Vue.js util for intelligently merging data passed to functional components. (1K => 0.5K gzipped)

• Getting Started
• Why do I need this util?
• Scoped Styles
• Performance

Load the util from npm:

# NPM:
npm i vue-functional-data-merge

# Yarn:
yarn add vue-functional-data-merge

Now import and use it in your functional component declaration:

// MyFunctionalComponent.js

// ESM
import { mergeData } from "vue-functional-data-merge";
// Common JS
const { mergeData } = require("vue-functional-data-merge/dist/lib.common.js");

export default {
  name: "my-functional-component",
  functional: true,
  props: ["foo", "bar", "baz"],
  render(h, { props, data, children }) {
    const componentData = {
      staticClass: "fn-component", // concatenates all static classes
      class: {
        // object|Array|string all get merged and preserved
        active: props.foo,
        "special-class": props.bar,
      },
      attrs: {
        id: "my-functional-component", // now overrides any id placed on the component
      },
      on: {
        // Event handlers are merged to an array of handlers at each event.
        // The last data object passed to `mergeData` will have it's event handlers called first.
        // Right-most arguments are prepended to event handler array.
        click(e) {
          alert(props.baz);
        },
      },
    };

    return h("div", mergeData(data, componentData), children);
  },
};

When writing functional Vue components, the render function receives a context.data object (see vue docs). This object that contains the entire data object passed to the component (the shape of which can be found here). In order to write flexible components, the data object used to create the component must be merged with the data received. If not, only the properties defined by the component will be rendered.

Consider this example:

// MyBtn.js
export default {
  name: "my-btn",
  props: ["variant"],
  functional: true,
  render(h, { props, children }) {
    return h(
      "button",
      {
        staticClass: "btn",
        class: [`btn-${props.variant}`],
        attrs: { type: "button" },
      },
      children
    );
  },
};

This exports a functional button component that applies a base .btn class and a .btn-<variant> class based on the variant prop passed to the component. It's just a simple wrapper around some Bootstrap styling to make repetitive usage simpler. Usage would look like this:

<template>
  <form>
    <input type="text" placeholder="Name" required />
    <input type="email" placeholder="email" required />
    <my-btn
      variant="primary"
      type="submit"
      id="form-submit-btn"
      @click="onClick"
      >Submit</my-btn
    >
  </form>
</template>

We've used our Bootstrap button component in a form and conveniently applied the primary variant, but we also wanted to change the button type from button to submit, give it an id, and attach a click handler. This won't work because we haven't passed the attributes, listeners, etc. to the create element call in the component's render function.

To fix this, we might extract out props, merge listeners/attributes, etc. This works well, but gets verbose fast when attempting to support all dom attributes, event listeners, etc. One might think to simply use Object spread or Object.assign to solve this like so:

return h("button", { ...context.data, ...componentData }, children);

Now when we try to add any dom attributes, Object spread is essentially performing something like this:

Object.assign(
	{},
	{
		props: { variant: "primary" },
		attrs: { id: "form-submit-btn", type: "submit" }
		on: { click: onClick }
	},
	{
		staticClass: "btn",
		class: [`btn-${props.variant}`],
		attrs: { type: "button" },
		on: {
			click() {
				alert("Hello from MyBtn!")
			}
		}
	}
)

The component data will wipe out all the context's attrs and on handlers as Object.assign merges these properties. This is where the mergeData util can help you. It will dig into the nested properties of the context.data and apply different merge strategies for each data property. mergeData works like a nested Object.assign in that the util has a variadic argument length—you can pass any number of arguments to it, and they will all be merged from left to right (the right most arguments taking merge priority). You don't have to pass a new target object as the first argument, as the return value will always be a fresh object.

You may run into cases where you are using a functional component in another component with scoped styles. This would look something like this:

<template>
  <button class="my-class">
    <slot></slot>
  </button>
</template>
<style scoped>
  .my-class {
    text-align: center;
  }
</style>

This will generate data attributes on the component elements and the css selector.

<style>
  .my-class[data-v-f3f3eg9] {
    text-align: center;
  }
</style>

<button data-v-f3f3eg9 class="my-class">Click me!</button>

When a parent component with scoped styles makes use of a functional component, the data attribute won't be passed down automatically. Instead, you must pull this attribute out manually and add it to the VNodeData used in a render function's createElement call. Doing this requires reaching into Vue internals, which can be risky due to the private nature of the API and its potential to change. For that reason, this is not supported in this util.

However, this util can make that manual merging easier by conforming to the VNodeData shape required by mergeData and Vue itself. Here is an example of a helper function to manually extract a parent's style scope id and conditionally apply it in the functional component's render function.

const FunctionalComponent = {
  functional: true,
  render(createElement, context) {
    let { parent, data, children } = context;
    let componentData = { class: "my-class" };

    return createElement(
      "button",
      mergeData(data, getScopedStyleData(parent), componentData),
      children
    );
  },
};

/**
 * @param {Vue} parent
 * @returns {VNodeData}
 */
export function getScopedStyleData(parent) {
  let data = { attrs: {} };

  if (parent.$options._scopeId) {
    data.attrs[`data-v-${parent.$options._scopeId}`] = "";
  }

  return data;
}