Inversion Method

Introduction

The inversion method uses to reduce nesting in code. This involves taking a nested code block and inverting the conditional statement that controls it.

Tips

Always start inversion from the upper block and then go down.

Example

Write a function that takes a date as a string and returns the year if it's greater than 2015, and throw an exception on each failed case.

Original nested code:

function sumOdd(dateStr) {
  if (typeof dateStr === "string") {
    if (Date.parse(dateStr)) {
      const year = new Date(dateStr).getFullYear();
      if (year > 2016) {
        return year;
      } else {
        throw new Error("The provided date is not greater than 2016.");
      }
    } else {
      throw new Error("The provided argument is not a valid date.");
    }
  } else {
    throw new Error("The provided argument is not a string.");
  }
}

Inverted code:

function sumOdd(dateStr) {
  if (!typeof dateStr === "string") {
    throw new Error("The provided argument is not a string.");
  }

  if (!Date.parse(dateStr)) {
    throw new Error("The provided argument is not a valid date.");
  }

  const year = new Date(dateStr).getFullYear();
  if (year < 2016) {
    throw new Error("The provided date is not greater than 2016.");
  }

  return year;
}