ECMAScript
5 and later let scripts opt in to a new
strict mode
, which alters the semantics of JavaScript in several ways to improve its resiliency and which make it easier to understand what's going on when there are problems.
The normal, non-strict mode of JavaScript is sometimes referred to as
sloppy mode
. This isn't an official designation, but you are likely to come across it if you spend time doing serious JavaScript code.
