ECMAScript 5 and later let scripts opt in to a newstrict mode, which alters the semantics of JavaScript in several ways to improve its resilience and make errors more understandable.

The normal, non-strict mode of JavaScript is sometimes referred to assloppy mode. This isn't an official designation, but you are likely to come across it if you spend time doing serious JavaScript code.

See also

• "Strict Mode" in chapter 7 ("JavaScript Syntax") in the book SpeakingJavaScript.