docs: simplify Ignoring Files From .eslintignore. (#1013)

The README advised ESLint >= 7 users to install node-filter-async.
Replace this suggestion with a simpler implementation requiring the same
number of lines of code but no dependencies. Rename the variable
eslintCli to eslint since it is an instance of the ESLint Node.js API as
distinct from the ESLint CLI. Avoid instantiating the API when the files
staged don't necessitate invocation of ESLint.

README.md

@@ -700,21 +700,19 @@ module.exports = {
 
 In versions of ESLint > 7, [isPathIgnored](https://eslint.org/docs/developer-guide/nodejs-api#-eslintispathignoredfilepath) is an async function and now returns a promise. The code below can be used to reinstate the above functionality.
 
-This particular code uses a tiny package, [node-filter-async](https://www.npmjs.com/package/node-filter-async), to filter the files array with an async function. If you prefer to not have an extra dependency, it is quite simple to write a similar function.
-
 Since [10.5.3](https://github.com/okonet/lint-staged/releases), any errors due to a bad ESLint config will come through to the console.
 
 ```js
 const { ESLint } = require('eslint')
-const filterAsync = require('node-filter-async').default
-
-const eslintCli = new ESLint()
 
 const removeIgnoredFiles = async (files) => {
-  const filteredFiles = await filterAsync(files, async (file) => {
-    const isIgnored = await eslintCli.isPathIgnored(file)
-    return !isIgnored
-  })
+  const eslint = new ESLint()
+  const isIgnored = await Promise.all(
+    files.map((file) => {
+      return eslint.isPathIgnored(file)
+    })
+  )
+  const filteredFiles = files.filter((_, i) => !isIgnored[i])
   return filteredFiles.join(' ')
 }