Creating a javascript array with one to many type relationship

Sometimes I need to build an array of lists with a one to many type relationship.

For example this data:
1. item1 has a parent of key1.
2. item2 has a parent of key2.
3. item3 has a parent of key1.

Using PHP after packing the array I will end up with something like this:

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<?php
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$m = array();
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$m['key1'][] = 'item1';
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$m['key2'][] = 'item2';
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$m['key1'][] = 'item3';
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print_r($m);
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?>

Result:

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Array
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(
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    [key1] => Array
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        (
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            [0] => item1
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            [1] => item3
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        )
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    [key2] => Array
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        (
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            [0] => item2
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        )
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)

In javascript there is some options to do something similar. I ended up with something like this:
packedArr[“key1”] = “item1,item3”
packedArr[“key2”] = “item2”

Not exactly array in array[key] but the comma separated list did what I needed.

Here is the javascript itself. To test it you can use online javascript testers or just make up a html page with a button and maybe have onclick display document.getElementById(“demo”).innerHTML = output;

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<script>
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function packArr() {
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var pA = Array();
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var inA = [
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  ["child1","parent1"],
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  ["child2","parent2"],
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  ["child3","parent1"],
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  ["child4","parent1"],
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  ["child5","parent2"]
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  ];
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var inALen = inA.length;
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for (var i = 0; i < inALen; i++) {
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    //alert(inA[i]);
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  if (pA[inA[i][1]] == null) {
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     pA[inA[i][1]] = inA[i][0];
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  } else {
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    pA[inA[i][1]] = pA[inA[i][1]] + "," + inA[i][0];
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  }
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}
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res="";
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for ( var key in pA ) {
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  res = res + key + ": " + pA[key] + "<br>";
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};
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//res = myArray["parent1"];
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document.getElementById("demo").innerHTML = res;
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}
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</script>