How to stringify event object?

Question

JSON.stringify(eventObject);

gives:

TypeError: Converting circular structure to JSON

---

dojox.json.ref.toJson(eventObject);

gives:

TypeError: Accessing selectionEnd on an input element that cannot have a selection.

---

Is there some library/code ready to use to accomplish it ?

Solution 1

You won't be able to serialize an event object with JSON.stringify, because an event object contains references to DOM nodes, and the DOM has circular references all over the place (e.g. child/parent relationships). JSON can't handle these by default, so you're a bit out of luck there.

I'd suggest to look at How to serialize DOM node to JSON even if there are circular references? which has a few suggestions on how to serialize a DOM node. Also, the following questions seem to have useful information:

• How to save an object with circular references?
• Stringify (convert to JSON) a JavaScript object with circular reference

JSON libraries able to handle circular references seem to be

• JSON-js (see cycle.js)
• dojox.json.ref

Alternatively, you could delete all references to DOM nodes if you don't need them, and then serialize the object. You shouldn't do this after all. See @PointedEars comment :)

Solution 2

Use the "replacer" function to avoid errors:

JSON.stringify(evt, function(k, v) {
    if (v instanceof Node) {
        return 'Node';
    }
    if (v instanceof Window) {
        return 'Window';
    }
    return v;
}, ' ');

Update 2019: the browser API has changed some way, here is a method to expose all available keys in Event prototype chain

function stringifyEvent(e) {
  const obj = {};
  for (let k in e) {
    obj[k] = e[k];
  }
  return JSON.stringify(obj, (k, v) => {
    if (v instanceof Node) return 'Node';
    if (v instanceof Window) return 'Window';
    return v;
  }, ' ');
}

Solution 3

I had a similar problem and wrote a simple event serializer with a helper method to cleanup the event's path attribute. The approach for this solution to transform data from the event to a serializable object:

• Copy over primitive attributes
• Copy outerHTML for element attributes in the event object
• Calculate selector path for the path attribute (this avoids copying the outerHTML of the entire HTML page)

// Calculate a string representation of a node's DOM path.
var pathToSelector = function(node) {
  if (!node || !node.outerHTML) {
    return null;
  }

  var path;
  while (node.parentElement) {
    var name = node.localName;
    if (!name) break;
    name = name.toLowerCase();
    var parent = node.parentElement;

    var domSiblings = [];

    if (parent.children && parent.children.length > 0) {
      for (var i = 0; i < parent.children.length; i++) {
        var sibling = parent.children[i];
        if (sibling.localName && sibling.localName.toLowerCase) {
          if (sibling.localName.toLowerCase() === name) {
            domSiblings.push(sibling);
          }
        }
      }
    }

    if (domSiblings.length > 1) {
      name += ':eq(' + domSiblings.indexOf(node) + ')';
    }
    path = name + (path ? '>' + path : '');
    node = parent;
  }

  return path;
};

// Generate a JSON version of the event.
var serializeEvent = function(e) {
  if (e) {
    var o = {
      eventName: e.toString(),
      altKey: e.altKey,
      bubbles: e.bubbles,
      button: e.button,
      buttons: e.buttons,
      cancelBubble: e.cancelBubble,
      cancelable: e.cancelable,
      clientX: e.clientX,
      clientY: e.clientY,
      composed: e.composed,
      ctrlKey: e.ctrlKey,
      currentTarget: e.currentTarget ? e.currentTarget.outerHTML : null,
      defaultPrevented: e.defaultPrevented,
      detail: e.detail,
      eventPhase: e.eventPhase,
      fromElement: e.fromElement ? e.fromElement.outerHTML : null,
      isTrusted: e.isTrusted,
      layerX: e.layerX,
      layerY: e.layerY,
      metaKey: e.metaKey,
      movementX: e.movementX,
      movementY: e.movementY,
      offsetX: e.offsetX,
      offsetY: e.offsetY,
      pageX: e.pageX,
      pageY: e.pageY,
      path: pathToSelector(e.path && e.path.length ? e.path[0] : null),
      relatedTarget: e.relatedTarget ? e.relatedTarget.outerHTML : null,
      returnValue: e.returnValue,
      screenX: e.screenX,
      screenY: e.screenY,
      shiftKey: e.shiftKey,
      sourceCapabilities: e.sourceCapabilities ? e.sourceCapabilities.toString() : null,
      target: e.target ? e.target.outerHTML : null,
      timeStamp: e.timeStamp,
      toElement: e.toElement ? e.toElement.outerHTML : null,
      type: e.type,
      view: e.view ? e.view.toString() : null,
      which: e.which,
      x: e.x,
      y: e.y
    };

    console.log(JSON.stringify(o, null, 2));
  }
};

