I need to get the ID of an element but the value is dynamic with only the beginning of it is the same always.

Heres a snippet of the code.

<form class="form-poll" id="poll-1225962377536" action="/cs/Satellite">

The ID always starts with poll- then the numbers are dynamic.

How can I get the ID using just J开发者_Python百科avaScript and not jQuery?

You can use the querySelector for that:

document.querySelector('[id^="poll-"]').id;

The selector means: get an element where the attribute [id] begins with the string "poll-".

^ matches the start
* matches any position
$ matches the end

jsfiddle

Try this.

function getElementsByIdStartsWith(container, selectorTag, prefix) {
    var items = [];
    var myPosts = document.getElementById(container).getElementsByTagName(selectorTag);
    for (var i = 0; i < myPosts.length; i++) {
        //omitting undefined null check for brevity
        if (myPosts[i].id.lastIndexOf(prefix, 0) === 0) {
            items.push(myPosts[i]);
        }
    }
    return items;
}

Sample HTML Markup.

<div id="posts">
    <div id="post-1">post 1</div>
    <div id="post-12">post 12</div>
    <div id="post-123">post 123</div>
    <div id="pst-123">post 123</div>
</div>

Call it like

var postedOnes = getElementsByIdStartsWith("posts", "div", "post-");

Demo here: http://jsfiddle.net/naveen/P4cFu/

querySelectorAll with modern enumeration

polls = document.querySelectorAll('[id ^= "poll-"]');
Array.prototype.forEach.call(polls, callback);

function callback(element, iterator) {
    console.log(iterator, element.id);
}

The first line selects all elements in which id starts ^= with the string poll-. The second line evokes the enumeration and a callback function.

Given that what you want is to determine the full id of the element based upon just the prefix, you're going to have to do a search of the entire DOM (or at least, a search of an entire subtree if you know of some element that is always guaranteed to contain your target element). You can do this with something like:

function findChildWithIdLike(node, prefix) {
    if (node && node.id && node.id.indexOf(prefix) == 0) {
        //match found
        return node;
    }

    //no match, check child nodes
    for (var index = 0; index < node.childNodes.length; index++) {
        var child = node.childNodes[index];
        var childResult = findChildWithIdLike(child, prefix);
        if (childResult) {
            return childResult;
        }
    }
};

Here is an example: http://jsfiddle.net/xwqKh/

Be aware that dynamic element ids like the ones you are working with are typically used to guarantee uniqueness of element ids on a single page. Meaning that it is likely that there are multiple elements that share the same prefix. Probably you want to find them all.

If you want to find all of the elements that have a given prefix, instead of just the first one, you can use something like what is demonstrated here: http://jsfiddle.net/xwqKh/1/

I'm not entirely sure I know what you're asking about, but you can use string functions to create the actual ID that you're looking for.

var base = "common";
var num = 3;

var o = document.getElementById(base + num);  // will find id="common3"

If you don't know the actual ID, then you can't look up the object with getElementById, you'd have to find it some other way (by class name, by tag type, by attribute, by parent, by child, etc...).

Now that you've finally given us some of the HTML, you could use this plain JS to find all form elements that have an ID that starts with "poll-":

// get a list of all form objects that have the right type of ID
function findPollForms() {
    var list = getElementsByTagName("form");
    var results = [];
    for (var i = 0; i < list.length; i++) {
        var id = list[i].id;
        if (id && id.search(/^poll-/) != -1) {
            results.push(list[i]);
        }
    }
    return(results);
}

// return the ID of the first form object that has the right type of ID
function findFirstPollFormID() {
    var list = getElementsByTagName("form");
    var results = [];
    for (var i = 0; i < list.length; i++) {
        var id = list[i].id;
        if (id && id.search(/^poll-/) != -1) {
            return(id);
        }
    }
    return(null);
}

You'll probably have to either give it a constant class and call getElementsByClassName, or maybe just use getElementsByTagName, and loop through your results, checking the name.

I'd suggest looking at your underlying problem and figure out a way where you can know the ID in advance.

Maybe if you posted a little more about why you're getting this, we could find a better alternative.

You use the id property to the get the id, then the substr method to remove the first part of it, then optionally parseInt to turn it into a number:

var id = theElement.id.substr(5);

or:

var id = parseInt(theElement.id.substr(5));

<form class="form-poll" id="poll-1225962377536" action="/cs/Satellite" target="_blank">

The ID always starts with 'post-' then the numbers are dynamic.

Please check your id names, "poll" and "post" are very different.

As already answered, you can use querySelector:

var selectors = '[id^="poll-"]';
element = document.querySelector(selectors).id;

but querySelector will not find "poll" if you keep querying for "post": '[id^="post-"]'

If you need last id, you can do that:

var id_list = document.querySelectorAll('[id^="image-"]')
var last_id = id_list.length
alert(last_id)