An Introduction Flashcards
How is JS different than Java?
JavaScript was initially created to “make web pages alive”.
The programs in this language are called scripts. They can be written right in a web page’s HTML and run automatically as the page loads.
Scripts are provided and executed as plain text. They don’t need special preparation or compilation to run.
In this aspect, JavaScript is very different from another language called Java.
How was JS named?
When JavaScript was created, it initially had another name: “LiveScript”. 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.
EcmaScript
JavaScript became a fully independent language with its own specification called ECMAScript, and now it has no relation to Java at all.
JS Engine
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.
How does JS work on browser work then?
The browser has an embedded engine sometimes called a “JavaScript virtual machine”.
Different Engine Names
V8 – in Chrome and Opera.
SpiderMonkey – in Firefox.
How do Engines work?
Engines are complicated. But the basics are easy.
The engine (embedded if it’s a browser) reads (“parses”) the script.
Then it converts (“compiles”) the script to the machine language.
And then the machine code runs, pretty fast.
The engine applies optimizations at each step of the process. 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.
Why is Javascript safe?
It does not provide low-level access to memory or CPU, because it was initially created for browsers which do not require it.
JS capabilities
JavaScript’s capabilities greatly depend on the environment it’s running in. For instance, Node.js supports functions that allow JavaScript to read/write arbitrary files, perform network requests, etc.
What can In-browser js do?
Add new HTML to the page, change the existing content, modify styles.
React to user actions, run on mouse clicks, pointer movements, key presses.
Send requests over the network to remote servers, download and upload files (so-called AJAX and COMET technologies).
Get and set cookies, ask questions to the visitor, show messages.
Remember the data on the client-side (“local storage”).
What cant in-browser JS do?
JS - LImitation 1
JavaScript on a webpage may not read/write arbitrary files on the hard disk, copy them or execute programs. It has no direct access to OS functions.
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.
JS- Lim 2
Different tabs/windows generally do not know about each other. Sometimes they do, for example when one window uses JavaScript to open the other one. 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).
This is called the “Same Origin Policy”. To work around that, both pages must agree for data exchange and contain a special JavaScript code that handles it. We’ll cover that in the tutorial.
This limitation is, again, for the user’s safety. A page from http://anysite.com which a user has opened must not be able to access another browser tab with the URL http://gmail.com and steal information from there.
JS - Lim 3
JavaScript can easily communicate over the net to the server where the current page came from. But its ability to receive data from other sites/domains is crippled. Though possible, it requires explicit agreement (expressed in HTTP headers) from the remote side. Once again, that’s a safety limitation.
What makes JS unique?
Full integration with HTML/CSS.
Simple things are done simply.
Support by all major browsers and enabled by default.
Transpilation
Languages are transpiled (converted) to JavaScript before they run in the browser.
