Using 'for in' to loop through an object

It is considered a bad practice, and if you are writing JavaScript in an environment that enforces Strict mode then it would actually not work.

When you don't add var before the variable the variable will automatically be created as a global variable, even if the variable is created within a function. Meaning that it exists in all functions in all JS files loaded on the site. Use of global variables are generally discouraged since it makes naming conflicts and bugs more likely to occur.

When you do add var the variable will only exist within the function it was created in. Since the loop you are writing in this video is not placed within a function the variable would actually end up as a global variable even if you did use the var keyword. So in this specific case it actually doesn't change anything, but being explicit when creating a variable is still considered good practice.