JavaScript: The Definitive Guide, Fourth Edition

Case Sensitivity

JavaScript is a case-sensitive language. This means that language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters. The while keyword, for example, must be typed “while”, not “While” or “WHILE”. Similarly, online, Online, OnLine, and ONLINE are four distinct variable names.

Note, however, that HTML is not case-sensitive. Because of its close association with client-side JavaScript, this difference can be confusing. Many JavaScript objects and properties have the same names as the HTML tags and attributes they represent. While these tags and attribute names can be typed in any case in HTML, in JavaScript they typically must be all lowercase. For example, the HTML onclick event handler attribute is commonly specified as onClick in HTML, but it must be referred to as onclick in JavaScript code.

While core JavaScript is entirely and exclusively case-sensitive, exceptions to this rule are allowed in client-side JavaScript. In Internet Explorer 3, for example, all client-side objects and properties were case-insensitive. This caused problematic incompatibilities with Netscape, however, so in Internet Explorer 4 and later, client-side objects and properties are case-sensitive.