About

In a browser, the document is:

• a DOM document (in-memory tree)
• of a XML document (generally a HTML dom document)

The document is provided by the browser via its DOM webapi (Not by the Javascript engine, DOM is not a standard javascript library)

Variable

The document variable exists as a global variable

The document variable is:

• is a property of the window object
• accessible globally through an alias

Example

• Showing the document type

type = document.contentType;
console.log("The document type is "+type);

Browser Detection

The presence of the document object tells you that Javascript is running in:

• a browser
• or a library that supports DOM such as JsDom

if (typeof document !== 'undefined') {
  // We are in a browser context
} else {
  // We are in a non-browser context (node without dom library)
}