TypeScript - Node.js

The Lekko Node.js SDK lets you add dynamic configuration to your Node.js apps.

This SDK is intended to be used with Lekko’s code transformation tools. Make sure your project is set up with Lekko by following our Getting Started guide.

Prerequisites

Node.js version 20 or greater
TypeScript version 5 or greater
Set up Lekko in your project using the Get started guide.

Install

In your terminal, run the following:

npm install @lekko/js-sdk @lekko/ts-transformer

Set up code transformation

Lekko uses TypeScript Compiler API (opens in a new tab) to make lekkos dynamic. To integrate into the TypeScript build process Lekko uses ts-patch (opens in a new tab), which is a wrapper around the TypeScript compiler (tsc) that allows running source code transformations during the build.

Add the following to your tsconfig.json to install Lekko source transformer:

tsconfig.json

{
  "compilerOptions": {
    ...
    "plugins": [
      {
        "transform": "@lekko/ts-transformer"
      }
    ]
  }
}

If you use ts-node you also need to add this to your tsconfig.json:

tsconfig.json

{
  ...
  "ts-node": { "compiler": "ts-patch/compiler" }
}

Then use tspc instead of tsc in your build script, for example:

package.json

{
  ...
  "scripts": {
    "build": "tspc"
    ...
  },
}

Initialize the Lekko client

The Lekko client connects to Lekko's services to periodically refresh lekkos and send evaluation events. In your application entry file:

src/server.ts

import { initLekko } from "@lekko/js-sdk";
 
// Before starting your application, create the global Lekko client
// that manages updates and evaluation events.
await initLekko({ updateIntervalMs: 5000 });

Usage

The following is an example of a lekko file under the lekko directory specified in your project’s .lekko.

src/lekko/example.ts

/** Whether to enable beta features */
export function getEnableBeta({ segment }: { segment: string }): boolean {
  if (segment.startsWith("enterprise")) {
    return true;
  }
  return false;
}
 
/** Return text based on environment */
export function getText({ env }: { env: string }): string {
  if (env === "production") {
    return "hello from prod";
  }
  return "hello from dev";
}

To use your lekkos, simply call them as normal functions:

src/server.ts

import { getText } from "./lekko/example.js";
 
const server = createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader("Content-Type", "text/plain");
  res.end(getText({ env: process.env.NODE_ENV ?? "" }));
});

Environment variables

For the client initialization to successfully connect to Lekko, you must provide the the following environment variables when running the project.

.env.local

LEKKO_API_KEY=<YOUR KEY>
LEKKO_REPOSITORY_OWNER=<GITHUB ORG>
LEKKO_REPOSITORY_NAME=lekko-configs

node --env-file .env.local <app entry>

Debug mode

You can set the LEKKO_DEBUG environment to true to enable debug logging of evaluations in your project.

Example:

.env.local

LEKKO_DEBUG=true

This allows you to see whenever lekkos are evaluated in your code and what contexts they are evaluated with.

[lekko] Debug mode is enabled, unset `LEKKO_DEBUG` environment variable to disable it.
[lekko] Loaded remote lekkos from: lekkodev/lekko-configs commit hash: 9fdc4319340a1ea55aa3a611cbcae98da9672db6.
[lekko] Connected to lekkodev/lekko-configs using API key "lekko_703adc..."
[lekko] Evaluated configs/text using the following context: {some_key: "foo"} to get: "best app ever"

TypeScript (Client)Python