Strict Mode
Opt-in using the pragma "use strict"
Applied to the top of a function makes the whole function run in strict mode. Applied to the top of a script makes the whole script run in strict mode. es6 modules run implicitly in strict mode.
| Sloppy Mode | Strict Mode | |
|---|---|---|
| this default binding | global Object | undefined |
accidental global variable
car = 42; | applies property to global object | throws Reference error |
| delete read-only prop | fails silently | throws TypeError |
| add prop to a non-extendible object | fails silently | throws TypeError |
| change property descriptor if configurable is false | fails silently | throws TypeError |
| with | works normally | throws Error |
octal notation:
let a = 05; | works | forbidden throws SyntaxError only
let a = 0o5;
works |
assign properties to primitives
(2).a = 3; window.undefined = true; "a".a = "a"; | fails silently | throws TypeError |
| eval and arguments used as variable/property names | works | throws TypeError |
delete plainNames
delete that; | works | throws SyntaxError |
addings variables to scope using eval
eval("var r = 3;") | adds variable to scope | doesnt not add a variable |