You can find the details in MDN's Promise and async function reference. Also, thanks to Mathias Bynens for updating various parts of the article. Learn Measure Blog About. We want to hear from you! We are looking for web developers to participate in user research, product testing, discussion groups and more. Apply now to join our WebDev Insights Community. JavaScript Promises: an introduction What's all the fuss about? Events aren't always the best way Promise terminology Promises arrive in JavaScript!
Developers, prepare yourself for a pivotal moment in the history of web development. Maybe you're not even sure what a "promise" is. You'd shrug, but the weight of glittery paper is weighing down on your shoulders.
If so, don't worry about it, it took me ages to work out why I should care about this stuff. You probably want to begin at the beginning. You punch the air! About time right? You've used these Promise things before but it bothers you that all implementations have a slightly different API.
You probably want to begin with the terminology. You knew about this already and you scoff at those who are jumping up and down like it's news to them. Take a moment to bask in your own superiority, then head straight to the API reference.
What's all the fuss about? You've probably used events and callbacks to get around this. Events aren't always the best way Events are great for things that can happen multiple times on the same object— keyup , touchstart etc. If HTML image elements had a "ready" method that returned a promise, we could do this: img1. It cannot succeed or fail twice, neither can it switch from success to failure or vice versa.
Promise terminology Domenic Denicola proof read the first draft of this article and graded me "F" for terminology. Despite that, I still get a lot of the terminology mixed up, but here are the basics: A promise can be: fulfilled - The action relating to the promise succeeded rejected - The action relating to the promise failed pending - Hasn't fulfilled or rejected yet settled - Has fulfilled or rejected The spec also uses the term thenable to describe an object that is promise-like, in that it has a then method.
Promises arrive in JavaScript! Here's how you use that promise: promise. Compatibility with other libraries The JavaScript promises API will treat anything with a then method as promise-like or thenable in promise-speak sigh , so if you use a library that returns a Q promise, that's fine, it'll play nice with the new JavaScript promises.
Complex async code made easier Right, let's code some things. Say we want to: Start a spinner to indicate loading Fetch some JSON for a story, which gives us the title, and urls for each chapter Add title to the page Fetch each chapter Add the story to the page Stop the spinner … but also tell the user if something went wrong along the way.
Chaining then isn't the end of the story, you can chain then s together to transform values or run additional async actions one after another. We could alter our get function to use the JSON responseType , but we could also solve it in promises land: get 'story. Queuing asynchronous actions You can also chain then s to run async actions in sequence. For example: getJSON 'story. Error handling As we saw earlier, then takes two arguments, one for success, one for failure or fulfill and reject, in promises-speak : get 'story.
Note that the two code examples above do not behave the same, the latter is equivalent to: get 'story. Take the following: asyncThing1. Everything from new organizational structures and payment schemes to new expectations, skills, and tools will shape the future of the firm. Stay on top of the emerging tools, trends, issues, and context necessary for making informed decisions about business and technology.
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Services Learn More. The first parameter is the name you want the downloaded file to be named , the second parameter is the file data. If you set the third parameter to boolean true, then the actual file MIME type based on the filename extension will be sent, so that if your browser has a handler for that type - it can use it. The response object MUST be returned for the download to be sent to the client. This allows the response to be passed through all after filters before being sent to the client.
Built into the HTTP specification are tools help the client often the web browser cache the results. This are handled through the Cache-Control and ETag headers. This guide is not the proper place for a thorough introduction to all of the cache headers power, but you can get a good understanding over at Google Developers. The options and exact circumstances are too varied for us to be able to create a good default other than turning it off.
You are free to set all of the options exactly as you need for your specific situation. While most of the options are applied to the Cache-Control header, it intelligently handles the etag and last-modified options to their appropriate header. This sounds complex, and on some sites, can definitely be challenging. As this is a complex subject, this user guide will not go over all of the details.
For more information, you should visit the following sites:. By default, support for this is off. With CSP enabled, two header lines are added to the HTTP response: a Content-Security-Policy header, with policies identifying content types or origins that are explicitly allowed for different contexts, and a Content-Security-Policy-Report-Only header, which identifies content types or origins that will be allowed but which will also be reported to the destination of your choice.
Our implementation provides for a default treatment, changeable through the reportOnly method. When an additional entry is added to a CSP directive, as shown below, it will be added to the CSP header appropriate for blocking or preventing. That can be overridden on a per call basis, by providing an optional second parameter to the adding method call.
The class holds a number of methods that map pretty clearly to the appropriate header value that you need to set.
Examples are shown below, with different combinations of parameters, though all accept either a directive name or an array of them.
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