merge-meta.ts

import type { V2_HtmlMetaDescriptor, V2_MetaFunction } from "@remix-run/node";

export const mergeMeta = (
  overrideFn: V2_MetaFunction,
  appendFn?: V2_MetaFunction,
): V2_MetaFunction => {
  return arg => {
    // get meta from parent routes
    let mergedMeta = arg.matches.reduce((acc, match) => {
      return acc.concat(match.meta || []);
    }, [] as V2_HtmlMetaDescriptor[]);

    // replace any parent meta with the same name or property with the override
    let overrides = overrideFn(arg);
    for (let override of overrides) {
      let index = mergedMeta.findIndex(
        meta =>
          ("name" in meta &&
            "name" in override &&
            meta.name === override.name) ||
          ("property" in meta &&
            "property" in override &&
            meta.property === override.property) ||
          ("title" in meta && "title" in override),
      );
      if (index !== -1) {
        mergedMeta.splice(index, 1, override);
      }
    }

    // append any additional meta
    if (appendFn) {
      mergedMeta = mergedMeta.concat(appendFn(arg));
    }

    return mergedMeta;
  };
};

usage.ts

import { mergeMeta } from "./merge-meta";

export const meta = mergeMeta(
  // these will override the parent meta
  ({ data }) => {
    return [{ title: data.project.name }];
  },
  
  // these will be appended to the parent meta
  ({ matches }) => {
    return [{ name: "author", content: "Ryan Florence" }];
  },
);

// both functions get the same arguments as a normal meta function

For posterities: to make TS to work with loaded data in <2 remix versions, I had to change the signature as follows:

export const mergeMeta = <
  Loader = unknown,
  ParentsLoaders extends Record<string, unknown> = Record<string, unknown>,
>(
  overrideFn: MetaFunction<Loader, ParentsLoaders>,
  appendFn?: MetaFunction<Loader, ParentsLoaders>,
): MetaFunction<Loader, ParentsLoaders> => {
  return (arg) => {
  …