JavaScript Strict_mode Guide
Strict mode opts into a restricted JS variant that eliminates silent errors and improves performance. ES6 modules are automatically strict. The guide walks through Key Differences. `"use strict"` at file/function top. Changes: undeclared variables throw, `this` in functions is undefined, assignments to non-writable throw, delete of non-configurable throws, duplicate params not allowed, `arguments.callee` forbidden, octal literals invalid. The guide is organized into 1 sections that build on each other, each pairing a prose explanation with runnable code. Related guides: Arrays, Strings, Numbers, Math, Date. This is one of 145 JavaScript guides on JS Reference — see the full list on the guides page. Practice what you learn in the JavaScript playground, and look up any unfamiliar term in the JavaScript glossary.