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To generate all combinations of elements in a single array in pairs with JavaScript, we call the flatMap and slice method.

For instance, we write

const array = ["apple", "banana", "lemon", "mango"];
const result = array.flatMap((v, i) =>
  array.slice(i + 1).map((w) => v + " " + w)
);

to call array.flatMap with a callback that returns a sliced version of the array with slice with items from index i + 1 to the end of the array.

And then we call map on the sliced array and combined the item v and w into a string.

flatMap will flatten the arrays returned into a single level array with the results.

To apply a function to each object in a JavaScript array, we use the map method.

For instance, we write

const newArray = oldArray.map((e) => {
  e.data = e.data.split(",");
  return e;
});

to call oldArray.map with a callback that sets the e.data property in the oldArray to a new value.

Then we return new object e.

An array with each object e in oldArray modified is returned.

To use Array.includes() to find object in array with JavaScript, we use the some method.

For instance, we write

const arr = [{ a: "b" }];
console.log(arr.some((item) => item.a === "b"));

to call arr.some with a callback that checks if item in arr has property a equal to 'b'.

If any object in arr has property a equal to 'b' then it returns true.

To split an array into two based on an index in JavaScript, we use the slice method.

For instance, we write

const ar = [1, 2, 3, 4, 5, 6];
const p1 = ar.slice(0, 4);
const p2 = ar.slice(4);

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

And we call ar.slice with 4 to return an array with the items in ar from index 4 to the end of the ar array.

To filter null from an array in JavaScript, we use the filter method.

For instance, we write

const filtered = [1, "", null, NaN, 2, undefined, 4, 5, 6].filter((x) => x);

to call [1, "", null, NaN, 2, undefined, 4, 5, 6].filter with a callback that returns the parameter to return an array that only includes the truthy values in [1, "", null, NaN, 2, undefined, 4, 5, 6].