Payload CMS dynamic variables with `{{` and `}}`. You can also add modifiers, e.g. `{{ my_var:millions }}` and divide the value by 1e6.

replace-variables-hook.ts

import payload from "payload";
import { CollectionAfterReadHook } from "payload/types";
import { Variable } from "../payload-types";

const pattern = /{{\s*(.*?)\s*}}/g;
const modifiers: { [key: string]: (input: string) => string } = {
	millions: (value) => Math.floor(parseInt(value) / 1e6).toString(),
};

/**
 * Recursively loops over all properties of the `input` object and invokes
 * `callback` for each one of them. If the callback returns a string, the looped
 * value is also replaced with that string.
 */
function deepIterate(
	input: object,
	callback: (value: string) => void | string
) {
	for (let k in input) {
		if (!input.hasOwnProperty(k)) {
			continue;
		}

		let val: string = input[k];

		if (typeof val === "string") {
			let result = callback(val);

			if (typeof result === "string") {
				input[k] = result;
			}
		} else if (val && typeof val === "object") {
			deepIterate(val, callback);
		}
	}
}

export const replaceVariablesHook: CollectionAfterReadHook = async ({
	doc,
	req,
}) => {
	// Don't do anything if `?replace` is missing from the request URL.
	if (!req.query.replace) {
		return doc;
	}

	let variables: string[] = [];

	deepIterate(doc, (value) => {
		for (let matches of value.matchAll(pattern)) {
			variables.push(matches[1].split(":")[0]);
		}
	});

	const query = await payload.find<Variable>({
		collection: "variables",
		limit: 0,
		where: {
			key: {
				in: variables,
			},
		},
	});

	deepIterate(doc, (value) => {
		return value.replace(pattern, (_, name: string) => {
			const parts = name.split(":");
			const key = parts.shift();
			let value = query.docs.find((doc) => doc.key === key)?.value;

			if (!value) {
				return "";
			}

			if (parts.length) {
				parts.forEach((modifierName) => {
					if (modifiers[modifierName]) {
						value = modifiers[modifierName](value);
					}
				});
			}

			return value;
		});
	});

	return doc;
};

variables-collection.ts

import { CollectionConfig } from "payload/types";

export const Variables: CollectionConfig = {
	slug: "variables",
	admin: {
		useAsTitle: "key",
	},
	fields: [
		{
			name: "key",
			type: "text",
		},
		{
			name: "value",
			type: "text",
		},
	],
};