JavaScript function that can be called only once

function add(a, b) {
    return a+b;
}

function once(func) {
    return function () {
        var f = func;
        func = null;
        return f.apply(
            this,
            arguments
        );
    };
}

var add_once = once(add);
console.log(add_once(3, 4));  // 7
console.log(add_once(3, 4));  // throws as f is now null

JSFiddle: http://jsfiddle.net/stevenhollidge/uA6TH/

But I prefer the following code as it’s more readable:

function add(a, b) {
    return a+b;
}

function once(func) {
    var usageCount = 0;

    return function () {
        if (usageCount === 1) {
            throw 'Already used';
        }
        usageCount += 1;
        return func.apply(
            this,
            arguments
        );     

    }
}

var add_once = once(add);
console.log(add_once(3, 4));  // 7
console.log(add_once(3, 4));  // throws

JSFiddle: http://jsfiddle.net/stevenhollidge/eqnU3/