Question

1: Why is the result of foo && baz not 1? Because true is 1.

var foo = 1;
var baz = 2;

foo && baz;   // returns 2, which is true

2: There are two pluses in the console.log(foo + +bar);, what’s the meaning of them?

var foo = 1;
var bar = '2';
console.log(foo + +bar);

Answer 1

That’s because the && (logical AND) operator returns the value of the last operand it evaluated. Since foo is true, it has to evaluate bar to determine the outcome of the expression (it will only be true if bar is also true).

The opposite would happen with the || (logical OR) operator. In that case, since foo is true, the outcome of the expression is known to be true without having to evaluate bar, so the value of foo will be returned.

Concerning your second question, the unary + operator allows to convert the string '2' into the number 2.