# Passing Dynamic Attributes/Properties

# Summary

This proposal adds mechanism to pass a dynamic attributes or properties bag to elements in an LWC template.

# Example

// x/myComponent.js
export default class MyComponent extends LightningElement {
    childProps = { 
        name: 'My Child',
        style: 'width: 50px',
        class: 'my-class',
        onclick: this.handleClick,
    };
    
    handleClick() {
        console.log('Clicked')
    }
}

// x/myComponent.html
<template>
    <x-child lwc:spread={childProps}></x-child>
</template>

The new lwc:spread directive passes childProps as properties/attributes to x-child.

# Motivation

In LWC, there is no way to pass props/attributes to a child component when the component author doesn't know in advance all the props/attributes that are needed. Adding this feature can be helpful in creating wrapper components that extend the functionality of an existing component while being oblivious to all the props that component requires.

# Prior Art

• React (JSX Spread Operator)
• Vue (v-bind)
• Svelte (Spread Props)
• Lit (Discussion for support)

# Detailed Design

A new LWC directive, lwc:spread will be introduced that can be used to spread the props/attributes to the children elements.

# Structure

It accepts an object with keys as the attribute names and values as the corresponding attribute values.

LWC Template compiler separates all attributes into:

1. Attributes
2. Properties
3. style attribute
4. class attribute

The same algorithm will be used for separating those attributes at runtime.

# Disallowed keys

Some keys (e.g. 'for:each') alter the compiled HTML output itself, so they are not allowed. lwc:spread does not allow LWC directives in general.

# Overriding attributes/props

<template>
  <c-child name="Hello" lwc:spread={childArgs}></c-child>
</template>

childArgs = { name: "World" };

The order in which attributes are specified matters. If the explicit attribute is passed before the attribute bag, the attribute bag takes precedence. Otherwise, the explicit attribute takes precedence. Much like Object.assign.

# Open Questions

1. How do we handle deleting properties? i.e., if a property is missing in an updated property bag, how do we "remove" the property of the child component?