The class example is misleading in terms of the line of code. The first example would have the componentDidMount and componentDidUpdate to call the same function

  componentDidMount() {
    const newData = getDataWithinRange(this.props.dateRange)
    this.setState({data: newData})
  }
  componentDidUpdate(prevProps) {
    if (prevProps.dateRange != this.props.dateRange) {
      const newData = getDataWithinRange(this.props.dateRange)
      this.setState({data: newData})
    }
  }

Could be:

  componentDidMount() {
     get(this.props.dateRange);
  }
  componentDidUpdate(prevProps) {
    if (prevProps.dateRange != this.props.dateRange) {
        get(this.props.dateRange);
    }
  }

  get(dateRange){
    const newData = getDataWithinRange(dateRange)
    this.setState({data: newData})
  }

It removes the duplication.