Convert `for` array iterations to array functions

From Airbnb style guide:

Use map() / every() / filter() / find() / findIndex() / reduce() / some() / … to iterate over arrays

const numbers = [1, 2, 3, 4, 5];

// bad
let sum = 0;
for (let num of numbers) {
  sum += num;
}
sum === 15;

// good
let sum = 0;
numbers.forEach((num) => {
  sum += num;
});
sum === 15;

// best (use the functional force)
const sum = numbers.reduce((total, num) => total + num, 0);
sum === 15;

// bad
const increasedByOne = [];
for (let i = 0; i < numbers.length; i++) {
  increasedByOne.push(numbers[i] + 1);
}

// good
const increasedByOne = [];
numbers.forEach((num) => {
  increasedByOne.push(num + 1);
});

// best (keeping it functional)
const increasedByOne = numbers.map(num => num + 1);

Ref: https://github.com/Khan/KaTeX/pull/1469#discussion_r203921764

[kevinbarabash]

Things like filter don’t work quite as expect if you’re trying to filter out elements of a certain type in the hope that the resultant array will be typed correctly. If you know how to get around this and other similar issues I would be forever grateful.

[kevinbarabash]

@VinVaz got for it. 😃