how can i access variables values from a netlify function to call API?

Question

hope someone can help me. I'm trying to deploy a web app on netlify, but i don't know how to call the API the right way, providing the right value.

this is my netlify function file:

const fetch = require('node-fetch');

exports.handler = async event => {
  const API_KEY =  process.env.API_KEY;
  const response = await fetch(`https://api.waqi.info/feed/${process.env.CITY}/?token=${API_KEY}`);
  const data = await response.json();

  const pass = (body) => {
    return {
      statusCode: 200,
      body: JSON.stringify(body)
    }
  };

  return pass(data);
}

My problem is about providing the right city value to the API call.

i've also tried to make city an env var on netlify, but even if i change its value, the file lambda.js provides me always the same value probably because it runs just one time at the start.

Here's the code in index.js:

let CITY = process.env.CITY;

async function getCityPollution(city) {
  let response = await fetch("/.netlify/functions/lambda");
  let result = await response.json();
  if(response.status == 200 && result.status == 'ok') {
    await dataHandler(result);
    console.log(result);
  } else if (result.status == 'error'){
    console.log(`${result.status}: ${result.data}`);
    setTimeout(() => {
      dataParagraph.innerHTML = `Unfortunately we have no datas for ${city} station (incredible but true),
                                insert coords to check datas from the nearest station or try with another city.
                                Go check <a href="https://waqi.info/">https://waqi.info/</a> to see our coverage`;
      $("html, body").animate({ scrollTop: document.body.scrollHeight }, "slow");
     }
    , 50);
  } else {
    console.log(response.status);
  }
}

// getting city input and call output function
let getCity = document.querySelector('#getCity');
getCity.onclick = async () => {
  CITY = cityInput.value;
  if (!CITY) {
    emptyFields(cityInput);
  } else {
    await getCityPollution(CITY);
    coordInput[0].value = '';
    coordInput[1].value = '';
    console.log(CITY) //it works, the value changes
  }
}

Obviously this is the try with the netlify env var. This way i get always the same value. There's a way to pass the right value of CITY everytime i need it? Even without using env variables.

Thank you in advance