JavaScript Why is there not one array with all the items in it, but an array for every item?

I want to push all the item-data into one array. However, when I try, it just makes an array for every item.

async function final() {
  const response = await fetch('/api');
  const data = await response.json();
  console.log(data)

  for (item of data) {
    const num = item.atomicNumber;
    const arr = []
    arr.push(num)

    console.log(arr)
  }

}

Answer:1

Because you are creating a new array inside the loop. Analyzing your own code step by step and understanding what every line does is mandatory to be a good developer :-). Let's analyze it:

async function final() {
  const response = await fetch('/api');
  const data = await response.json();
  console.log(data)

  // Until here everything is fine

  // You define a loop which will run from this point each time
  for (item of data) {
    const num = item.atomicNumber;

    // You are creating a new array inside the loop. The loop, as its name says, will run one time per item.
    const arr = []
    arr.push(num)

    console.log(arr)
  }
}

To fix this, just move the array outside the loop, so it only runs one time:

async function final() {
  const response = await fetch('/api');
  const data = await response.json();
  console.log(data)

  // We create the array outside the loop
  const arr = []

  // Then define a loop which will run from this point each time
  for (let item of data) { // Don't forget to define variables with the proper keyword (in this case, "let" is enough).
    const num = item.atomicNumber;
    arr.push(num)
  }

  // We log the array when the loop has ended, so it logs only one time
  console.log(arr)
}

Answer:2

you need to put your empty array const arr = [] out of the loop, otherwise you're re-declaring the array at every iteration.

async function final() {
  const response = await fetch('/api');
  const data = await response.json();
  console.log(data)
  const arr = []

  for (item of data) {
    const num = item.atomicNumber;
    arr.push(num)
  }
  console.log(arr)
}

Answer:3

You're re-declaring arr in every iteration of the loop. Move the declaration out of it, and just push into it every iteration:

async function final() {
  const response = await fetch('/api');
  const data = await response.json();
  console.log(data)
  const arr = [] // arr declared outside the loop

  for (item of data) {
    const num = item.atomicNumber;
    arr.push(num) // but used inside it

    console.log(arr)
  }
}

Answer:4

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