code that triggers this rule:

const concatFlat = [].concat(...array);

const reduceFlat = array.reduce((p, n) => p.concat(n), []);

Code that doesn't trigger this rule:

const flattened = array.flat();

const reverseFlat = array.reduce((p, n) => n.concat(p), []);

const otherReduce = array.reduce((p, n) => n + p, 0);

Configurations

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": [
    "plugin:array-func/recommended"
  ]
}

array-func/all Configuration

The recommended configuration does not include all rules, since some Array methods were added after ES2015. The all configuration enables all rules the plugin contains and sets the ECMA version appropriately.

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

License

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

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