JavaScript is the only browser technology that combines these three things.
But its ability to receive data from other sites/domains is crippled. JavaScript can easily communicate over the net to the server where the current page came from. A page from which a user has opened must not be able to access another browser tab with the URL and steal information from there. This limitation is, again, for the user’s safety.
DO I HAVE JAVASCRIPT INSTALLED CODE
To work around that, both pages must agree for data exchange and contain a special JavaScript code that handles it. But even in this case, JavaScript from one page may not access the other if they come from different sites (from a different domain, protocol or port). Sometimes they do, for example when one window uses JavaScript to open the other one. So a JavaScript-enabled page may not sneakily enable a web-camera, observe the surroundings and send the information to the NSA.ĭifferent tabs/windows generally do not know about each other. There are ways to interact with camera/microphone and other devices, but they require a user’s explicit permission. Modern browsers allow it to work with files, but the access is limited and only provided if the user does certain actions, like “dropping” a file into a browser window or selecting it via an tag. JavaScript on a webpage may not read/write arbitrary files on the hard disk, copy them or execute programs. The aim is to prevent an evil webpage from accessing private information or harming the user’s data. JavaScript’s abilities in the browser are limited for the sake of the user’s safety.
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Send requests over the network to remote servers, download and upload files (so-called AJAX and COMET technologies).React to user actions, run on mouse clicks, pointer movements, key presses.Add new HTML to the page, change the existing content, modify styles.In-browser JavaScript can do everything related to webpage manipulation, interaction with the user, and the webserver.įor instance, in-browser JavaScript is able to: For instance, Node.js supports functions that allow JavaScript to read/write arbitrary files, perform network requests, etc.
JavaScript’s capabilities greatly depend on the environment it’s running in. It does not provide low-level access to memory or CPU, because it was initially created for browsers which do not require it. Modern JavaScript is a “safe” programming language. It even watches the compiled script as it runs, analyzes the data that flows through it, and further optimizes the machine code based on that knowledge. The engine applies optimizations at each step of the process.
And then the machine code runs, pretty fast.Then it converts (“compiles”) the script to the machine language.The engine (embedded if it’s a browser) reads (“parses”) the script.For instance, if “a feature X is supported by V8”, then it probably works in Chrome and Opera.Įngines are complicated. The terms above are good to remember because they are used in developer articles on the internet. …There are other codenames like “Chakra” for IE, “JavaScriptCore”, “Nitro” and “SquirrelFish” for Safari, etc.The browser has an embedded engine sometimes called a “JavaScript virtual machine”.ĭifferent engines have different “codenames”. Today, JavaScript can execute not only in the browser, but also on the server, or actually on any device that has a special program called the JavaScript engine. But Java was very popular at that time, so it was decided that positioning a new language as a “younger brother” of Java would help.īut as it evolved, JavaScript became a fully independent language with its own specification called ECMAScript, and now it has no relation to Java at all.
When JavaScript was created, it initially had another name: “LiveScript”.
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