Definitely NodeJS, and Express, as mentioned by Tonny Hideyori. I would go a bit further to say that this is a good starting point, due to the lack of barrier to entry for writing simple queries and understanding the basics of writing a REST API.
A simple Hello World API looks like this.
var express = require('express')
var app = express()
// respond with "hello world" when a GET request is made to the homepage
app.get('/', function (req, res) {
res.send('hello world')
})
app.listen(3000);
Not much to it, easy to understand, easy to expand upon.