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Currency exchange web app in ~100 lines using the CRA (Create React App)

epranka profile image Edvinas Pranka ・5 min read

This tutorial describes how to create a simple currency exchange app in just about 100 lines of code using the Create React App boilerplate.

To see it in action check out this live demo.

🛠️ Preparation

First, create the react app. I call it "rates".

$ yarn create react-app rates

Run the command in your root

$ yarn start

Your web app will be available on http://localhost:3000

⚓ Fetching the currencies rate

For currency exchange data we will use free API on exchangeratesapi.io

We will load the latest data of the currencies rates.

Install the swr and unfetch libraries. It helps to easily fetch API.

$ yarn add swr unfetch

Create the fetcher function outside the App component

// src/App.js
import React from "react";
import "./App.css";
import fetch from "unfetch";

const API_URL = "https://api.exchangeratesapi.io";

const fetcher = async path => {
  const res = await fetch(API_URL + path);
  const json = await res.json();
  return json;
};

// function App...

Add useSWR in App component to fetch data

import React from "react";
import "./App.css";
import fetch from "unfetch";
import useSWR from "swr";

 // API_URL = ...

 // const fetcher = ...

 function App() {
  const { data: currencies } = useSWR("/latest?base=EUR", fetcher);

  return <div>Welcome to your Currency exchange app!</div>;
 }

Now we have the currencies rates in currencies.rates variable.

To see the JSON data of currencies rates you can open the API url directly into your browser: https://api.exchangeratesapi.io/latest?base=EUR

The ?base=EUR query is used to get all rates relative to EUR currency.

💄 Add the UI

For ui, we will use the material-ui design system. Install it:

$ yarn add @material-ui/core

Create the UI for our currency exchange app in the App component.

// ...
import {
  Container,
  Paper,
  Grid,
  TextField,
  Select,
  MenuItem
} from "@material-ui/core";

// ...

function App() {
  const { data: currencies } = useSWR("/latest?base=EUR", fetcher);

  if (!currencies) {
    return null;
  }

  return (
    <Container className="currency-exchange-container" fixed>
      <h1>Currency exchange</h1>
      <Paper
        className="currency-exchange-paper"
        variant="outlined"
        elavation={1}
      >
        <Grid container spacing={3}>
          <Grid item xs={6}>
            <TextField type="number" />
          </Grid>
          <Grid item xs={6}>
            <TextField type="number" />
          </Grid>
          <Grid item xs={6}>
            <Select>
              <MenuItem value={"EUR"}>EUR</MenuItem>
              {Object.keys(currencies.rates).map((rate, key) => (
                <MenuItem key={key} value={rate}>
                  {rate}
                </MenuItem>
              ))}
            </Select>
          </Grid>
          <Grid item xs={6}>
            <Select>
              <MenuItem value={"EUR"}>EUR</MenuItem>
              {Object.keys(currencies.rates).map((rate, key) => (
                <MenuItem key={key} value={rate}>
                  {rate}
                </MenuItem>
              ))}
            </Select>
          </Grid>
        </Grid>
      </Paper>
    </Container>
  );
}

Now, look at the following code snippet. We create the currency selection box by iterating over all possible rates from our API data. Since we fetch rates relative to EUR, so we should manually add the EUR item, because it doesn't exist in rates object.

  <Select>
    <MenuItem value={"EUR"}>EUR</MenuItem>
    {Object.keys(currencies.rates).map((rate, key) => (
      <MenuItem key={key} value={rate}>
        {rate}
      </MenuItem>
    ))}
  </Select>

When the currencies are not loaded, we just return null, because of currencies rates is undefined at that moment.

  if (!currencies) {
    return null;
  }

Add some styles to App.css

h1 {
  font-weight: 300;
  color: #636363;
  margin-bottom: 3rem;
}

.currency-exchange-container {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  height: 100vh;
}

.currency-exchange-paper {
  max-width: 350px;
  padding: 30px 30px 40px 30px;
}

.MuiInput-root {
  width: 100%;
}

⚙️ Add logic

Now we add the inputs state to our App component

function App() {
  const { data: currencies } = useSWR("/latest?base=EUR", fetcher);

  const [fromValue, setFromValue] = useState(1);
  const [toValue, setToValue] = useState(1);

  const [fromCurrency, setFromCurrency] = useState("EUR");
  const [toCurrency, setToCurrency] = useState("EUR");

  const handleFromCurrencyChange = e => {
    setFromCurrency(e.target.value);
  };

  const handleToCurrencyChange = e => {
    setToCurrency(e.target.value);
  };

  const handleFromValueChange = e => {
    setFromValue(parseFloat(e.target.value));
  };

  const handleToValueChange = e => {
    setToValue(parseFloat(e.target.value));
  };

  if (!currencies) {
    return null;
  }

  return (
    <Container className="currency-exchange-container" fixed>
      <h1>Currency exchange</h1>
      <Paper
        className="currency-exchange-paper"
        variant="outlined"
        elavation={1}
      >
        <Grid container spacing={3}>
          <Grid item xs={6}>
            <TextField
              type="number"
              value={fromValue}
              onChange={handleFromValueChange}
            />
          </Grid>
          <Grid item xs={6}>
            <TextField
              type="number"
              value={toValue}
              onChange={handleToValueChange}
            />
          </Grid>
          <Grid item xs={6}>
            <Select value={fromCurrency} onChange={handleFromCurrencyChange}>
              <MenuItem value={"EUR"}>EUR</MenuItem>
              {Object.keys(currencies.rates).map((rate, key) => (
                <MenuItem key={key} value={rate}>
                  {rate}
                </MenuItem>
              ))}
            </Select>
          </Grid>
          <Grid item xs={6}>
            <Select value={toCurrency} onChange={handleToCurrencyChange}>
              <MenuItem value={"EUR"}>EUR</MenuItem>
              {Object.keys(currencies.rates).map((rate, key) => (
                <MenuItem key={key} value={rate}>
                  {rate}
                </MenuItem>
              ))}
            </Select>
          </Grid>
        </Grid>
      </Paper>
    </Container>
  );
}

