Svelte - how to wait for data that is being passed in from parent component?

I'm learning Svelte and I want to use data from one JSON API in three components. The data looks like this:

{
  "stats": {
    "currentYear": {
      "total": 6,
      "success": 6
    },
    "thirty": {
      "total": 30,
      "success": 28
    },
    "hundred": {
      "total": 100,
      "success": 92
    },
    "allTime": {
      "total": 789,
      "success": 728
    }
  },
  "heatmap": {
     ...
  },
  "other": {
     ...
  }
}

I retrieve the data via onMount in the App.svelte main component via async fetch, this works well. Then I want to pass each object to its corresponding component, so the stats object gets passed to Stats.svelte, the heatmap object to Heatmap.svelte etc.

To illustrate my issue, in Stats.svelte I am trying to display percentage values for each time period, for example:

current year: 100%
last thirty days: 93%
last 100 days: 92%
all time: 92%

Also, the CSS class for each will be based on some threshold values to change the colour (x >= 95: green, 95 > x >= 90: yellow, x < 90: red).

So some basic computation is needed which I wanted to have in a generic function, like shown below.

The stats object does get passed in from the parent component App.svelte, and if all I wanted to do is to show its values in the HTML via the {#await} block, this would work fine. However, I want to do some calculations, so I wanted to call a function that would use the stats object's data, but I do not know how to call this function at the right moment. Calling it on onMount does not work, because it's too early, the data coming in from the parent component has not yet been received.

<script>
    import { onMount } from "svelte"
    
    export let stats

    let currentYearClass, currentYearStat

    const calcPercentage = async (period) => {
        currentYearStat = stats[period].currentYearSuccess * 100 / stats[period].currentYearTotal
        currentYearClass = 'green'
    }

    onMount( async () => {
        calcPercentage('currentYear')
    })
</script>
<div id="stats">
{#await stats}
    <div>Waiting for stats ...</div>
{:then stats}
    <div class="{currentYearClass}" id="currentYear">{currentYearStat}</div>
    ...
    ...
{/await}
</div>

Solution

There are several ways to do this, but one way would be to have the calcPercentage take in the stats as an argument and then call it reactively.

export let stats
let currentYearClass, currentYearStat

const calcPercentage = (stats, period) => {
   currentYearStat = stats[persion}......
   currentYearClass = 'green'
}

$: stats && calcPercentage(stats, 'currentYear')

Edit: Some Explaining

The first problem with the original solution, something you probably noticed is that the component is mounted without the correct data, making stats to be undefined.

The above solution works in two steps:

$: stats && calcPercentage(stats, 'currentYear')

This first part defines a reactive statement, it will check if stats evaluates to true, something it will do unless it is undefined, false or 0. If stats is true it will then execute the function.

The second part is a the same function as before, I added the stats argument here although it is strictly not required in this situation, as the function will, because of the earlier execute everytime stats changes and is a true-like value.

With those two in place, when mounting the reactive statement will fail because stats is undefined, the function is not executed. Once the data comes in, it will be re-evaluated, stats is no longer undefined and the function fires.

Extra

When the reactive statement would be of the form:

$: myfunction(myvar)

It will execute for every value change of myvar, even during the mounting (consider it going from non exisiting to undefined ?). This means you would have to move your check into the function itself, for some scenarios this might be desired, an example is where this actually part of an assignment and the function itself is defined outside the component

import heavyCalc from 'heavy/calc/function`

$: value = heavyCalc(otherValue)