React - how to map nested object values?

Question

I am just trying to map nested values inside of a state object. The data structure looks like so:

I want to map each milestone name and then all tasks inside of that milestone. Right now I am trying to do so with nested map functions but I am not sure if I can do this.

The render method looks like so:

render() {
    return(
      <div>
        {Object.keys(this.state.dataGoal).map( key => {
            return <div key={key}>>

                     <header className="header">
                       <h1>{this.state.dataGoal[key].name}</h1>
                     </header>
                     <Wave />

                     <main className="content">
                       <p>{this.state.dataGoal[key].description}</p>

                         {Object.keys(this.state.dataGoal[key].milestones).map( (milestone, innerIndex) => {
                             return <div key={milestone}>
                                      {milestone}
                                      <p>Index: {innerIndex}</p>
                                    </div>
                         })}
                     </main>

                   </div>
        })}
      </div>
    );
  }

I think that I could somehow achieve that result by passing the inner index to this line of code: {Object.keys(this.state.dataGoal[key].milestones) so it would look like: {Object.keys(this.state.dataGoal[key].milestones[innerIndex]).

But I am not sure how to pass the innerIndex up. I have also tried to get the milestone name by {milestone.name} but that doesn't work either. I guess that's because I have to specify the key.

Does anybody have an idea? Or should I map the whole object in a totally different way?

Glad for any help, Jakub

Solution 1

You can use nested maps to map over the milestones and then the tasks array:

 render() {
  return (
    <div>
      {Object.keys(this.state.dataGoal.milestones).map((milestone) => {
        return (
          <div>
            {this.state.dataGoal.milestones[milestone].tasks.map((task, idx) => {
              return (
              //whatever you wish to do with the task item
              )
            })}
          </div>
        )
     })}
    </div>
  )
}

Solution 2

What you want is flatMap. flatMap takes an array and a function that will be applied to each element in the array, which you can use to (for example) access properties inside each object in the array. It then returns a new array with the returned values from its lambda:

function flatMap(arr, lambda) {
  return Array.prototype.concat.apply([], arr.map(lambda))
}

In our case, we don't have an array, we have an object so we can't use flatMap directly. We can convert the object to an array of its properties' values with Object.values and then make a function that accesses the object with the passed key:

function tasksFromDataGoal(key) {
  return flatMap(Object.values(dataGoal[key].milestones), milestone => milestone.tasks)
}

Working example:

function flatMap(arr, lambda) {
  return Array.prototype.concat.apply([], arr.map(lambda))
}

function tasksFromDataGoal(key) {
  return flatMap(Object.values(dataGoal[key].milestones), milestone => milestone.tasks)
}

const dataGoal = { 123: { milestones: { milestone1: { tasks: ['a', 'b'] }, milestone2: { tasks: ['c', 'd'] } } } }

alert(tasksFromDataGoal('123'))

Author of this implementation of flatMap: https://gist.github.com/samgiles/762ee337dff48623e729