Possibility to declare variable from function in IF statement?

I'm back again. I had this little question about a possibility that I could declare a variable in the IF condition from a function, and to use it inside the statement itself.

Ok so, I'm curious to see if there's a way to declare a variable inside an IF statement's condition, and use that further along the statement as follows:

function SomeFunc() {return true}
if (let n = SomeFunc()) {
    console.log(n); // true
    // n is truthy
} else {
    // Would never run, because it always returns true.
    // This is just an example though, where it can return a truthy or falsy value dependent on the input context I could give it.
    console.log(n); // false (if it actually returned 'false')
    // n is falsy
}

Is there any way to do this, without having to run the function twice and not have it run outside the IF statement?

(Not like this though):

let n = SomeFunc();
if (n) { ... } else { ... }

// Or this:
if (SomeFunc()) {
    let n = SomeFunc();
} else { ... }

I'd like to have one function being declared inside the condition, to minimalise line usage and have it - for me - clean. I hope there's a way to declare a variable inside of an IF condition.

Thank you in advance. ~Q