How Javascript decides when new line character separates statements?

Question

For example, following code prints 1, which means that new line is treated as statement separator:

function f1() {
    return 1
    2
}

console.log(f1());

However, this example prints 3, which means that new line is ignored:

function f1() {
    return 1
    +2
}

console.log(f1());

How does Javascript decides when to ignore new line character, and when use it as a statement separator?