This was given to me as an interview question -- didn't get the job, but I still want to figure it out.
The objective is to write two querySelectorAll functions: one called qsa1 which works for selectors consisting of a single tag name (e.g. div or span) and another called qsa2 which accepts arbitrarily nested tag selectors (such as p span or ol li code).
I got the first one easily enough, but the second one is a bit trickier.
I suspect that, in order to handle a variable number of selectors, the proper solution might be recursive, but I figured I'd try to get something working that is iterative first. Here's what I've got so far:
qsa2 = function(node, selector) {
var selectors = selector.split(" ");
var matches;
var children;
var child;
var parents = node.getElementsByTagName(selectors[0]);
if (parents.length > 0) {
for (var i = 0; i < parents.length; i++) {
children = parents[i].getElementsByTagName(selectors[1]);
if (children.length > 0) {
for (var i = 0; i < parents.length; i++) {
child = children[i];
matches.push(child); // somehow store our result here
}
}
}
}
return matches;
}
The first problem with my code, aside from the fact that it doesn't work, is that it only handles two selectors (but it should be able to clear the first, second, and fourth cases).
The second problem is that I'm having trouble returning the correct result. I know that, just as in qsa1, I should be returning the same result as I'd get by calling the getElementsByTagName() function which "returns a live NodeList of elements with the given tag name". Creating an array and pushing or appending the Nodes to it isn't cutting it.
How do I compose the proper return result?
(For context, the full body of code can be found here)
Here's how I'd do it
function qsa2(selector) {
var next = document;
selector.split(/\s+/g).forEach(function(sel) {
var arr = [];
(Array.isArray(next) ? next : [next]).forEach(function(el) {
arr = arr.concat( [].slice.call(el.getElementsByTagName(sel) ));
});
next = arr;
});
return next;
}
Assume we always start with the document as context, then split the selector on spaces, like you're already doing, and iterate over the tagnames.
On each iteration, just overwrite the outer next variable, and run the loop again.
I've used an array and concat to store the results in the loop.
This is somewhat similar to the code in the question, but it should be noted that you never create an array, in fact the matches variable is undefined, and can't be pushed to.
You have syntax errors here:
if (parents.length > 0) {
for (var i = 0; i < parents.length; i++) {
children = parents[i].getElementsByTagName(selectors[1]);
if (children.length > 0) {
for (var i = 0; i < parents.length; i++) { // <-----------------------
Instead of going over the length of the children, you go over the length of the parent.
As well as the fact that you are reusing iteration variable names! This means the i that's mapped to the length of the parent is overwritten in the child loop!
On a side note, a for loop won't iterate over the elements if it's empty anyway, so your checks are redundant.
It should be the following:
for (var i = 0; i < parents.length; i++) {
children = parents[i].getElementsByTagName(selectors[1]);
for (var k = 0; k < children.length; i++) {
Instead of using an iterative solution, I would suggest using a recursive solution like the following:
var matches = [];
function recursivelySelectChildren(selectors, nodes){
if (selectors.length != 0){
for (var i = 0; i < nodes.length; i++){
recursivelySelectChildren(nodes[i].getElementsByTagName(selectors[0]), selectors.slice(1))
}
} else {
matches.push(nodes);
}
}
function qsa(selector, node){
node = node || document;
recursivelySelectChildren(selector.split(" "), [node]);
return matches;
}
Related
So i want to delete SVGs of my iframe, this is my code :
var parent = document.querySelectorAll("#main");
var child = parent[0].childNodes;
var lengthOfNodes = child.length;
for (var j = 0; j < lengthOfNodes; j++) {
child[j].parentNode.removeChild(child[j]);
}
child is an array of my svg element.
It works, but sometimes this algo throw me "Cannot read property 'parentNode' of undefined" and i don't know why... I need to relaunch this algo to get it work.
This way it would be too easy to remove elements you are iterating through
and take advantage of refrences
var parent = document.querySelectorAll("#main");
var child = parent[0].childNodes;
child.forEach(c => c.remove());
or if you want to break from a loop somewhere in future then ForEach loop is not going to help then
var parent = document.querySelectorAll("#main");
var child = parent[0].childNodes;
for(var c of child){
//can get out of loop anytime
c.remove();
}
Every time you remove a child, there is one less elements in the children array, so a loop like for (let j=0; j<length; j++) will make j too big at some point.
