Samples of loops over an array that seem to show an implicit and explicit undefined for arrays

implicit-explicit.arrays.js

function ifIn(array) {
  console.group(`ifIn======`);
    let i = 0;
    while (i < array.length) {
      if (i++ in array) {
        console.log(`${i - 1} is in the array`);
      }
    }
  console.groupEnd();
}

function forIn(array) {
  console.group(`forIn======`);
  let i = 0;
  for (item in array) {
    console.log(array[item], `index = ${i++}`);
  }
  console.groupEnd();
}

function forOf(array) {
  console.group(`forOf======`);
  let i = 0;
  for (item of array) {
    console.log(item, `index = ${i++}`);
  }
  console.groupEnd();
}

function forEach(array) {
  console.group(`forEach======`);
  let i = 0;

  array.forEach(item => console.log(item, `index =  ${i++}`));
  
  console.groupEnd();
}

function mapIt(array) {
    console.group(`map======${array}`);
    let i = 0;

  const newArr = array.map(item => { console.log(item, `index = ${i++}`); return item;})
  console.groupEnd();
}

function testArray(array, label) {
  console.group(label, array)
  ifIn(array);
  forIn(array);
  forOf(array)
  forEach(array);
  mapIt(array);
  console.groupEnd();
}

console.clear();


const slotted = [,,'slotted one'];
const explicit = [undefined, undefined, 'explicit one'];
const gapped = [];
gapped[2] = 'gapped one';
const presized = new Array(3);
presized[2] = 'presized one';

testArray(slotted, `const slotted = [,,'slotted one'];`);
testArray(explicit, `const explicit = [undefined, undefined, 'explicit one'];`);
testArray(gapped, `const gapped = [];
c[2] = 'gapped one';`);
testArray(presized, `const presized = new Array(3);`);