# html-element-property-mixins

Installation

$ npm install html-element-property-mixins

Introduction

html-element-property-mixins is a collection of mixins extending HTMLElement with properties, powering custom elements.

1. ObservedProperties enables observed properties (just like built-in observedAttributes).
2. DOMProperties enables attribute to property synchonisation.
3. ReflectedProperties enables property to attribute synchonisation.
4. Properties combines all three above.

Furthermore, we created a bunch of addons:

1. PropertiesChangedCallback Debounces / batches property changes for efficient DOM-rendering.
2. PropertyChangedHandler enables change handlers methods for property changes.
3. PropertiesChangedHandler enables change handlers methods for multiple property changes.

Mixins

ObservedProperties

import { ObservedProperties } from 'html-element-property-mixins';

Observing

By default, Custom Elements can observe attribute value changes whitelisted in the observedAttributes Array. ObservedProperties offers a similar solution for DOM properties using observedProperties. When a property has changed, propertyChangedCallback is called, passing the property name, the old value and the new value.'

class DemoElement extends ObservedProperties(HTMLElement) {

  static get observedProperties() {
    return ['firstName', 'lastName', 'age']
  }

  propertyChangedCallback(propName, oldValue, newValue) {
    console.info(`${propName} changed from ${oldValue} to ${newValue}`);
  }
  
}

If you like you can add your own getter / setter pairs:

static get observedProperties() {
  return ['initials']
}

get initials() {
  return this._initials;
}

set initials(val) {
  this._initials = val.toUpperCase();
}

constructor() {
  this.initials = 'a.b.c.';
}

propertyChangedCallback(propName, oldValue, newValue) {
  console.info(`${propName} changed to ${newValue}`); //initials changed to A.B.C;
}

Accessors don't require a getter / setter pair. Keep in mind though that by default, private property values are assigned using the following pattern: #${propName}.

static get observedProperties() {
  return ['firstName']
}

get firstName() {
  return this['#firstName'].toLowerCase()
}

DOMProperties

import { DOMProperties } from 'html-element-property-mixins';

Some native properties (e.g. input value) can be set using a DOM attribute. This mixin adds exactly this behavior: attribute to property sync:

class DemoElement extends DOMProperties(HTMLElement) {

  static get DOMProperties() {
    return ['firstname', 'lastname']
  }

}

<demo-element id="demo" firstname="Adewale" lastname="King"></demo-element>
<script>
  console.info(demo.firstname, demo.lastname); // Adewale, King
</script>

By default, attributes are lowercased property names (e.g. 'myPropName' becomes 'mypropname'). You can configure custom attribute mappings using 'propertyAttributeNames':

static get DOMProperties() {
  return ['myBestFriend']
}

static get propertyAttributeNames() {
  return {
    myBestFriend: 'my-best-friend',
  }
}

<demo-element id="demo" my-best-friend="Hellen"></demo-element>

Attribute Converters

Attribute values are always strings. If you wish to set attributes based on properties taht have a specific type, you can confifure converters using propertyFromAttributeConverters:

static get DOMProperties() {
  return ['married', 'friends']
}

static get propertyFromAttributeConverters() {
  return {
    married: function(value) {
      if(value === '') return true;
      return false;
    },
    friends: function(value) {
      if(!value) return null;
      return JSON.parse(value);
    }
  }
}

<demo-element id="demo" married friends='["Gabriella","Anik","Linda"]'></demo-element>
<script>
  console.info(demo.married, demo.friends); //true, ['Gabriella','Anik','Linda'];
</script>

html-element-property-mixins come with a set of attribute converters for boolean, string, number and object types:

  import { StringConverter, NumberConverter, BooleanConverter, ObjectConverter } from 'html-element-property-mixins/utils/attribute-converters';

  static get propertyFromAttributeConverters() {
    return {
      firstName: StringConverter.fromAttribute,
      age: NumberConverter.fromAttribute,
      married: BooleanConverter.fromAttribute,
      friends: ObjectConverter.fromAttribute,
    }
  }

ReflectedProperties

import { ReflectedProperties, ObservedProperties } from 'html-element-property-mixins';

This enables property to attribute sync. Using the 'reflectedProperties' object, one can map properties (keys) to attributes (values). The ObservedProperties mixin is required.

class DemoElement extends ReflectedProperties(ObservedProperties(HTMLElement)) {

  static get observedProperties() {
    return ['firstname', 'lastname', 'age']
  }

  static get reflectedProperties() {
    return ['firstname', 'lastname', 'age']
  }

  constructor() {
    this.firstname = 'Amira';
    this.firstname = 'Arif';
    this.age = 24;
  }

}

By default, attributes are lowercased property names (e.g. 'myPropName' becomes 'mypropname'). You can configure custom attribute mappings using 'propertyAttributeNames':

static get reflectedProperties() {
  return ['firstName']
}

static get propertyAttributeNames() {
  return {
    firstName: 'first-name',
  }
}

<demo-element first-name="Amira"></demo-element>

Attribute Converters

Attribute values are always strings. If you wish to set attributes based on properties taht have a specific type, you can confifure converters using propertyToAttributeConverters:

static get reflectedProperties() {
  return ['married', 'friends']
}

static get propertyToAttributeConverters() {
  return {
    married: function(value) {
      if(value === '') return true;
      return false;
    },
    friends: function(value) {
      if(!value) return null;
      return JSON.parse(value);
    }
  }
}

