Global environment

So far, we have discussed JavaScript keywords, a few essential functions, and the concepts of scope and environment. What is the global environment and what does it contain? The answer depends on the particular implementation of JavaScript.

In a browser, JavaScript runs in a restricted environment for security reasons. Allowing JavaScript in your browser means allowing code written in unknown locations to run in your browser and therefore on your computer. There have been countless attacks where such code has stolen sensitive files, sent spam, inspected a company's internal network, or spied on other JavaScript code to find credit card numbers or passwords. Browsers have made great efforts to limit these risks and, as a result, provide libraries with limited functionality: it is therefore not possible for a JavaScript program running in a browser to read the user's...