Click "I'll be careful, I promise" if a warning message appears. In the search box, search for javascript. Click on the "Reload current page" button of the web browser to refresh the page. In the "Settings" section click on the "Show advanced settings When the dialog window opens, look for the "JavaScript" section and select "Allow all sites to run JavaScript recommended ". Click on the "OK" button to close it. Close the "Settings" tab. Click on the "Reload this page" button of the web browser to refresh the page.
Opera 1. Click on Opera icon "Menu" and than "Settings". Click on "Websites" and then choose "Allow all sites to run JavaScript recommended " 3. Top Rated Most Recent. Accept Solution Reject Solution. Add a dot to indicate that the relative path starts from current directory, i. Posted Apr pm Peter Leow. Peter Leow Apr am. Thank you, Karthik. PeMaCN Apr am. No,this is not working.
What is not working, be more specific. Where did you place the external js file? Sorry, it's working. I had one bracket which was causing javascript not working.
It's okay. All's well that ends well. Posted Apr pm Passion4Code. Add your solution here. OK Paste as. Treat my content as plain text, not as HTML. Existing Members Sign in to your account.
Refresh the page in the browser, and you will get an output in the console of "Response value:", plus the same error message we saw before. The console. A very common problem with async requests like this is when you try to do something with the response object before it has actually been returned from the network. Let's fix this problem by running the code once the load event has been fired — remove the console. To summarize, anytime something is not working and a value does not appear to be what it is meant to be at some point in your code, you can use console.
Unfortunately, we still have the same error — the problem has not gone away. Let's investigate this now, using a more sophisticated feature of browser developer tools: the JavaScript debugger as it is called in Firefox. The main feature of such tools is the ability to add breakpoints to code — these are points where the execution of the code stops, and at that point you can examine the environment in its current state and see what is going on.
Let's get to work. The error is now being thrown at line Click on line number 51 in the center panel to add a breakpoint to it you'll see a blue arrow appear over the top of it. At this point, the right-hand side will update to show some very useful information. We'd like you to try fixing this problem yourself. If you get stuck, consult our fixed-ajax. Note: The debugger tab has many other useful features that we've not discussed here, for example conditional breakpoints and watch expressions.
For a lot more information, see the Debugger page. As your apps get more complex and you start to use more JavaScript, you may start to run into performance problems, especially when viewing apps on slower devices.
Performance is a big topic, and we don't have time to cover it in detail here. Some quick tips are as follows:. In this section, we'll look at some of the more common cross-browser JavaScript problems. We'll break this down into:.
In the previous article we described some of the ways in which HTML and CSS errors and unrecognized features can be handled due to the nature of the languages. There are a number of modern JavaScript language features defined in recent versions of the specs that won't work in older browsers. Some of these are syntactic sugar basically an easier, nicer way of writing what you can already do using existing features , and some offer interesting new possibilities.
There are also many new APIs appearing in recent browsers, which don't work in older browsers, for example:. There are a few strategies for handling incompatibilities between browsers relating to feature support; let's explore the most common ones. Note: These strategies do not exist in separate silos — you can, of course combine them as needed.
For example, you could use feature detection to determine whether a feature is supported; if it isn't, you could then run code to load a polyfill or a library to handle the lack of support. The idea behind feature detection is that you can run a test to determine whether a JavaScript feature is supported in the current browser, and then conditionally run code to provide an acceptable experience both in browsers that do and don't support the feature.
As a quick example, the Geolocation API which exposes available location data for the device the web browser is running on has a main entry point for its use — a geolocation property available on the global Navigator object.
Therefore, you can detect whether the browser supports geolocation or not by using something like the following:. You could also write such a test for a CSS feature, for example by testing for the existence of element.
But for both CSS and JavaScript, it is probably better to use an established feature detection library rather than writing your own all the time. Modernizr is the industry standard for feature detection tests. As a last point, don't confuse feature detection with browser sniffing detecting what specific browser is accessing the site — this is a terrible practice that should be discouraged at all costs. See Using bad browser sniffing code , later on, for more details. Note: Some features are known to be undetectable — see Modernizr's list of Undetectables.
Note: Feature detection will be covered in a lot more detail in its own dedicated article, later in the module. JavaScript libraries are essentially third party units of code that you can attach to your page, providing you with a wealth of ready-made functionality that can be used straight away, saving you a lot of time in the process. A lot of JavaScript libraries probably came into existence because their developer was writing a set of common utility functions to save them time when writing future projects, and decided to release them into the wild because other people might find them useful too.
JavaScript libraries tend to come in a few main varieties some libraries will serve more than one of these purposes :. When choosing a library to use, make sure that it works across the set of browsers you want to support, and test your implementation thoroughly. Also make sure that the library is popular and well-supported, and isn't likely to just become obsolete next week. Talk to other developers to find out what they recommend, see how much activity and how many contributors the library has on Github or wherever else it is stored , etc.
Library usage at a basic level tends to consist of downloading the library's files JavaScript, possibly some CSS or other dependencies too and attaching them to your page e. You will have to read the libraries' individual install pages for more information. Whereas libraries are often usable for solving individual problems and dropping into existing sites, frameworks tend to be more along the lines of complete solutions for developing complex web applications.
Polyfills also consist of 3rd party JavaScript files that you can drop into your project, but they differ from libraries — whereas libraries tend to enhance existing functionality and make things easier, polyfills provide functionality that doesn't exist at all.
Polyfills use JavaScript or other technologies entirely to build in support for a feature that a browser doesn't support natively. For example, you might use a polyfill like es6-promise to make promises work in browsers where they are not supported natively. Again, you should research them before you use them — make sure they work and are maintained. Let's work through an exercise — in this example we will use a Fetch polyfill to provide support for the Fetch API in older browsers; however we also need to use the es6-promise polyfill, as Fetch makes heavy use of promises, and browsers that don't support them will still be in trouble.
Note: You can find our finished version at fetch-polyfill-finished. Note: Again, there are many different ways to make use of the different polyfills you will encounter — consult each polyfill's individual documentation.
One thing you might be thinking is "why should we always load the polyfill code, even if we don't need it? It makes sense to only load files as needed. Doing this requires some extra setup in your JavaScript.
You need some kind of a feature detection test that detects whether the browser supports the feature we are trying to use:. So first we run a conditional that checks whether the function browserSupportsAllFeatures returns true. If it does, we run the main function, which will contain all our app's code.
Here we are testing whether the Promise object and fetch function exist in the browser. If both do, the function returns true. If the function returns false , then we run the code inside the second part of the conditional — this runs a function called loadScript , which loads the polyfills into the page, then runs main after the loading has finished.
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