custom attribute works only with element.getAttribute("attribute") but not "element.attribute"

Question

I have just noticed, that if I give a custom attribute to an html element, for example:

<input type="button" id="my_button" custom_attr="custom_attr_text" value="value_text" />

then i can retrieve it like this:

document.getElementById("my_button").getAttribute("custom_attr");

and it will return "custom_attr_text", but if I do

document.getElementById("my_button").custom_attr;

then it returns undefined!

I also noticed that with a built in attribute (for example value or id) both of the above works fine! Could somebody please explain why is this happening?

Answer 1

Only certain standard attributes are directly mapped to properties. This is not the defined behavior for non-standard (custom) attributes.

The forward compatible way of using custom attributes is to prefix them with data-.

<input ... data-custom_attr="custom_attr_text" ... />

Then they become available in HTML5 compliant browsers as:

element.dataset.custom_attr

But in legacy browsers, you'll still need to use .getAttribute().