REACT.js variables not accepted in api request

Question

I'm trying to make my code more efficient and reusable by adding variables on my requests and console.log. But for some reason its not working and I can't figure out why: full code can be found at: https://codesandbox.io/s/wispy-lake-6h051

This works:

  state = {
    lastWeek: '2020-11-09',
    today: '2020-11-12',
    selectedBase: 'USD',
    firstDateValues: null,
    fifthDateValues: null
  };

  getAPI = async() => {
    const START_DATE = this.state.lastWeek;
    const END_DATE = this.state.today;
    const BASE = this.state.selectedBase;
    
    const response = await fixer.get(`?start_at=${START_DATE}&end_at=${END_DATE}&base=${BASE}`, {
    });
    console.log(BASE) ---> output: USD
    console.log(response.data.rates[START_DATE].USD) --> correct data
}

But this doesn't:

  state = {
    lastWeek: '2020-11-09',
    today: '2020-11-12',
    selectedBase: 'USD',
    firstDateValues: null,
    fifthDateValues: null
  };

  getAPI = async() => {
    const START_DATE = this.state.lastWeek;
    const END_DATE = this.state.today;
    const BASE = this.state.selectedBase;
    
    const response = await fixer.get(`?start_at=${START_DATE}&end_at=${END_DATE}&base=${BASE}`, {
    });
    console.log(BASE) ---> output: USD
    console.log(response.data.rates[START_DATE].BASE) --> output: undefined 
}

Why?

Answer 1

What you need to do is to change

response.data.rates[START_DATE].BASE

to

esponse.data.rates[START_DATE][BASE]

because if you are using .BASE , it means BASE key, not your key from BASE variable