Shopping cart Quantity Component

Note, This post is for beginners

In this post we see how to design a e-commerce shopping app quantity / qty component with regex validation.

This is the basic page and basic functions, hope you know them already.

import React, { useState } from 'react';
import {
  View,
  Text,
  StyleSheet,
  TextInput,
  TouchableOpacity,
} from 'react-native';

export default function App() {
  const [qty, setQty] = useState(0);

  return (
    <View style={styles.container}>
      <View style={styles.qtyLine}>
        <TouchableOpacity
          onPress={() => qty < 100 && setQty(qty + 1)}
          style={styles.btn}>
          <Text style={styles.btnText}>{'+'}</Text>
        </TouchableOpacity>
        <TextInput
          keyboardType={'number-pad'}
          maxLength={3}
          style={styles.qtyInput}
          value={'' + qty}
          onChangeText={(txt) => {
            let num = Number(txt);
            if (!isNaN(num) && num > 0 && num < 101) {
              setQty(num);
            } else setQty(0);
          }}
        />
        <TouchableOpacity
          onPress={() => qty > 0 && setQty(qty - 1)}
          style={styles.btn}>
          <Text style={styles.btnText}>{'-'}</Text>
        </TouchableOpacity>
        <TouchableOpacity
          onPress={() => setQty(0)}
          style={[styles.btn, { marginLeft: 5 }]}>
          <Text style={styles.btnText}>{'x'}</Text>
        </TouchableOpacity>
      </View>
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: 'snow',
    alignItems: 'center',
    justifyContent: 'center',
    padding: 10,
    paddingTop: 20,
  },
  qtyLine: {
    flexDirection: 'row',
    alignItems: 'center',
    justifyContent: 'center',
  },
  qtyInput: {
    marginHorizontal: 10,
    borderWidth: 0.5,
    borderColor: 'dodgerblue',
    padding: 5,
    textAlign: 'center',
    width: '20%',
  },
  btn: {
    backgroundColor: 'dodgerblue',
    paddingVertical: 5,
    paddingHorizontal: 10,
    borderRadius: 3,
  },
  btnText: {
    color: 'white',
  },
});

Number(txt)
// also converts empty string and blank space(s) as 0

At present in TextInput onChangeText event we are checking 3 conditions. Checking a text is a number, number is > 0 and number is < 101. But this can be simplified in one step by using regex validation.

Lets touch the TextInput component. In onChangeText event we are using regex to validate the given string is a number and the value is between 1 to 100.

TextInput with RegEx pattern

const qtyRgPtn = /^([0-9][0-9]?|100)$/;
. . .
<TextInput onChangeText={txt => 
qtyRgPtn.test(txt) && setQty(Number(txt))} />

so simple isn't it?

Now in addition to that we are showing a warning message for short time when the user inputs invalid qty.

const qtyRgPtn = /^([0-9][0-9]?|100)$/;
export default function App() {
  const [qty, setQty] = useState(0);
  const [qtyError, setQtyError] = useState('');
. . .

  <TextInput 
   onChangeText={(txt) => {
            /*
            let num = Number(txt);
            if (!isNaN(num) && num > 0 && num < 101) {
              setQty(num);
            } else {
              setQty(0);
            }
            */
            if (qtyRgPtn.test(txt)) setQty(Number(txt));
            else {
              setQty(0);
              if (!txt || !txt.trim().length) {
                setQtyError('Qty should be in between 1-100');
                setTimeout(() => setQtyError(''), 2000);
              }
            }
          }}
  />
. . . 
{/* final child of container */}
      {qtyError ? (
        <Text style={{ color: 'salmon', fontSize: 12, marginTop: 5 }}>
          {qtyError}
        </Text>
      ) : null}

Good! we are done :)

Hope this post will be useful. Source code here. Thank you.