Javascript array reduce promise pattern

let userIDs = [1,2,3];

userIDs.reduce( async (previousPromise, nextID) => {
  await previousPromise;
  return methodThatReturnsAPromise(nextID);
}, Promise.resolve());

Ref: https://css-tricks.com/why-using-reduce-to-sequentially-resolve-promises-works/

Looping over a NodeList in Javascript

Using Javascript array iteration functions to loop over a NodeList requires some special handling.

The easiest way is to use Array.from()

Array.from(document.querySelectorAll('div'))
    .map(div => div.style.border = '1px solid crimson')

Older info from this source:

let divs = document.querySelectorAll('div')

[].forEach.call(divs, function(div) {
  // do whatever
  div.style.color = "red"
});

Array.prototype.slice can also now do the conversion:

Array.prototype.slice.call(document.childNodes)

which can be shortened to

[].slice.call(document.childNodes)

Remove duplicates from array in Javascript

arr.filter((val, i, self) => self.indexOf(val) === i)

Javascript equivalent of python `range` function

let size = 10, startAt = 1;
[...Array(size).keys()].map(i => i + startAt);

produces

[ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]

or, simply for a range starting at zero

[...Array(9).keys()];

[ 0, 1, 2, 3, 4, 5, 6, 7, 8 ]

See this posting for a more complete discussion.

Flatten an array

let arrays = [
  ["$6"],
  ["$12"],
  ["$25"],
  ["$25"],
  ["$18"],
  ["$22"],
  ["$10"]
]
let merged = [].concat.apply([], arrays)
console.log(merged);

The posting at https://stackoverflow.com/a/10865042 gives the following explanation:

Using the .apply() method of .concat() takes the second param as an array, so the last line effectively does this:

let merged2 = [].concat(
    ["$6"], ["$12"], ["$25"], ["$25"],
    ["$18"], ["$22"], ["$10"]
)

Rotate an array

Forwards:

arr.push(arr.shift())

and in reverse:

arr.unshift(arr.pop())

Intersection of two arrays

let intersect = arr1.filter(e => arr2.includes(e))

Difference of two arrays

let diff = arr1.filter(e => !arr2.includes(e))