Why does a number convert into a string when setting it as a Object value, and how to avoid it

Question

I have this object:

const someobj = {
  100: 'a',
  200: 'b',
  300: 'c'
}

for (const prop in someobj) {
  console.log(prop)
  console.log({ prop: prop })
}

When running it i get this output:

100
{ prop: '100' }
200
{ prop: '200' }
300
{ prop: '300' }

Why are the prop values converted from a number to a string? Is there anyway to avoid this besides doing Number(prop)?

Have you tried `parseInt(prop)`? – Udo E. Jul 03 '19 at 23:43 — Udo E., Jul 03 '19 at 23:43

score 1 · Accepted Answer · answered Jul 03 '19 at 23:28

Object properties are always strings (or symbols). Even though you can define an object with a numeric property, or by putting a number into bracket notation, the property itself will still be a string.

const obj = { 1: 'one' };
const [prop] = Object.keys(obj);
console.log(prop, typeof prop);

When you extract a property from an object using for..in, or Object.keys, or any other method, it will be a string (or a symbol). To make it be a number instead, you need to cast it to a number explicitly, like you're doing in your question.

Why does a number convert into a string when setting it as a Object value, and how to avoid it

1 Answers1