Prefer a code structured like this:
// This code removes all <li> nodes
const parent = document.querySelector('ul');
// Converts parent.children to an array,
// then use forEach which automatically handles varying length
[...parent.children].forEach(child => parent.removeChild(child));
<ul>
<li>xxx</li>
<li>xxx</li>
<li>xxx</li>
<li>xxx</li>
</ul>
Note that in order to wipe out the content of a parent node you might prefer the more simpler parent.innerHTML = ''!
You are changing the length of child itself in every iteration, Try this:
for (var i=0, j = child.length; i<j; i++)
{
child[i].parentNode.removeChild(child[i]);
}
Sup, and what do you think about a simple while loop instead of for loop ? Like this :
var parent = document.querySelectorAll("#main");
var child = parent[0].childNodes;
var j = 0;
while (obj.length > 0) {
child[j].parentNode.removeChild(child[j]);
}
Much easier for delete all elements. But if we want to iterate on a livelist with multiple conditions, we can do a reverse loop :
for (var i = obj.length - 1; i >= 0; i--) {
parent.removeChild(child[i]);
}
If the parent is undefined, at the moment you call parent[0] the array is empty. Are the SVG's being dynamically loaded? If so you might want to wrap this in a callback.
I've ditched jquery about 9(ish) months ago and needed a selector engine (without all the hassle and don't mind ie<7 support) so i made a simplified version of document.querySelectorAll by creating this function:
// "qsa" stands for: "querySelectorAll"
window.qsa = function (el) {
var result = document.querySelectorAll(el)[0];
return result;
};
This works perfectly fine for 95% of the time but I've had this problem for a while now and i have researched mdn, w3c, SO and not to forget Google :) but have not yet found the answer as to why I only get the first element with the requested class.
And I know that only the first element being returned is caused by the "[0]" at the end, but the function won't work if I remove it so I've tried to make a for loop with an index variable that increases in value depending on the length of elements with that class like this:
window.qsa = function (el) {
var result, el = document.querySelectorAll(el);
for(var i = 0; i < el.length; ++i) {
result = el[i];
}
return result;
};
Again that did not work so I tried a while loop like this:
window.qsa = function (el) {
var result, i = 0, el = document.querySelectorAll(el);
while(i < el.length) {
i++;
}
result = el[i];
return result;
};
By now I'm starting to wonder if anything works? and I'm getting very frustrated with document.querySelectorAll...
But my stubborn inner-self keeps going and I keep on failing (tiering cycle) so I know that now is REALLY the time to ask these questions :
Why is it only returning the first element with that class and not all of them?
Why does my for loop fail?
Why does my while loop fail?
And thank you because any / all help is much appreciated.
Why is it only returning the first element with that class and not all of them?
Because you explicitly get the first element off the results and return that.
Why does my for loop fail?
Because you overwrite result with a new value each time you go around the end of loop. Then you return the last thing you get.
Why does my while loop fail?
The same reason.
If you want all the elements, then you just get the result of running the function:
return document.querySelectorAll(el)
That will give you a NodeList object containing all the elements.
Now that does what you say you want, I'm going to speculate about what your real problem is (i.e. why you think it doesn't work).
You haven't shown us what you do with the result of running that function, but my guess is that you are trying to treat it like an element.
It isn't an element. It is a NodeList, which is like an array.
If you wanted to, for instance, change the background colour of an element you could do this:
element.style.backgroundColor = "red";
If you want to change the background colour of every element in a NodeList, then you have to change the background colour of each one in turn: with a loop.
for (var i = 0; i < node_list.length; i++) {
var element = node_list[i];
element.style.backgroundColor = "red";
}
You are returning a single element. You can return the array. If you want to be able to act on all elements at once, jQuery style, you can pass a callback into your function;
window.qsa = function(query, callback) {
var els = document.querySelectorAll(query);
if (typeof callback == 'function') {
for (var i = 0; i < els.length; ++i) {
callback.call(els[i], els[i], i);
}
}
return els;
};
qsa('button.change-all', function(btn) {
// You can reference the element using the first parameter
btn.addEventListener('click', function(){
qsa('p', function(p, index){
// Or you can reference the element using `this`
this.innerHTML = 'Changed ' + index;
});
});
});
qsa('button.change-second', function(btn) {
btn.addEventListener('click', function(){
var second = qsa('p')[1];
second.innerHTML = 'Changed just the second one';
});
});
<p>One</p>
<p>Two</p>
<p>Three</p>
<button class='change-all'>Change Paragraphs</button>
<button class='change-second'>Change Second Paragraph</button>
Then you can call either use the callback
qsa('P', function(){
this.innerHTML = 'test';
});
Or you can use the array that is returned
var pList = qsa('p');
var p1 = pList[0];
This loop
for(var i = 0; i < el.length; ++i) {
result = el[i];
}
overwrites your result variable every time. That's why you always get only one element.
