Replace multiple if statements with a lookup table

Assume that we are going to determine the weekday of a date. Given a `date` object, we can get the day of the week:
const day = date.getDay();
The day is zero-based index, and starts with Sunday. It's easy to write a few `if` statements such as:
let weekDay = '';
if (day === 0) {
weekDay = 'Sunday';
} else if (day === 1) {
weekDay = 'Monday';
} else if (day === 2) {
// More if statements are omitted
// ...
} else if (day === 6) {
weekDay = 'Saturday';
Since all the `if` statements above have the same left-hand side expression, we can make a better version using the `switch` statement:
// Better version
let weekDay = '';
switch (day) {
case 0:
weekDay = 'Sunday';
case 1:
weekDay = 'Monday';
// More cases are omitted
case 6:
weekDay = 'Saturday';
Using the `switch` statement makes the code more readable, but it seem that the number of line-of-codes in both approaches are the same.
In pure English, we map 0 with Sunday, 1 with Monday, and so forth. Why don't we create a lookup table that quickly returns the matching item based on given key?
// The best version
const lookup = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];
const weekDay = lookup[day];
The code is short, and only uses `const` for variable declarations meaning that it doesn't have side effects and easy to test.
In reality, a lookup table can be an array, object literal or a `Map`. The data structure varies depending on your business logic.
As long as you get the idea, the following example uses the same practice. It is a function that converts a given number to the Roman numeral representation.
const lookup = [
['M', 1000],
['CM', 900],
['D', 500],
['CD', 400],
['C', 100],
['XC', 90],
['L', 50],
['XL', 40],
['X', 10],
['IX', 9],
['V', 5],
['IV', 4],
['I', 1],
const convertToRoman = (number) =>
lookup.reduce((curr, [key, value]) => {
curr += key.repeat(Math.floor(number / value));
number = number % value;
return curr;
}, '');
convertToRoman(20); // 'XX'
convertToRoman(21); // 'XXI'
convertToRoman(2021); // 'MMXXI'
If a function uses a fixed lookup table, it's recommended to move the lookup to outside of the function

See also