Create objects, EACH with unique ids using for loop

Question

So I'm trying to generate an array that will have multiple objects, one of them with another array. Each object should have its own unique id, but when using for loop, each batch has objects with the same repeated ids. using i variable or even adding random number value seems to be not working. Is this even achievable in javascript?

Test.js

const testFunc = () => {
  let resultArray = []
  let testObj = {}
  let testArray = []

  const rn = () => {
    return ~~(Math.random() * 1000)
  }

  for (let i = 0; i < 3; i++) {
    testObj = { id: rn() }
    testArray.push(testObj)
    testObj = {}
  }

  for (let i = 0; i < 3; i++) {
    resultArray.push(testArray)
  }

  console.log(resultArray)
  return resultArray
}

Expected result:

[
  [
    { id: 859, text: 0 },
    { id: 232, text: 1 },
    { id: 240, text: 2 },
    { id: 638, text: 3 },
    { id: 393, text: 4 }
  ],
  [
    { id: 681, text: 0 },
    { id: 384, text: 1 },
    { id: 413, text: 2 },
    { id: 721, text: 3 },
    { id: 985, text: 4 }
  ],
  [
    { id: 348, text: 0 },
    { id: 911, text: 1 },
    { id: 255, text: 2 },
    { id: 816, text: 3 },
    { id: 797, text: 4 }
  ]
]

Result I get:

[
  [
    { id: 859, text: 0 },
    { id: 232, text: 1 },
    { id: 240, text: 2 },
    { id: 638, text: 3 },
    { id: 393, text: 4 }
  ],
  [
    { id: 859, text: 0 },
    { id: 232, text: 1 },
    { id: 240, text: 2 },
    { id: 638, text: 3 },
    { id: 393, text: 4 }
  ],
  [
    { id: 859, text: 0 },
    { id: 232, text: 1 },
    { id: 240, text: 2 },
    { id: 638, text: 3 },
    { id: 393, text: 4 }
  ]
]

You'll have a lot of trouble with id collisions if you're picking a random number from 0 to 999. It's called the birthday paradox. In your case, you only need 38 items before you have 50% chance of having 2 of the same ids.Either keep a counter, or use uuid() from the npm uuid package. — Charlie Bamford
– Charlie Bamford, Commented May 7, 2021 at 23:34
@CharlesBamford Thanks for the comment! I tried uuid() as well, but the result was exactly the same - except that it's not in numbers only. — Agamotto
– Agamotto, Commented May 8, 2021 at 5:36

Ori Drori · Accepted Answer · 2021-05-07 23:34:53Z

1

You push the testArray thrice to the resultArray. Instead generate testArray in a loop, push it to the resultArray, and add the items:

const testFunc = () => {
  const resultArray = []

  const rn = () => {
    return ~~(Math.random() * 1000)
  }

  for (let i = 0; i < 3; i++) {
    const testArray = []
    resultArray.push(testArray)
    
    for (let i = 0; i < 3; i++) {
      const testObj = { id: rn() }
      testArray.push(testObj)
    }
  }

  return resultArray
}

console.log(testFunc())

answered May 7, 2021 at 23:34

Ori Drori

196k32 gold badges243 silver badges233 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Agamotto Over a year ago

So I was just pushing three identical items. No wonder they all have the same ids. Thank you!

CertainPerformance · Accepted Answer · 2021-05-07 23:35:42Z

0

You're only ever creating a single sub-array. The line

let testArray = []

only runs once, and you do

resultArray.push(testArray)

3 times - so in the result, all sub-arrays in resultArray refer to the same testArray.

Create a new array explicitly instead, and use a nested loop so that there are 3x3 iterations total:

let resultArray = []
let testObj = {}

const rn = () => {
  return ~~(Math.random() * 1000)
}

for (let i = 0; i < 3; i++) {
  const testArray = []
  for (let i = 0; i < 3; i++) {
    const testObj = {
      id: rn()
    }
    testArray.push(testObj)
  }
  resultArray.push(testArray)
}


console.log(resultArray)

answered May 7, 2021 at 23:35

CertainPerformance

374k55 gold badges354 silver badges359 bronze badges

1 Comment

Agamotto Over a year ago

Thank you for the comment! Your explanation cleared my head a lot

Collectives™ on Stack Overflow

Create objects, EACH with unique ids using for loop

2 Answers 2

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related