Why these syntactically identical functions produce different results?

Question

function foo1() {
    return {
        bar: "hello"
    };
}

function foo2() {
    return
    {
        bar: "hello"
    };
}

console.log(foo1());
console.log(foo2());

Can I get an explanation why the two functions print out different results even though the code looks the same?

Answer 1

Automatic semicolon insertion

Quoting the specification,

When a continue, break, return, throw, or yield token is encountered and a LineTerminator is encountered before the next token, a semicolon is automatically inserted after the continue, break, return, throw, or yield token.

So the code will become like this

function foo2() {
    return;          // Note the `;` after `return`
    {
        bar: "hello"
    };
}

The return statement terminates and then there is an object after that, which is basically unreachable code. Since the return statement doesn't return anything explicitly, undefined will be returned.