ImgMinify/minify.js

const gm = require('gm').subClass({ imageMagick: true });
const fs = require('fs');
const runExtnames = ["png", "jpg", "jpeg", "gif", "bmp"]

// 检查格式是否符合
const extnameCheck = name => ~runExtnames.indexOf(name.toLocaleLowerCase())

/**
 * 对图片进行缩放
 * @param {*} width Number 缩放宽度 
 *            height Number 缩放高度
 */
function zoom({ width, height }) {
  let seft = this

  width = Number(width) || 0;
  width = width < 0 ? 0 : width;
  height = Number(height) || 0;
  height = height < 0 ? 0 : height;

  if (!width && !height) throw 'Invalid zoom size'

  if (width && height) {
    let { origin_width: oWdith, origin_height: oHeight } = seft
    let porp = width / height, oPorp = oWdith / oHeight, left, top, cWidth, cHeight;

    if (oPorp <= porp) {
      seft.resize(width);
      cWidth = width;
      cHeight = cWidth / oPorp;
    } else if (oPorp > porp) {
      seft.resize(undefined, height);
      cHeight = height;
      cWidth = cHeight * oPorp;
    }


    left = (cWidth - width) / 2;
    top = (cHeight - height) / 2;
    seft = seft.crop(width, height, left, top);
  } else {
    seft = seft.resize(width || undefined, height || undefined);
  }

  return seft;
}


/**
 * 初始化缩略图对象
 * @param {*} fileUri string 要进行缩放的图片本地路径
 *            stream readStream 要进行缩放的图片的可读流
 *            fileFormat string 要缩放的图片的后缀名
 * @return Promise 
 */
const init = ({ fileUri, stream }) => {
  if (fileUri) {
    stream = fs.createReadStream(fileUri);
  } else if (!stream) {
    throw 'Unknown file';
  }

  return new Promise((resolve, reject) => {
    gm(stream).size({ bufferStream: true }, function (err, val) {
      if (err) {
        reject(err)
      } else {
        let result = Object.create(this);
        result.origin_width = val.width;
        result.origin_height = val.height;
        result.zoom = zoom;
        resolve(result);
      }
    });
  });
}

module.exports = exports = {
  runExtnames,
  init,
  extnameCheck
};