For a JavaScript beginner, Is it efficient to learn with or use "Strict Mode" while building stuff?

I don’t think it makes a huge difference. But please us ESLint, it grealt improves your learning process :)

Definitely use strict mode. It should be default but they cannot do that because of the need for support of older systems.

I'd even say start learning TypeScript and enable its --strict setting in the tsconfig.

I'd come to hate the weak types of plain JavaScript and the compile time checks provide more sanity.

I would say it is efficient, but the question of using eslint or not is far more important.

You'll save a lot of headaches using "use strict" but eslint will save you a lot more, and will warn you on lint time, before runtime.

Also your code will be a lot better. I would still hate writing js if not for eslint, it's that important.