crazy-array-functions.md

• map => Takes an array, and then applies a function to every element of that array.
• filter => Takes an array, and passes every element through a function. It only keeps the element if the function returns true.
• sort => By default, it sorts the elements of an array in alphabetical order. But, if you give it a function, it will use it to sort the array in the following way:
  • The function takes two arguments to compare, a and b.
  • The function returns a negative number if a is lower than b.
  • The function returns zero if a has the same value as b.
  • The function returns a positive number is a is higher than b.
• reduce => Starts with an accumulator. It starts with the first element of the array. Then it goes through the rest of the elements, and for each one, it applies a function to the accumulator and the element and puts the result back into the accumulator. The function returns what the accumulator is at the end.

Examples

• Let's say you have [1, 2, 4, 6].map(x => x*2). It applies the doubling function to every element of the array, so it returns [2, 4, 8, 12].
• Let's say you have ["a", "hello", "b", "goodbye"] and you want to filter out all single-letter strings. You can call filter with the following function: x => x.length > 1. That will only return true if the string has more than one letter.
• You want to sum all the elements of [6, 5, 3, 1, 8, 7, 2, 4]. You can call reduce with this function: (acc, val) => acc + val:
  • It starts with an accumulator value of 6, the first value of the array.
  • It goes to the next element, 5, and applies the function to 6 and it. It adds 6 + 5 = 11. That's the new accumulator value.
  • It goes to the next element, 3, and applies the function to 11 and it. It adds 11 + 3 = 14. That's the new accumulator value.
  • It goes to the next element, 1, and applies the function to 14 and it. It adds 14 + 1 = 15. That's the new accumulator value.
  • And so on, until we get to the accumulator value of 36. That's the answer.
  • This method works because this is just a convoluted way of just adding on every element of the array one by one.
• You have [9, 4, 5, 3, 1].sort((a, b) => a - b). The function returns a negative number if a < b, and a positive number if a > b. So it will sort it from lowest to highest: [1, 3, 4, 5, 9].
• You have [9, 4, 5, 3, 1].sort((a, b) => b - a). The function returns a negative number if a > b, and a positive number if a < b. So it will sort it from highest to lowest: [9, 5, 4, 3, 1].