Set custom attribute using JavaScript

Question

Use the setAttribute method:

document.getElementById('item1').setAttribute('data', "icon: 'base2.gif', url: 'output.htm', target: 'AccessPage', output: '1'");

But you really should be using data followed with a dash and with its property, like:

<li ... data-icon="base.gif" ...>

And to do it in JS use the dataset property:

document.getElementById('item1').dataset.icon = "base.gif";

Please use dataset

var article = document.querySelector('#electriccars'),
    data = article.dataset;

// data.columns -> "3"
// data.indexnumber -> "12314"
// data.parent -> "cars"

so in your case for setting data:

getElementById('item1').dataset.icon = "base2.gif";

For people coming from Google, this question is not about data attributes - OP added a non-standard attribute to their HTML object, and wondered how to set it.

However, you should not add custom attributes to your properties - you should use data attributes - e.g. OP should have used data-icon, data-url, data-target, etc.

In any event, it turns out that the way you set these attributes via JavaScript is the same for both cases. Use:

ele.setAttribute(attributeName, value);

to change the given attribute attributeName to value for the DOM element ele.

For example:

document.getElementById("someElement").setAttribute("data-id", 2);

Note that you can also use .dataset to set the values of data attributes, but as @racemic points out, it is 62% slower (at least in Chrome on macOS at the time of writing). So I would recommend using the setAttribute method instead.