You can use the result outside though, and iterate through it. Kinda like
var result = window.qsa(el)
for(var i = 0; i < result.length; ++i) {
var workOn = result[i];
// Do something with workOn
}
Is there a good way to add a certain value to each element in an array in javascript? Essentially, this should be a better way of writing the following:
a = [1,2,3,4];
for (i = 0; i < a.length; i++) {
a[i] += 7;
}
Maybe using map (but not necessarily)?
Edit:
Or a more interesting example:
a = [{'x':1},{'x':2},{'x':3},{'x':4}];
for (i = 0; i < a.length; i++) {
a[i].x += 7;
}
You can use map to do it:
a = a.map(function(entry) {
return entry + 7;
});
I'm not seeing how it's "better" to create a new array rather than update the one you have.
You can also use forEach:
a.forEach(function(entry, index) {
a[index] += 7;
});
It's still a bunch of function calls (but that's not a problem), but you have the advantage (over a for loop) of not having to declare the indexing variable, and you're modifying the existing array rather than replacing it.
Edit: Your "most interesting" example says even more that map is not really the best choice.
a.forEach(function(entry) {
entry.x += 7;
});
Yes, you can use .map but it will not modify the array in-place, so you must assign the result to a:
a = a.map(function(x) { return x+7 });
I want to select all elements with the css class
.arrow-down
Sorry but i simply dont find the correct answer, for my problem!
I have an javascript code:
document.getElementsByClassName("arrow-down")[0].style.borderTopColor=""+blu+"";
so how do i select not the first but [all] or is there a way to [1;2;3;]??
getElementsByClassName("arrow-down")[all]
getElementsByClassName("arrow-down")[1;2...]
I tried many things but simply dont get it!
Greetings from germany!
You need to iterate over the list of returned results.
var elements = document.getElementsByClassName("arrow-down");
for (var i = 0, len = elements.length; i < len; i++){
elements[i].style.borderTopColor = blu;
}
If you want to only do a specific subset based on the index, then you can add a condition that checks the value of i. I'm also assuming that blu here is a variable you have defined somewhere?
for (var i = 0, len = elements.length; i < len; i++){
if (i === 1 || i === 2 || i === 3){
elements[i].style.borderTopColor = blu;
}
}
Unfortunately, JavaScript does not have a shorthand for accessing a specific subset of array values, or for applying changes to multiple elements at once. That is something that jQuery does automatically for you. For instance, with jQuery you could write this as:
$('.arrow-down').css('borderTopColor', blu);
document.getElementsByClassName("arrow-down") does select all of such elements.
These are returned in a node list (which can be treated as an array), which is why using [0] on that returns the first element.
Loop over the different elements that the expression returns and act on them:
var elements = document.getElementsByClassName("arrow-down");
var elementsNum = elements.length)
for(var i = 0; i < elementsNum; i++)
{
var anElement = elements[i];
// do something with anElement
}
I'm trying to loop over ALL elements on a page, so I want to check every element that exists on this page for a special class.
So, how do I say that I want to check EVERY element?
You can pass a * to getElementsByTagName() so that it will return all elements in a page:
var all = document.getElementsByTagName("*");
for (var i=0, max=all.length; i < max; i++) {
// Do something with the element here
}
Note that you could use querySelectorAll(), if it's available (IE9+, CSS in IE8), to just find elements with a particular class.
if (document.querySelectorAll)
var clsElements = document.querySelectorAll(".mySpeshalClass");
else
// loop through all elements instead
This would certainly speed up matters for modern browsers.
Browsers now support foreach on NodeList. This means you can directly loop the elements instead of writing your own for loop.
document.querySelectorAll('*').forEach(function(node) {
// Do whatever you want with the node object.
});
Performance note - Do your best to scope what you're looking for by using a specific selector. A universal selector can return lots of nodes depending on the complexity of the page. Also, consider using document.body.querySelectorAll instead of document.querySelectorAll when you don’t care about <head> children.
