Example of getting Salesforce access token using JWT bearer flow

index.js

const jwt = require('jsonwebtoken');

const {
	SALESFORCE_CLIENT_ID,
	SALESFORCE_AUTH_KEY,
	SALESFORCE_USERNAME,
	SALESFORCE_AUTH_HOST,
} = process.env;

function async getSalesforceAuthData() {
	const payload = {
		iss: SALESFORCE_CLIENT_ID,
		aud: SALESFORCE_AUTH_HOST,
		sub: SALESFORCE_USERNAME,
		exp: Date.now() + (3 * 60 * 1000), // +3 minutes
	};
	
	const token = jwt.sign(payload, SALESFORCE_AUTH_KEY, { algorithm: 'RS256' });
	const url = `${SALESFORCE_AUTH_HOST}/services/oauth2/token?grant_type=urn:ietf:params:oauth:grant-type:jwt-bearer&assertion=${token}`;
	const response = await fetch(url, { 
		method: 'POST', 
		headers: {
			accept: 'application/json', 
		},
	});
	
	return response.json();
}