promises-in-parallel.js

/*
* this is an example of how to limit the amount of promise tasks in parallel.
*/

let urls = getUrls();				// the array of image urls.
const maxTasks = 5;					// the max amount of promise task in parallel.
let uploadAttemps = 0; 				// upload attemps

const uploadImage = () =>
	new Promise((resolve, reject) => {
		// gives up when upload attemps reach to 100.
		if(uploadAttemps >= 100) {
			reject();
			return;
		}

		const url = urls.shift();

		if(!url) {
			resolve();
			return;
		}

		uploadTries++;
		const uploadUrl = 'https://example.com/upload-path';

		$.ajax({
			url: uploadUrl,
			method: 'POST',
			data: { url }
		}).done(res => {
			if (res.code === 0) {
				console.log(`${url} is uploaded.`);
			} else {
				// puts current url back to the array.
				urls.unshift(url);
			}

			// continues to upload the rest images.
			uploadImage().then(res => resolve());
		});
	});

const promises = [...Array(3).keys()].map(x => uploadImage());
Promise.all(promises).then(res => console.log('work completed.'));