Methods of matching string input with numerical input in ReactJS?

You mean something like this might help?

import React, { Component } from "react";
import "./styles.css";

class App extends Component {
  state = {
    Items: {},
    CurItem: "",
    Quantity: 0
  };
  render() {
    return (
      <div className="App">
        <form
          onSubmit={(e) => {
            e.preventDefault();
            const Items = { ...this.state.Items };
            if (typeof Items[this.state.CurItem] === "undefined") {
              Items[this.state.CurItem] = +this.state.Quantity;
            } else {
              Items[this.state.CurItem] += +this.state.Quantity;
            }
            this.setState({
              Items,
              CurItem: "",
              Quantity: 0
            });
          }}
        >
          <label>
            <strong>Item Name:</strong>
            <input
              type="text"
              value={this.state.CurItem}
              onChange={(e) => this.setState({ CurItem: e.target.value })}
            />
          </label>
          <label>
            <strong>Quantity:</strong>
            <input
              type="number"
              value={this.state.Quantity}
              onChange={(e) => this.setState({ Quantity: e.target.value })}
            />
          </label>
          <input type="submit" />
        </form>
        <table border="1" width="50%">
          <thead>
            <tr>
              <th>Item</th>
              <th>Quantity</th>
            </tr>
          </thead>
          <tbody>
            {Object.keys(this.state.Items).map((item) => (
              <tr>
                <td>{item}</td>
                <td>{this.state.Items[item]}</td>
              </tr>
            ))}
          </tbody>
        </table>
      </div>
    );
  }
}

export default App;

See demo here: https://j6fv7.csb.app/

Preview