What I think he means is that if you "use strict" you will get an error if you reference a undeclared variable, but due to the hoisting you will not get an error if you use a variable before it has been declared - if it's declared later in the scope. But if you use let, you will get an error! Such errors are trivial to detect even for beginners though ... Something more nasty is when you have an undeclared (undefined), maybe misspelled object property, then neither "use strict" block scope, or even const will help. It's also hard to find such bugs via static analysis, Typescript nor Eslint will detect it! This is a feature in JavaScript though, and has nothing to do (but often confused) with block scope vs function scope, or loose type vs strict types. In other languages you would need several lines of boilerplate just to do something like "foo.bar=1", and I think the convenience outweigh the possible hard to debug errors, and I'm also working on a static analysis tool that will detect some of those errors.