JavaScript snoop properties of objects. Bind callbacks to getters and setters for all properties of an object. Useful for observing how a page element has its properties accessed

snoopProperty.js

/**
 * Snoops on an object, monitors all attempts to access and set properties
 * @param {Object} obj - the object to monitor
 * @param {string[]=} props - list of properties to watch, watches all properties by default
 * @param {Function=} onset - callback when the property is set (newValue is passed)
 * @param {Function=} onget - callback when the property is accessed
 * @param verbose
 */
function snoopProperty(obj, props = [], onset = (val) => null, onget = () => null, verbose = true) {
    if (typeof onset !== 'function') onset = (val) => null;
    if (typeof onget !== 'function') onget = () => null;

    var snooper = {
        object: obj,
        props: props,
        records: {
            sets: [],
            gets: [],
        },
        /**
         * @param props the properties to snoop
         */
        snoop: function (props) {
            if (props.length === 0) { // default to all properties
                props = Object.keys(obj);
            } else if (!!props && !props.length) {
                props = [props];
            }

            for (const prop of props) {
                obj['__' + prop] = obj[prop]; // actual property

                snooper.object.__defineSetter__(prop, function (newValue) {
                    if(verbose) console.warn(
                        'Snooper: [', prop, '] was just changed from "', snooper.object['__' + prop], '" to -> "', newValue, '"',
                        '\non:', snooper.object
                    );
                    snooper.records.sets.push({timeStamp: Date.now(), from: snooper.object['__' + prop], to: newValue});
                    onset.call(snooper.object, newValue);
                    snooper.object['__' + prop] = newValue;
                });

                snooper.object.__defineGetter__(prop, function () {
                    if(verbose) console.warn(
                        'Snooper: [', prop, '] was just accessed "', snooper.object['__' + prop], '"',
                        '\non:', snooper.object
                    );
                    snooper.records.gets.push({timeStamp: Date.now(), value: snooper.object['__' + prop]});
                    onget.call(snooper.object);

                    return snooper.object['__' + prop];
                });
            }
        },
        abort: function (props) {
            if (props.length === 0) { // default to all properties
                props = Object.keys(obj);
            } else if (!!props && !props.length) {
                props = [props];
            }

            for (const prop of props) {
                snooper.object.__defineSetter__(prop, newValue => {
                    snooper.object['__' + prop] = newValue;
                });
                snooper.object.__defineGetter__(prop, () => snooper.object['__' + prop]);
            }
        }
    };

    snooper.snoop(props);

    return snooper;
}