App.jsx

handleCheckBox(e) {

    const newSelection = e.target.value;
    let newSelectionArray;

    if(this.state.newUser.skills.indexOf(newSelection) > -1) {
      newSelectionArray = this.state.newUser.skills.filter(s => s !== newSelection)
    } else {
      newSelectionArray = [...this.state.newUser.skills, newSelection];
    }

      this.setState( prevState => ({ newUser:
        {...prevState.newUser, skills: newSelectionArray }
      })
      )
}