What does "use strict" do in JavaScript and why is it needed?

Hello everyone! I started learning JavaScript and noticed that in some code examples at the beginning of a file or function they write `'use strict';`. Honestly, I don't quite understand why this is needed and what it actually does. I tried writing a simple script without this directive, and everything worked. For example, I accidentally created a variable without `let`, `const`, or `var`: ```javascript function test() { myVar = 10; // I forgot to write let console.log(myVar); } test(); // Outputs 10 ``` But if I add `'use strict';`: ```javascript 'use strict'; function test() { myVar = 10; // Error! console.log(myVar); } test(); // ReferenceError: myVar is not defined ``` Now the code throws an error. It seems like without `'use strict'` everything worked more simply, but I've heard that experienced developers