<demo-element id="demo" married friends='["Gabriella","Anik","Linda"]'></demo-element>
<script>
  console.info(demo.married, demo.friends); //true, ['Gabriella','Anik','Linda'];
</script>

html-element-property-mixins come with a set of attribute converters for boolean, string, number and object types. Attributes are set based on the return value of these functions: when false or undefined, removeAttribute is called. Otherwise, setAttribute is called using the return value.

import { StringConverter, NumberConverter, BooleanConverter, ObjectConverter } from 'html-element-property-mixins/utils/attribute-converters';

static get reflectedProperties() {
  return ['firstName', 'age', 'married', 'friends']
}

static get propertyToAttributeConverters() {
  return {
    firstName: StringConverter.toAttribute,
    age: NumberConverter.toAttribute,
    married: BooleanConverter.toAttribute,
    friends: ObjectConverter.toAttribute,
  }
}

NOTE: ObservedProperties is required for ReflectedProperties.

Properties

import { Properties } from 'html-element-property-mixins';

This wraps all property mixins into a single properties configuration object.

class DemoElement extends Properties(HTMLElement) {

  static get properties() {
    return {
      firstName: {
        observe: true, //add to `observedProperties` array
        DOM: true, //add to `DOMProperties` array
        reflect: true, //add to `reflectedProperties` array
        attributeName: 'first-name', //map to custom attribute name,
        toAttributeConverter: StringConverter.toAttribute, //run when converting to attribute
        fromAttributeConverter: StringConverter.fromAttribute //run when converting from attribute
      }
    }
  }

}

If you use the PropertyChangedHandler addon, you can add 'changedHandler' to your config:

class DemoElement extends PropertyChangedHandler(Properties(HTMLElement)) {

  static get properties() {
    return {
      age: {
        observe: true,
        changedHandler: '_firstNameChanged',
      }
    }
  }

  _firstNameChanged(oldValue, newValue) {
    //custom handler here!
  }

}

Addons

PropertiesChangedCallback

import { ObservedProperties } from 'html-element-property-mixins';
import { PropertiesChangedCallback } from 'html-element-property-mixins/src/addons';

When declaring observed properties using the observedProperties array, property changes are fired each time a a property changes using the propertyChangedCallback. For efficiency reasons (e.g. when rendering DOM), the propertiesChangedCallback (plural!) can be used. This callback is debounced by cancel / requestAnimationFrame on every property change. In the following example, render is invoked only once:

import { PropertiesChangedCallback } from 'html-element-property-mixins/src/addons';
import { ObservedProperties } from 'html-element-property-mixins';

class DemoElement extends PropertiesChangedCallback(ObservedProperties(HTMLElement)) {

  constructor() {
    super();
    this._renderCount = 0;
  }

  static get observedProperties() {
    return ['firstName', 'lastName', 'age'];
  }

  propertiesChangedCallback(propNames, oldValues, newValues) {
    this._renderCount++;
    this.render();
  }

  render() {
    this.innerHTML = `
      Hello, ${this.firstName} ${this.lastName} (${this.age} years).<br>
      Render Count = ${this._renderCount}. 
    `
  }

  constructor() {
    super();
    this.firstName = 'Amina';
    this.lastName = 'Hamzaoui';
    this.age = 24;
  }

}

PropertyChangedHandler

import { ObservedProperties } from 'html-element-property-mixins';
import { PropertyChangedHandler } from 'html-element-property-mixins/src/addons';

Value changes to properties whitelisted in the observedProperties array are always notified using propertyChangedCallback. PropertyChangedHandler provides for custom callbacks for property changes:

class DemoElement extends PropertyChangedHandler(ObservedProperties((HTMLElement)) {
  static get observedProperties() {
    return ['firstName']
  }

  static get propertyChangedHandlers() {
    return {
      firstName: function(newValue, oldValue) {
        console.info('firstName changed!', newValue, oldValue);  
      }
    }
  }
}

Alternatively, callbacks can be passed as string references:

static get propertyChangedHandlers() {
  return { firstName: '_firstNameChanged' }
}

_firstNameChanged(newValue, oldValue) {
  console.info('firstName changed!', newValue, oldValue);
}

Note: PropertyChangedHandler should always be used in conjunction with ObservedProperties.

PropertiesChangedHandler

import { ObservedProperties } from 'html-element-property-mixins';
import { PropertiesChangedHandler } from 'html-element-property-mixins/src/addons';

Its plural companion propertiesChangedHandlers can be used to invoke a function when one of many properties have changed. Key / value pairs are now swapped. A key refers to the handler function, the value holds an array of the observed properties.

class DemoElement extends PropertiesChangedHandler(ObservedProperties((HTMLElement)) {
  static get observedProperties() {
    return ['firstName', 'lastName']
  }

  static get propertiesChangedHandlers() {
    return {
      _nameChanged: ['firstName', 'lastName']
    }
  }

  _nameChanged(propNames, newValues, oldValues) {
    console.info(newValues.firstName, newValues.lastName);
  }

}

Note: PropertiesChangedHandler should always be used in conjunction with ObservedProperties.