// Create a mock event for this example
var evt = new MouseEvent("click", {
  bubbles: true,
  cancelable: true,
  view: window
});
var cb = document.getElementById("clicker");

// Add a click listener
cb.addEventListener("click", serializeEvent);

// Fire the event
cb.dispatchEvent(evt);

<div>
  <button id="clicker" /> JSONify my click!
</div>

Solution 4

Improved version of the code given by Alexander Shutau, as this one handles multi level objects (ES6):

function stringify_object(object, depth=0, max_depth=2) {
    // change max_depth to see more levels, for a touch event, 2 is good
    if (depth > max_depth)
        return 'Object';

    const obj = {};
    for (let key in object) {
        let value = object[key];
        if (value instanceof Node)
            // specify which properties you want to see from the node
            value = {id: value.id};
        else if (value instanceof Window)
            value = 'Window';
        else if (value instanceof Object)
            value = stringify_object(value, depth+1, max_depth);

        obj[key] = value;
    }

    return depth? obj: JSON.stringify(obj);
}

Just call it like this:

stringify_object(event, 2);

For example, on a touchstart event, I'm getting this:

touchstart : {"isTrusted":true,"touches":{"0":{"identifier":0,"target":{"id":"screen"},"screenX":548,"screenY":281.5,"clientX":498.1817932128906,"clientY":185.90908813476562,"pageX":498.1817932128906,"pageY":185.90908813476562,"radiusX":29.77272605895996,"radiusY":27.954544067382812,"rotationAngle":0,"force":0.5},"length":1,"item":{}},"targetTouches":{"0":{"identifier":0,"target":{"id":"screen"},"screenX":548,"screenY":281.5,"clientX":498.1817932128906,"clientY":185.90908813476562,"pageX":498.1817932128906,"pageY":185.90908813476562,"radiusX":29.77272605895996,"radiusY":27.954544067382812,"rotationAngle":0,"force":0.5},"length":1,"item":{}},"changedTouches":{"0":{"identifier":0,"target":{"id":"screen"},"screenX":548,"screenY":281.5,"clientX":498.1817932128906,"clientY":185.90908813476562,"pageX":498.1817932128906,"pageY":185.90908813476562,"radiusX":29.77272605895996,"radiusY":27.954544067382812,"rotationAngle":0,"force":0.5},"length":1,"item":{}},"altKey":false,"metaKey":false,"ctrlKey":false,"shiftKey":false,"view":"Window","detail":0,"sourceCapabilities":{"firesTouchEvents":true},"which":0,"initUIEvent":{},"NONE":0,"CAPTURING_PHASE":1,"AT_TARGET":2,"BUBBLING_PHASE":3,"type":"touchstart","target":{"id":"screen"},"currentTarget":{"id":"screen"},"eventPhase":2,"bubbles":true,"cancelable":true,"defaultPrevented":false,"composed":true,"timeStamp":192516.7899999651,"srcElement":{"id":"screen"},"returnValue":true,"cancelBubble":false,"path":{"0":{"id":"screen"},"1":{"id":"back"},"2":{"id":""},"3":{"id":""},"4":{},"5":"Window"},"composedPath":{},"stopPropagation":{},"stopImmediatePropagation":{},"preventDefault":{},"initEvent":{}}

Solution 5

Not sure if it helps, but I just stumbled upon this in the Angular JS documentation:

*Source: https://code.angularjs.org/1.5.5/docs/guide/expression#-event-

/*
 * return a copy of an object with only non-object keys
 * we need this to avoid circular references
 */
function simpleKeys (original) {
  return Object.keys(original).reduce(function (obj, key) {
    obj[key] = typeof original[key] === 'object' ? '{ ... }' : original[key];
    return obj;
  }, {});
}

Now you could do something like:

JSON.stringify(simpleKeys(eventObject));

View more solutions

Comments

[PointedEars]

Event objects are host objects. Do not mess with them, such as trying to delete their properties or assign to properties that are not supposed to be assigned to.

[THE JOATMON]

When using this on a touchevent it only returns a single element {isTrusted: true}.

[Blightbuster]

Same for all other MouseEvent's

[Blightbuster]

this also results in only {isTrusted: true} for MouseEvent's

[Preston Badeer]

I'm downvoting this only because of the limited use of a function that returns "Node" instead of all the info you would normally want from an event object.

[Alexander Shutau]

@PrestonBadeer You can create a custom algorithm that will handle circular references, return a constructor name, a selector etc. It's up to you.