Why does this Integer become a string in JavaScript?

Question

So this little snippet of code takes an array A of integers and makes the integers strings when they become the keys of pairs. Why? Python is happy for them to remain integers, but JavaScript seems to only want strings for keys.

    var pairs = {};
    for(var i=0; i < A.length; ++i) {
        var value = A[i];
        if(pairs[value]) {
            delete pairs[value];
        } else {
            pairs[value] = true;
        }
    }
    console.log(A, pairs)

Produces:

Example test:    [9, 3, 9, 3, 9, 7, 9]
Output:
[ 9, 3, 9, 3, 9, 7, 9 ] { '7': true }
OK

Your test case:  [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Output:
[ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 1, 2, 3, 4, 5, 6, 7, 8, 9 ] { '10': true }
Returned value: 10

All the keys are integers when they are added to the pairs object, but they become strings when they are used as keys. This is a good bit different from Python.

they have to be strings. see this question: https://stackoverflow.com/questions/6066846/keys-in-javascript-objects-can-only-be-strings — zim, May 08 '19 at 17:25
See [Keys in Javascript objects can only be strings?](https://stackoverflow.com/questions/6066846/keys-in-javascript-objects-can-only-be-strings) and [the documentation on it](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Property_Accessors) ==> _Property names are Strings or Symbols. Any other value, including a number, is coerced to a string._ — cb64, May 08 '19 at 17:25
it's considered a `key` in a object, so, not a integer as in a value. — Anuga, May 08 '19 at 17:25
https://stackoverflow.com/questions/1450957/pythons-json-module-converts-int-dictionary-keys-to-strings#1450981 — chevybow, May 08 '19 at 17:29

score 0 · Answer 1 · answered May 08 '19 at 17:29

By definition, object keys are always strings (or Symbols), if you use something else as a key it will be casted to a string. To have a Map in JS, there is Map, or a Set which will be enough in this case:

  const pairs = new Set();

 for(const value of [1, 2, 1])
   if(!pairs.delete(value)) pairs.add(value);

 console.log([...pairs]);

score 0 · Answer 2 · edited Apr 16 '23 at 15:03

0

In JavaScript, {} is a literal expression for the Object type. According to the documentation objects map string names to values.

A datatype that accepts arbitrary values as keys is Map.

edited Apr 16 '23 at 15:03

Peter Mortensen

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answered May 08 '19 at 17:29

ctt

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1

There is. `Map` – Jonas Wilms May 08 '19 at 17:30
2

Actually, `Map` _is_ a built-in dictionary type that accepts anything, including symbols, as keys. – Faust May 08 '19 at 17:30
Edited the answer. – ctt May 08 '19 at 17:32

score 0 · Answer 3 · answered May 08 '19 at 17:30

Object keys are always strings. E.g.

let x = {};
x.a = 1;
x[2] = 2;
x[x] = 3;

console.log(x);

On the other hand, if you want integer indices, you can use arrays instead

let A = [9, 3, 9, 3, 9, 7, 9];
var pairs = [];
for (var i = 0; i < A.length; ++i) {
  var value = A[i];
  if (pairs[value]) {
    delete pairs[value];
  } else {
    pairs[value] = true;
  }
}
console.log(A, pairs.reduce((acc, v, i) => {
  if (v) acc.push(i);
  return acc;
}, []));

Why does this Integer become a string in JavaScript?

3 Answers3