Also, add the two following functions for currency exchange

  const convertFromTo = () => {
    const fromRate =
      fromCurrency === "EUR" ? 1 : currencies.rates[fromCurrency];
    const valueInEur = fromValue / fromRate;
    const toRate = toCurrency === "EUR" ? 1 : currencies.rates[toCurrency];
    setToValue(valueInEur * toRate);
  };

  const convertToFrom = () => {
    const toRate = toCurrency === "EUR" ? 1 : currencies.rates[toCurrency];
    const valueInEur = toValue / toRate;
    const fromRate =
      fromCurrency === "EUR" ? 1 : currencies.rates[fromCurrency];
    setFromValue(valueInEur * fromRate);
  };

One function converts currencies forward, and other - backward. In both functions, firstly, we convert currencies to EUR, because all rates we fetched from API are relative to euros.

The last thing is to add the React hooks which run the rates exchange after the input change.

  useEffect(() => {
    convertFromTo();
  }, [fromValue, toCurrency]);

  useEffect(() => {
    convertToFrom();
  }, [toValue, fromCurrency]);

Here is a full App.js file

import React, { useState, useEffect } from "react";
import "./App.css";
import fetch from "unfetch";
import useSWR from "swr";
import {
  Container,
  Paper,
  Grid,
  TextField,
  Select,
  MenuItem
} from "@material-ui/core";

const API_URL = "https://api.exchangeratesapi.io";

const fetcher = async path => {
  const res = await fetch(API_URL + path);
  const json = await res.json();
  return json;
};

function App() {
  const { data: currencies } = useSWR("/latest?base=EUR", fetcher);

  const [fromValue, setFromValue] = useState(1);
  const [toValue, setToValue] = useState(1);

  const [fromCurrency, setFromCurrency] = useState("EUR");
  const [toCurrency, setToCurrency] = useState("EUR");

  const handleFromCurrencyChange = e => {
    setFromCurrency(e.target.value);
  };

  const handleToCurrencyChange = e => {
    setToCurrency(e.target.value);
  };

  const handleFromValueChange = e => {
    setFromValue(parseFloat(e.target.value));
  };

  const handleToValueChange = e => {
    setToValue(parseFloat(e.target.value));
  };

  const convertFromTo = () => {
    const fromRate =
      fromCurrency === "EUR" ? 1 : currencies.rates[fromCurrency];
    const valueInEur = fromValue / fromRate;
    const toRate = toCurrency === "EUR" ? 1 : currencies.rates[toCurrency];
    setToValue(valueInEur * toRate);
  };

  const convertToFrom = () => {
    const toRate = toCurrency === "EUR" ? 1 : currencies.rates[toCurrency];
    const valueInEur = toValue / toRate;
    const fromRate =
      fromCurrency === "EUR" ? 1 : currencies.rates[fromCurrency];
    setFromValue(valueInEur * fromRate);
  };

  useEffect(() => {
    convertFromTo();
  }, [fromValue, toCurrency]);

  useEffect(() => {
    convertToFrom();
  }, [toValue, fromCurrency]);

  if (!currencies) {
    return null;
  }

  return (
    <Container className="currency-exchange-container" fixed>
      <h1>Currency exchange</h1>
      <Paper
        className="currency-exchange-paper"
        variant="outlined"
        elavation={1}
      >
        <Grid container spacing={3}>
          <Grid item xs={6}>
            <TextField
              type="number"
              value={fromValue}
              onChange={handleFromValueChange}
            />
          </Grid>
          <Grid item xs={6}>
            <TextField
              type="number"
              value={toValue}
              onChange={handleToValueChange}
            />
          </Grid>
          <Grid item xs={6}>
            <Select value={fromCurrency} onChange={handleFromCurrencyChange}>
              <MenuItem value={"EUR"}>EUR</MenuItem>
              {Object.keys(currencies.rates).map((rate, key) => (
                <MenuItem key={key} value={rate}>
                  {rate}
                </MenuItem>
              ))}
            </Select>
          </Grid>
          <Grid item xs={6}>
            <Select value={toCurrency} onChange={handleToCurrencyChange}>
              <MenuItem value={"EUR"}>EUR</MenuItem>
              {Object.keys(currencies.rates).map((rate, key) => (
                <MenuItem key={key} value={rate}>
                  {rate}
                </MenuItem>
              ))}
            </Select>
          </Grid>
        </Grid>
      </Paper>
    </Container>
  );
}

export default App;

✨ Finished!

Congratulations! You have done the currency exchange app using the CRA (Create React App).

The full source code you can find in my repository epranka/rates.

Live demo.

Thanks for reading this. I hope it was helpful to you. Feedback and questions are appreciated.

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Discussion

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Editor guide
 

There seems to be a bug in the app related to rounding errors, where entering something like 121 rounds it off to 120.9999999.....

 

Thanks for reporting it! I keep in mind this and fix it soon. But I think is not an issue for the tutorial itself :)