NewItem.jsx

import React from "react";

export default class NewItem extends React.Component {
  state = { value: "" };

  updateValue = evt => {
    this.setState({ value: evt.target.value });
  };

  addNewItem = () => {
    this.props.onAdd(this.state.value);
    this.setState({ value: "" });
  };

  render() {
    return (
      <div className="form">
        <input
          type="text"
          placeholder="add a new item..."
          value={this.state.value}
          onChange={this.updateValue}
          onBlur={this.addNewItem}
        />
      </div>
    );
  }
}