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Showing content from https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/dataset below:

HTMLElement: dataset property - Web APIs

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The dataset read-only property of the HTMLElement interface provides read/write access to custom data attributes (data-*) on elements. It exposes a map of strings (DOMStringMap) with an entry for each data-* attribute.

Note: The dataset property itself can be read, but not directly written. Instead, all writes must be to the individual properties within the dataset, which in turn represent the data attributes.

An HTML data-* attribute and its corresponding DOM dataset.property modify their shared name according to where they are read or written:

In HTML

The attribute name begins with data-. It can contain only letters, numbers, dashes (-), periods (.), colons (:), and underscores (_). Any ASCII capital letters (A to Z) are converted to lowercase.

In JavaScript

The property name of a custom data attribute is the same as the HTML attribute without the data- prefix. Single dashes (-) are removed, and the next ASCII character after a removed dash is capitalized to form the property's camel-cased name.

Details and examples of converting between the HTML and JavaScript forms is described in more detail in the next section.

In addition to the information below, you'll find a how-to guide for using HTML data attributes in our article Using data attributes.

dash-style to camelCase conversion

A custom data attribute name is transformed to a key for the DOMStringMap entry by the following:

1. Lowercase all ASCII capital letters (A to Z);
2. Remove the prefix data- (including the dash);
3. For any dash (U+002D) followed by an ASCII lowercase letter a to z, remove the dash and uppercase the letter;
4. Other characters (including other dashes) are left unchanged.

camelCase to dash-style conversion

The opposite transformation, which maps a key to an attribute name, uses the following:

1. Restriction: Before transformation, a dash must not be immediately followed by an ASCII lowercase letter a to z;
2. Add the data- prefix;
3. Add a dash before any ASCII uppercase letter A to Z, then lowercase the letter;
4. Other characters are left unchanged.

For example, a data-abc-def attribute corresponds to dataset.abcDef.

• Attributes can be set and read by the camelCase name/key as an object property of the dataset: element.dataset.keyname.
• Attributes can also be set and read using bracket syntax: element.dataset['keyname'].
• The in operator can check if a given attribute exists: 'keyname' in element.dataset. Note that this will walk the prototype chain of dataset and may be unsafe if you have external code that may pollute the prototype chain. Several alternatives exist, such as Object.hasOwn(element.dataset, 'keyname'), or just checking if element.dataset.keyname !== undefined.

• When the attribute is set, its value is always converted to a string. For example: element.dataset.example = null is converted into data-example="null".

• To remove an attribute, you can use the delete operator: delete element.dataset.keyname.

A DOMStringMap.

<div id="user" data-id="1234567890" data-user="carinaanand" data-date-of-birth>
  Carina Anand
</div>

const el = document.querySelector("#user");

// el.id === 'user'
// el.dataset.id === '1234567890'
// el.dataset.user === 'carinaanand'
// el.dataset.dateOfBirth === ''

// set a data attribute
el.dataset.dateOfBirth = "1960-10-03";
// Result on JS: el.dataset.dateOfBirth === '1960-10-03'
// Result on HTML: <div id="user" data-id="1234567890" data-user="carinaanand" data-date-of-birth="1960-10-03">Carina Anand</div>

delete el.dataset.dateOfBirth;
// Result on JS: el.dataset.dateOfBirth === undefined
// Result on HTML: <div id="user" data-id="1234567890" data-user="carinaanand">Carina Anand</div>

if (el.dataset.someDataAttr === undefined) {
  el.dataset.someDataAttr = "mydata";
  // Result on JS: 'someDataAttr' in el.dataset === true
  // Result on HTML: <div id="user" data-id="1234567890" data-user="carinaanand" data-some-data-attr="mydata">Carina Anand</div>
}

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