reentrancy.js

'use strict';

const x = (() => {

    // This is always >= 0
    let nonNegative = 0;

    return {

        // Return the value
        get: () => nonNegative,

        // Add some number to the value, and call the callback with the new value.
        add: (offset, callback) => {

            // The value is not allowed to go below 0!
            if (nonNegative + offset < 0) throw 'Too low!';

            // Let the listener know what the new value is
            if (callback) callback(nonNegative + offset);

            // Make the change
            nonNegative += offset;
        }
    };

})();

x.add(1, console.log);        // 1
x.add(-1, () => x.add(-1))
console.log(x.get());         // -1