12.6. Combining Math Methods

The Math methods provide useful actions, but each one is fairly specific in what it does (e.g. taking a square root). At first glance, this might seem to limit how often we need to call on Math. However, the methods can be manipulated or combined to produce some clever results.

12.6.1. Random Selection From an Array

To select a random item from the array happiness = ['Hope', 'Joy', 'Peace', 'Love', 'Kindness', 'Puppies', 'Kittens', 'Tortoise'], we need to randomly generate an index value from 0 to 7. Since Math.random() returns a decimal number between 0 and 1, the method on its own will not work.

The Math.random appendix page describes how to generate random integers by combining the random and floor methods. We will use this functionality now.

Let's define a function that takes an array as a parameter. Since we might not know how many items are in the array, we cannot multiply Math.round() by a specific value. Fortunately, we have the length property…


function randomSelection(arr){
   let index = Math.floor(Math.random()*arr.length);
   return arr[index];

let happiness = ['Hope','Joy','Peace','Love','Kindness','Puppies','Kittens','Tortoise'];

for (i=0; i < 8; i++){

Console Output


The happiness array has a length of 8, so in line 2 Math.floor(Math.random()*arr.length) evaluates as Math.floor(Math.random()*8), which generates an integer from 0 to 7. Line 3 then returns a random selection from the array.

12.6.2. Rounding to Decimal Places

The ceil, floor, and round methods all take a decimal value and return an integer, but what if we wanted to round 5.56789123 to two decimal places? Let's explore how to make this happen by starting with a simpler example.

Math.round(1.23) returns 1, but what if we want to round to one decimal place (1.2)? We cannot alter what round does---it always returns an integer. However, we CAN change the number used as the argument.

Let's multiply 1.23 by 10 (1.23*10 = 12.3) and then apply the method. Math.round(12.3) returns 12. Why do this? Well, if we divide 12 by 10 (12/10 = 1.2) we get the result of rounding 1.23 to one decimal place.

Combining these steps gives us Math.round(1.23*10)/10, which returns the value 1.2.

Let's return to 5.56789123 and step through the logic for rounding to two decimal places:

Step Description
Math.round(5.56789123*100)/100 Evaluate the numbers in () first: 5.56789123*100 = 556.789123
Math.round(556.789123)/100 Apply the round method to 556.789123
557/100 Perform the division 557/100 = 5.57

The clever trick for rounding to decimal places is to multiply the original number by some factor of 10, round the result, then divide the integer by the same factor of 10. The number of digits we want after the decimal are shifted in front of the '.' before rounding, then moved back into place by the division.

Rounding to Decimal Places
Decimal Places In Answer Multiply & Divide By Syntax
1 10 Math.round(number*10)/10
2 100 Math.round(number*100)/100
3 1000 Math.round(number*1000)/1000
etc. etc. etc.

12.6.3. Check Your Understanding


Which of the following correctly rounds 12.3456789 to 4 decimal places?

  1. Math.round(12.3456789)*100/100
  2. Math.round(12.3456789*100)/100
  3. Math.round(12.3456789*10000)/10000
  4. Math.round(12.3456789)*10000/10000