Enable environment variables in your configs with env-interpolation

I shipped env-interpolation, a tiny TS helper that interpolates ${VAR} and ${VAR:default} inside config values. It walks strings in objects/arrays and never touches keys, so shapes stay stable and predictable—very config-oriented.

Born out of mcp-tool-selector, where I needed layered config without leaking secrets or littering process.env calls. It turned into a sharp little utility, so I published it.

At a glance

• Resolves placeholders in values only (objects/arrays), keys untouched
• Supports defaults and multi-pass resolution
• Zero deps, TS-first, Node 18+, ESM/CJS

Docs & API: read the GitHub repo and the npm package page.

Quick taste: load JSON → interpolate

config.json

{
  "api": "${API_URL:https://api.example.com}",
  "timeoutMs": "${TIMEOUT:5000}",
  "flags": ["${PRIMARY:alpha}", "${SECONDARY:beta}"],
  "service": {
    "url": "${SERVICE_URL:${API_URL}/v1}",
    "headers": { "x-tenant": "${TENANT:public}" }
  }
}

load-config.ts

import { readFileSync } from "node:fs";
import { interpolate } from "env-interpolation";

const raw = readFileSync("config.json", "utf8");
const input = JSON.parse(raw);

const resolved = interpolate(input, {
  API_URL: "https://api.example.com",
  TIMEOUT: "8000",
  TENANT: "public",
});

console.log(resolved);

This is my second AI engineered project (the first one was mcp-server-giphy), built with multiple AI agents (Claude, Copilot, Gemini & Codex). If your configs span files, environments, and tools, this should remove a few papercuts.