So, I was going through this video : youtube.com/watch?&v=4CeTFW4agRw - which talks about form validation in React, but is really general JavaScript form validation implemented in React. It has a line of code val.length > 0 && (valid = false); which got me wondering since this is equivalent to

if (val.length > 0)
{
    valid = false;
}

Full Code :

const formValid = ({formErrors, ...rest}) =>
{
    let valid = true;

    Object.values(formErrors).forEach(val =>
    {
        val.length > 0 && (valid = false);
    });    

    Object.values(rest).forEach(val =>
    {
        val === null && (valid = false);
    });

    return valid;
}

form = {
    firstName: "J",
    lastName: "Smith",
    email: "j.com",
    password: "",
    formErrors: {
        firstName: "Min 3 characters required",
        lastName: "",
        email: "Invalid Email",
        password: ""
        }
    };

let err = formValid(form);
console.log(err);

In the one-liner it uses a boolean check and sets valid to false if val.length > 0. This works because a AND operator (.) with a FALSE (or 0) equates to FALSE.

AND can be represented by a dot - ·, TRUE as 1 and FALSE as 0. In Logic gates, The statement "A ∧ B is true if A and B are both true; otherwise, it is false." In simple 1 and 0 :

1.1 = 1
1.0 = 0

In the expression val.length > 0 && (valid = false), val.length = 25 which is greater than 0 for the first item, formErrors.firstName - which makes val.length > 0 equal to true. That is ANDed to (valid = false) which is false.

val.length > 0 is true or 1
(valid = false) = false or 0        
1 . 0 = 0 or false