render() {
  return (
    <button type="button" {this.state.submitting ? 'disabled' : ''}
      onClick={ this.handleSubmit }>Submit</button>
    );
}

<button type="button" disabled={this.state.submitting} onClick={ this.handleSubmit }>Submit</button>

<button type='button' disabled={this.state.submitting ? 'disabled' : null} onClick={this.handleSubmit}>Submit</button>

const disableBtnProps = {};
if (index !== 0) {
  disableBtnProps.disabled = false;
} else {
 disableBtnProps.disabled = true;
}

<Button {...disableBtnProps} className="btn"> my button </Button>