generate-thumbnails.js

const Jimp = require(`jimp`)
const imagemin = require('imagemin')
const imageminPngquant = require('imagemin-pngquant')
const { readdirSync } = require(`fs`)
const { join } = require(`path`)

const relativeToThisFile = relativePath => join(__dirname, relativePath)

const inputPath = relativeToThisFile(`../../book-cover-image`)
const outputPath = relativeToThisFile(`../../Markdown/Web/book-image/thumbnail`)

const files = readdirSync(inputPath)

// 900x1350

const toTransform = files.filter(file => /\.png$/.test(file))

toTransform.forEach(filename => {
	const path = join(inputPath, filename)

	Jimp.read(path).then(image => {
		console.log(`Writing`, filename)
		return new Promise((resolve, reject) => {
			const filePath = join(outputPath, filename)
			image
				.autocrop()
				.resize(400, 600)
				.crop(0, 0, 400, 400)
				.write(filePath, err => err ? reject(err) : resolve(filePath))
		})
	}).then(filePath => {
		imagemin([filePath], outputPath, {
			plugins: [
				imageminPngquant({ quality: '70-80' })
			]
		})
	})
})