How can I "reset" an array's keys?

This question already has an answer here:

Remove empty elements from an array in Javascript 36 answers

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For example, you can use Array.prototype.filter method.

var array = [];
array[30] = 1;
array.length; // 31

var compactArray = array.filter(function (item) {
    return item !== undefined;
});

compactArray.length; // 1

If it's an object, for..in loop will be usefull

var array = { 31: 1};
var compactArray = [];
for (var i in array) {
    compactArray.push(array[i]);
}

---

You can loop throw array and if its a valid item, then push in another array.

Also, when you do something like this,

var arr = [];
arr[3] = 15;

arr is actually [null, null, null, 15]

Following is an example.

(function() {
  var arr = [];
  var result = [];

  arr[3] = 15;
  arr[7] = 20;
  arr[19] = -1;

  console.log(arr);
  console.log(JSON.stringify(arr))

  arr.forEach(function(item) {
    if (item) {
      result.push(item);
    }
  })

  console.log(result);
  console.log(JSON.stringify(result));
})()

---

如果你的意思是你的对象是这样的

var obj = {3: 15, 7: 20, 19: -1};
var output = {};
var counter = 0;
for ( var id in obj )
{
   output[ String( counter ) ] = obj[ id ];
   counter++;
}
console.log( output );

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