Author: John Au-Yeung

To swap array elements with JavaScript, we use destructuring.

For instance, we write

[arr[0], arr[1]] = [arr[1], arr[0]];

to ass arr[1] to arr[0]and assignarr[0]toarr[1]` with destructuring.

To loop through an array containing objects and access their properties with JavaScript, we use the forEach method.

For instance, we write

yourArray.forEach((arrayItem) => {
  const x = arrayItem.prop + 2;
  console.log(x);
});

to call forEach with a callback that gets the prop property value from the arrayItem being looped through.

And then we add 2 to it assign the sum to x.

To reverse array in JavaScript without mutating original array, we copy the array before calling reverse.

For instance, we write

const newArray = [...array].reverse();

to copy the array with the spread operator.

And then we call reverse to reverse the copied array.

To sum a property value in an array with JavaScript, we use the map and reduce methods.

For instance, we write

const traveler = [
  { description: "Senior", amount: 50 },
  { description: "Senior", amount: 50 },
  { description: "Adult", amount: 75 },
  { description: "Child", amount: 35 },
  { description: "Infant", amount: 25 },
];

const sum = traveler
  .map((item) => item.amount)
  .reduce((prev, next) => prev + next);

to call traveler.map with a callback to get the amount property from each entry and put it in the returned array.

And then we call reduce with a callback to sum up the values from the array returned by map.

To count the occurrences / frequency of array elements with JavaScript, we use a for-of loop.

For instance, we write

const arr = [5, 5, 5, 2, 2, 2, 2, 2, 9, 4];
const counts = {};

for (const num of arr) {
  counts[num] = counts[num] ? counts[num] + 1 : 1;
}

to define the counts object.

Then we use a for-of loop to loop through the arr array and set counts[num] to counts[num] + 1 if it’s already set and 1 otherwise.