dumb-image-preloader/distribution/index.js

An image pre-loader that takes a bunch of image URLs and loads them into the browsers cache

'use strict';

Object.defineProperty(exports, "__esModule", {
  value: true
});
var MSG_REJECT = 'dumbImagePreloader: Parameter provided is not an array or is an empty array.';

/**
 * getImageFromBlob - takes a blob from a fetch response and returns image
 * @param  {object} blob - blob from response object
 * @return {object}      - image object or undefined
 */
var getImageFromBlob = exports.getImageFromBlob = function getImageFromBlob(blob) {
  var img = void 0;
  if (blob !== null) {
    img = new Image();
    img.src = URL.createObjectURL(blob);
  }
  return img;
};

/**
 * getBlobFromResponse - takes in a fetch response object and returns the blob
 * @param  {object} response - fetch response object
 * @return {object}          - null if response has error, blob otherwise
 */
var getBlobFromResponse = exports.getBlobFromResponse = function getBlobFromResponse(response) {
  return response.ok ? response.blob() : null;
};

/**
 * getRequestSettings - constructs a request settings object for Request
 * @return {object} - settings object
 */
var getRequestSettings = exports.getRequestSettings = function getRequestSettings() {
  var headers = new Headers();
  return { method: 'GET', headers: headers, mode: 'no-cors', cache: 'default' };
};

/**
 * requestImage fetches the image by URl
 * @param  {string} url      - Image request URL
 * @param  {object} settings - Request settings
 * @return {promise}         - Resolves whether request is successful / fails
 */
var requestImage = exports.requestImage = function requestImage(url, settings) {
  var request = new Request(url, settings);
  return fetch(request).then(function (response) {
    return getBlobFromResponse(response);
  });
};

/**
 * Pre-load images
 * @param {array} imageURLs - image URL array
 * @return {promise} - promise
 */
var dumbImagePreloader = function dumbImagePreloader(imageURLs) {
  if (!Array.isArray(imageURLs) || imageURLs.length === 0) {
    return Promise.reject(new Error(MSG_REJECT));
  }
  var settings = getRequestSettings();
  var imgReqPromiseArray = imageURLs.map(function (imageURL) {
    return requestImage(imageURL, settings);
  });
  return Promise.all(imgReqPromiseArray).then(function (responses) {
    return responses.map(getImageFromBlob);
  });
};

exports.default = dumbImagePreloader;