Native deep cloning
It's called "structured cloning", works experimentally in Node 11 and later, and hopefully will land in browsers. See this answer for more details.
Fast cloning with data loss - JSON.parse/stringify
If you do not use Date
s, functions, undefined
, Infinity
, RegExps, Maps, Sets, Blobs, FileLists, ImageDatas, sparse Arrays, Typed Arrays or other complex types within your object, a very simple one liner to deep clone an object is:
JSON.parse(JSON.stringify(object))
const a = {
string: 'string',
number: 123,
bool: false,
nul: null,
date: new Date(), // stringified
undef: undefined, // lost
inf: Infinity, // forced to 'null'
re: /.*/, // lost
}
console.log(a);
console.log(typeof a.date); // Date object
const clone = JSON.parse(JSON.stringify(a));
console.log(clone);
console.log(typeof clone.date); // result of .toISOString()
See Corban's answer for benchmarks.
Reliable cloning using a library
Since cloning objects is not trivial (complex types, circular references, function etc.), most major libraries provide function to clone objects. Don't reinvent the wheel - if you're already using a library, check if it has an object cloning function. For example,
- lodash -
cloneDeep
; can be imported separately via the lodash.clonedeep module and is probably your best choice if you're not already using a library that provides a deep cloning function
- AngularJS -
angular.copy
- jQuery -
jQuery.extend(true, { }, oldObject)
; .clone()
only clones DOM elements
- just library -
just-clone
; Part of a library of zero-dependency npm modules that do just do one thing.
Guilt-free utilities for every occasion.
ES6 (shallow copy)
For completeness, note that ES6 offers two shallow copy mechanisms: Object.assign()
and the spread syntax.
which copies values of all enumerable own properties from one object to another. For example:
var A1 = {a: "2"};
var A2 = Object.assign({}, A1);
var A3 = {...A1}; // Spread Syntax
Now most browsers support getBoundingClientRect method, which has become the best practice. Using an old answer is very slow, not accurate and has several bugs.
The solution selected as correct is almost never precise.
This solution was tested on Internet Explorer 7 (and later), iOS 5 (and later) Safari, Android 2.0 (Eclair) and later, BlackBerry, Opera Mobile, and Internet Explorer Mobile 9.
function isElementInViewport (el) {
// Special bonus for those using jQuery
if (typeof jQuery === "function" && el instanceof jQuery) {
el = el[0];
}
var rect = el.getBoundingClientRect();
return (
rect.top >= 0 &&
rect.left >= 0 &&
rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) && /* or $(window).height() */
rect.right <= (window.innerWidth || document.documentElement.clientWidth) /* or $(window).width() */
);
}
How to use:
You can be sure that the function given above returns correct answer at the moment of time when it is called, but what about tracking element's visibility as an event?
Place the following code at the bottom of your <body>
tag:
function onVisibilityChange(el, callback) {
var old_visible;
return function () {
var visible = isElementInViewport(el);
if (visible != old_visible) {
old_visible = visible;
if (typeof callback == 'function') {
callback();
}
}
}
}
var handler = onVisibilityChange(el, function() {
/* Your code go here */
});
// jQuery
$(window).on('DOMContentLoaded load resize scroll', handler);
/* // Non-jQuery
if (window.addEventListener) {
addEventListener('DOMContentLoaded', handler, false);
addEventListener('load', handler, false);
addEventListener('scroll', handler, false);
addEventListener('resize', handler, false);
} else if (window.attachEvent) {
attachEvent('onDOMContentLoaded', handler); // Internet Explorer 9+ :(
attachEvent('onload', handler);
attachEvent('onscroll', handler);
attachEvent('onresize', handler);
}
*/
If you do any DOM modifications, they can change your element's visibility of course.
Guidelines and common pitfalls:
Maybe you need to track page zoom / mobile device pinch? jQuery should handle zoom/pinch cross browser, otherwise first or second link should help you.
If you modify DOM, it can affect the element's visibility. You should take control over that and call handler()
manually. Unfortunately, we don't have any cross browser onrepaint
event. On the other hand that allows us to make optimizations and perform re-check only on DOM modifications that can change an element's visibility.
Never Ever use it inside jQuery $(document).ready() only, because there is no warranty CSS has been applied in this moment. Your code can work locally with your CSS on a hard drive, but once put on a remote server it will fail.
After DOMContentLoaded
is fired, styles are applied, but the images are not loaded yet. So, we should add window.onload
event listener.
We can't catch zoom/pinch event yet.
The last resort could be the following code:
/* TODO: this looks like a very bad code */
setInterval(handler, 600);
You can use the awesome feature pageVisibiliy of the HTML5 API if you care if the tab with your web page is active and visible.
TODO: this method does not handle two situations:
Best Solution
Though there's probably an easier way to do this using a JS Library, here's a working solution using vanilla js.