Update attributes in .env file in Node JS

I think the accepted solution will suffice for most use cases, but I encountered a few problems while using it personally:

• It will match keys that is prefixed with your target key if it is found first (e.g. if ENV_VAR is the key, ENV_VAR_FOO is also a valid match).
• If the key does not exist in your .env file, it will replace the last line of your .env file. In my case, I wanted to do an upsert instead of just updating existing env var.
• It will match commented lines and update them.

I modified a few things from Marc's answer to solve the above problems:

function setEnvValue(key, value) { // read file from hdd & split if from a linebreak to a array const ENV_VARS = fs.readFileSync(".env", "utf8").split(os.EOL); // find the env we want based on the key const target = ENV_VARS.indexOf(ENV_VARS.find((line) => { // (?<!#\s*) Negative lookbehind to avoid matching comments (lines that starts with #). // There is a double slash in the RegExp constructor to escape it. // (?==) Positive lookahead to check if there is an equal sign right after the key. // This is to prevent matching keys prefixed with the key of the env var to update. const keyValRegex = new RegExp(`(?<!#\\s*)${key}(?==)`); return line.match(keyValRegex); })); // if key-value pair exists in the .env file, if (target !== -1) { // replace the key/value with the new value ENV_VARS.splice(target, 1, `${key}=${value}`); } else { // if it doesn't exist, add it instead ENV_VARS.push(`${key}=${value}`); } // write everything back to the file system fs.writeFileSync(".env", ENV_VARS.join(os.EOL)); } 

Simple and it works: for typescript

import fs from 'fs' import os from 'os' import path from 'path' function setEnvValue(key: string, value: string): void { const environment_path = path.resolve('config/environments/.env.test') const ENV_VARS = fs.readFileSync(environment_path, 'utf8').split(os.EOL) const line = ENV_VARS.find((line: string) => { return line.match(`(?<!#\\s*)${key}(?==)`) }) if (line) { const target = ENV_VARS.indexOf(line as string) if (target !== -1) { ENV_VARS.splice(target, 1, `${key}=${value}`) } else { ENV_VARS.push(`${key}=${value}`) } } fs.writeFileSync(environment_path, ENV_VARS.join(os.EOL)) } 

var updateAttributeEnv = function(envPath, attrName, newVal){ var dataArray = fs.readFileSync(envPath,'utf8').split('\n'); var replacedArray = dataArray.map((line) => { if (line.split('=')[0] == attrName){ return attrName + "=" + String(newVal); } else { return line; } }) fs.writeFileSync(envPath, ""); for (let i = 0; i < replacedArray.length; i++) { fs.appendFileSync(envPath, replacedArray[i] + "\n"); } } 

I wrote this function to solve my issue.

By using the RegExp constructor we can achieve an even smaller solution than the accepted answer while also fixing the prefix problem mentioned by jp06:

const fs = require("fs"); function setEnvValue(key, value) { const file = fs.readFileSync(".env", "utf8"); // ^ Matches the start of a line due to the 'm' multiline flag // = Matches the literal equal sign character // .+ Matches any set of characters except for newlines const newFile = file.replace(new RegExp(`^${key}=.+`, 'm'), `${key}=${newValue}`); fs.writeFileSync(".env", newFile); } setEnvValue("A", "1");