Is it OK to have a function that returns true/nothing instead of true/false?

My examples use JavaScript, but I'm wondering about the general case, not attached to one specific language.

The subjects of my interest:

• It is OK from the practical side?
• Is it OK from the logical side?

I mean, some languages provide too much freedom and personally I don't like it. I think it is better to follow the way how it works in serious languages like C++ and Java, but I never worked with them.

var a = 1;
var b = 2;

function foo() {
    if (a === b)
        return true;
}

if (!foo())
    alert('error');

versus

function foo() {
    if (a === b)
        return true;
    else
        return false;
}

Is it OK to have a function that returns true/nothing instead of true/false?

My examples use JavaScript, but I'm wondering about the general case, not attached to one specific language.

The subjects of my interest:

• Is it OK from the practical side?
• Is it OK from the logical side?

I mean, some languages provide too much freedom and personally I don't like it. I think it is better to follow the way how it works in serious languages like C++ and Java, but I never worked with them.

var a = 1;
var b = 2;

function foo() {
    if (a === b)
        return true;
}

if (!foo())
    alert('error');

versus

function foo() {
    if (a === b)
        return true;
    else
        return false;
}