For loop setting all object values to last instance

Question

I'm trying to iterate through a for loop to sort an array 5 times.

let sortedCities = {};
for (let i = 0; i < 5; i++) {
  sortedCities[i] = darkSkyHelper.sortCities(info,unsortedCities,i);
  console.log(sortedCities[i]);
}
console.log(sortedCities);

static sortCities(info, cities, day) {
    let sortedCities = cities.sort(function(a,b) {
      return info[b].daily.data[day].precipProbability - info[a].daily.data[day].precipProbability;
    })
    return sortedCities;
  }

The first console.log displays the expected output for each loop while the second displays an object with each object value equaling the output from the last running of the loop

["cin", "cle", "col", "nya", "laa", "was", "tex"]
["was", "nya", "cle", "cin", "col", "laa", "tex"]
["cle", "cin", "was", "nya", "laa", "col", "tex"]
["laa", "col", "cle", "cin", "was", "nya", "tex"]
["col", "tex", "cle", "laa", "cin", "was", "nya"]

Object {..}
0: Array[7]
0: "col"
1: "tex"
2: "cle"
3: "laa"
4: "cin"
5: "was"
6: "nya"
length: 7
__proto__: Array[0]
1: Array[7]
0: "col"
1: "tex"
2: "cle"
3: "laa"
4: "cin"
5: "was"
6: "nya"
...

You've got five arrays shown, but you only loop through one in your code (this.state.days). Please include a minimal reproducible example if you'd like more help. — Heretic Monkey
– Heretic Monkey, Commented Apr 5, 2017 at 19:11
It sounds like darkSkyHelper.sortCities is returning a reference to the same array each time, just sorted differently. Hence each element in sortedCities is really the same array. — user27414
– user27414, Commented Apr 5, 2017 at 19:21
See How can you sort an array without mutating the original array? — JJJ
– JJJ, Commented Apr 5, 2017 at 19:30

user27414 · Accepted Answer · 2017-04-05 19:28:36Z

1

Each time you call sortCities you're returning a reference to the same array, just sorted differently. You need to create a new array each time. There are several ways to do this, including slice:

static sortCities(info, cities, day) {
    let sortedCities = cities.sort(function(a,b) {
      return info[b].daily.data[day].precipProbability - info[a].daily.data[day].precipProbability;
    })
    return sortedCities.slice(0);
}

answered Apr 5, 2017 at 19:28

user27414

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3 Comments

JJJ Over a year ago

Might be cleaner to sort a copy of the original array, instead of sorting the original (which mutates it) and then returning a copy. Also the new variable is unnecessary.

user27414 Over a year ago

@JJJ - perhaps. That would be return cities.slice().sort(...). The OP can implement however in whatever way suits their style.

P. Jack Over a year ago

Thank you both for your help!

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