Author: John Au-Yeung

To truncate an array with JavaScript, we use the slice method.

For instance, we write

let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
arr = arr.slice(0, 4);
console.log(arr);

to call arr.slice with 0 and 4 to return an array with the arr array items between index 0 and 3.

And we assign the returned array back to arr to update it.

To convert a JavaScript iterable to an array, we use the spread operator.

For instance, we write

const x = new Set([1, 2, 3, 4]);
const z = [...x];

to create a set with the Set constructor.

Then we convert the set to an array with the spread operator.

To compute a set difference using JavaScript arrays, we use the filter method.

For instance, we write

const a = [1, 2, 3, 4];
const b = [1, 3, 4, 7];

const diff = a.filter((x) => {
  return b.indexOf(x) < 0;
});

console.log(diff);

to call a.filter with a callback that checks if the item x in a isn’t in b with b.indexOf(x) < 0.

An array with the items in a but not in b is then returned.

To find index of all occurrences of element in array, we use the map and filter method.

For instance, we write

const nanoIndexes = cars
  .map((car, i) => (car === "Nano" ? i : -1))
  .filter((index) => index !== -1);

to call map with a callback that checks if the car value being iterated through is 'Nano'.

If it is, we return i and we return -1 otherwise.

Then we call filter with a callback that returns an array with the index value not equal to -1.

To create object from array with JavaScript, we can use the Object.fromEntries method.

For instance, we write

const dynamicArray = ["2007", "2008", "2009", "2010"];
const obj = Object.fromEntries(
  dynamicArray.map((year) => [
    year,
    {
      something: "based",
      on: year,
    },
  ])
);

console.log(obj);

to call Object.fromEntries with an array that we get from the map method called with a callback that returns arrays with the key and value of each property.

We use Object.fromEntries to convert the nested key-value pair array to an object.