Was looking for same. Well, not exactly. I only wanted to list all DOM Nodes.
var currentNode,
ni = document.createNodeIterator(document.documentElement, NodeFilter.SHOW_ELEMENT);
while(currentNode = ni.nextNode()) {
console.log(currentNode.nodeName);
}
To get elements with a specific class, we can use filter function.
var currentNode,
ni = document.createNodeIterator(
document.documentElement,
NodeFilter.SHOW_ELEMENT,
function(node){
return node.classList.contains('toggleable') ? NodeFilter.FILTER_ACCEPT : NodeFilter.FILTER_REJECT;
}
);
while(currentNode = ni.nextNode()) {
console.log(currentNode.nodeName);
}
Found solution on
MDN
As always the best solution is to use recursion:
loop(document);
function loop(node){
// do some thing with the node here
var nodes = node.childNodes;
for (var i = 0; i <nodes.length; i++){
if(!nodes[i]){
continue;
}
if(nodes[i].childNodes.length > 0){
loop(nodes[i]);
}
}
}
Unlike other suggestions, this solution does not require you to create an array for all the nodes, so its more light on the memory. More importantly, it finds more results. I am not sure what those results are, but when testing on chrome it finds about 50% more nodes compared to document.getElementsByTagName("*");
Here is another example on how you can loop through a document or an element:
function getNodeList(elem){
var l=new Array(elem),c=1,ret=new Array();
//This first loop will loop until the count var is stable//
for(var r=0;r<c;r++){
//This loop will loop thru the child element list//
for(var z=0;z<l[r].childNodes.length;z++){
//Push the element to the return array.
ret.push(l[r].childNodes[z]);
if(l[r].childNodes[z].childNodes[0]){
l.push(l[r].childNodes[z]);c++;
}//IF
}//FOR
}//FOR
return ret;
}
For those who are using Jquery
$("*").each(function(i,e){console.log(i+' '+e)});
Andy E. gave a good answer.
I would add, if you feel to select all the childs in some special selector (this need happened to me recently), you can apply the method "getElementsByTagName()" on any DOM object you want.
For an example, I needed to just parse "visual" part of the web page, so I just made this
var visualDomElts = document.body.getElementsByTagName('*');
This will never take in consideration the head part.
from this link
javascript reference
<html>
<head>
<title>A Simple Page</title>
<script language="JavaScript">
<!--
function findhead1()
{
var tag, tags;
// or you can use var allElem=document.all; and loop on it
tags = "The tags in the page are:"
for(i = 0; i < document.all.length; i++)
{
tag = document.all(i).tagName;
tags = tags + "\r" + tag;
}
document.write(tags);
}
// -->
</script>
</head>
<body onload="findhead1()">
<h1>Heading One</h1>
</body>
</html>
UPDATE:EDIT
since my last answer i found better simpler solution
function search(tableEvent)
{
clearResults()
document.getElementById('loading').style.display = 'block';
var params = 'formAction=SearchStocks';
var elemArray = document.mainForm.elements;
for (var i = 0; i < elemArray.length;i++)
{
var element = elemArray[i];
var elementName= element.name;
if(elementName=='formAction')
continue;
params += '&' + elementName+'='+ encodeURIComponent(element.value);
}
params += '&tableEvent=' + tableEvent;
createXmlHttpObject();
sendRequestPost(http_request,'Controller',false,params);
prepareUpdateTableContents();//function js to handle the response out of scope for this question
}
Getting all elements using var all = document.getElementsByTagName("*"); for (var i=0, max=all.length; i < max; i++); is ok if you need to check every element but will result in checking or looping repeating elements or text.
Below is a recursion implementation that checks or loop each element of all DOM elements only once and append:
(Credits to #George Reith for his recursion answer here: Map HTML to JSON)
function mapDOMCheck(html_string, json) {
treeObject = {}
dom = new jsdom.JSDOM(html_string) // use jsdom because DOMParser does not provide client-side Window for element access
document = dom.window.document
element = document.querySelector('html')
// Recurse and loop through DOM elements only once
function treeHTML(element, object) {
var nodeList = element.childNodes;
if (nodeList != null) {
if (nodeList.length) {
object[element.nodeName] = []; // IMPT: empty [] array for parent node to push non-text recursivable elements (see below)
for (var i = 0; i < nodeList.length; i++) {
console.log("nodeName", nodeList[i].nodeName);
if (nodeList[i].nodeType == 3) { // if child node is **final base-case** text node
console.log("nodeValue", nodeList[i].nodeValue);
} else { // else
object[element.nodeName].push({}); // push {} into empty [] array where {} for recursivable elements
treeHTML(nodeList[i], object[element.nodeName][object[element.nodeName].length - 1]);
}
}
}
}
}
treeHTML(element, treeObject);
}
Use *
var allElem = document.getElementsByTagName("*");
for (var i = 0; i < allElem.length; i++) {
// Do something with all element here
}
i think this is really quick
document.querySelectorAll('body,body *').forEach(function(e) {
You can try with
document.getElementsByClassName('special_class');