Not sure what exactly was the problem of your version (I'd change to $: if(max) {...}) (turns out it was using var instead of let) but if you put the functionality in the onMount function it works >> REPL

App.svelte

<script>
    import Component from './Component.svelte'
</script>

<Component max={20}/>

Component.svelte

<script>
    import {onMount} from 'svelte'
    export let max;
    let map = {};
    
    onMount(() => {         
      for(var i=0; i<max; i++){
        if(i<5) {
          map["id_"+i] = 1;
        }
        else {
          map["id_"+i] = -1;
        }
      }
    console.log(map)
    })

</script>

<p>{max}</p>

Corrl's version works, but has an important caveat: your array won't be updated if you change the max prop on the fly, because the component will not be destroyed & re-mounted, and hence your array routine will not run again.

Perhaps this is your intent, or perhaps it is not.

A reactive version will keep the array updated. However, when dealing with reactive statements, my advice is always to keep these statements as simple as possible.

A cleaner solution to your problem would be to move your array building routine into a simple function that accepts the max value as input and returns the initialized array:

function makeMap(max) {
  const map = {}
  for(var i=0; i<max; i++){
    if (i<5) {
      map["id_"+i] = 1
    } else {
      map["id_"+i] = -1
    }
  }
  return map
}

Then your reactive code becomes a very simple:

$: map = makeMap(max)

that respects the fundamental reactive assignment rule with the reactive variable (map) on the left hand side and the dependency (max) on the right hand side.

Here is a REPL demonstrating the difference between having your array updated through a reactive statement, and having it initialized through onMount. Have a look at the console while you change the max prop passed to the component by clicking on the + button.