• Recognize the limitations of math in JavaScript
• Employ operators to perform arithmetic and assign values to variables
• Explain what NaN is
• Use built-in objects like Math and Number to accomplish complex tasks

We're going to discuss a number of the common operators and objects we'll use to perform arithmetic operations in JavaScript.

In the browser's JavaScript console, we can test out all of the examples in this lesson. Remember that we can't redeclare variables previously declared with const or let, so the page may have to be refreshed (which wipes away all declared variables) or different variable names can be chosen than those in the examples.

Math is awesome! JavaScript has only a single, all-encompassing number type. While other languages might have distinct types for integers, decimals, and the like, JavaScript represents everything as a double-precision floating-point number, or float. This imposes some interesting technical limitations on the precision of the arithmetic we can perform with JavaScript. For example:

0.1 * 0.1;
//=> 0.010000000000000002

0.1 + 0.1 + 0.1;
//=> 0.30000000000000004

1 - 0.9;
//=> 0.09999999999999998

You shouldn't waste too much time diving into why this happens, but it basically boils down to the language, once again, trying to be too user-friendly. Under the hood, JavaScript stores numbers in binary (base-2) format, as a series of 1s and 0s, but it displays numbers in the more human-readable decimal (base-10) format. The problem that the above code snippet highlights is that it's really easy to represent something like 1/10 in decimal (0.1) but impossible to do it in binary (0.0001100110011...). It's the exact same problem that the decimal system has in trying to represent 1/3 as 0.33333333333....

The only time you'd really have to worry about this is if you needed to calculate something to a high degree of precision, like interest payments for a bank. However, for most of our day-to-day arithmetic needs, JavaScript is more than capable.

JavaScript employs a pretty standard set of arithmetic operators.

We've used the addition operator to concatenate strings, but it's also used to add numbers together:

40 + 2;
//=> 42

The subtraction operator returns the difference between two numbers:

9001 - 9000;
//=> 1

The multiplication operator returns the product of two numbers:

6 * 7;
//=> 42

The division operator returns the result of the left number divided by the right number:

9001 / 42;
//=> 214.3095238095238

The remainder operator returns the remainder when the left number is divided by the right number:

9001 % 42;
//=> 13

The exponentiation operator returns the left number raised to the power of the right number:

2 ** 8;
//=> 256

JavaScript evaluates compound arithmetic operations by following the standard order of operations used in basic math. Anything in parentheses has highest priority; exponentiation is second; then multiplication, division, and remainder; and, finally, addition and subtraction, in order from left to right. This is how the JavaScript compiler works. For example:

( ) → ** → * / % → + -

2 - (2 % 2) + (2 / 2 ** 2) * 2;
//=> 3

2 - ((2 % (2 + 2)) / 2 ** 2) * 2;
//=> 1

JavaScript also has a pair of operators that we can use to increment and decrement a numerical value stored in a variable.

The ++ operator increments the stored number by 1. If the ++ operator comes after the variable (e.g., counter++), the variable's value is returned first and then incremented:

let counter = 0;
//=> undefined

counter++;
//=> 0

counter;
//=> 1

If the ++ operator comes before the variable (e.g., ++counter), the variable's value is incremented first and then returned:

let counter = 0;
//=> undefined

++counter;
//=> 1

counter;
//=> 1

In both cases, counter contains the value 1 after incrementing. The difference is in whether we want the operation to return the original or incremented value.

The -- operator decrements the stored number by 1 and has the same pair of prefix and postfix options as the ++ operator:

let counter = 0;
//=> undefined

// Return the current value of 'counter' and then decrement by 1
counter--;
//=> 0

// Check the new value of 'counter'
counter;
//=> -1

// Decrement 'counter' and then return the new value
--counter;
//=> -2

// Check the new value of 'counter'
counter;
//=> -2

JavaScript has a number of operators for assigning a value to a variable. We've already used the most basic, =, but we can also couple it with an arithmetic operator to perform an operation and assign the value of the operation:

let counter = 0;
//=> undefined

counter += 10;
//=> 10

counter -= 2;
//=> 8

counter *= 4;
//=> 32

counter /= 2;
//=> 16

counter %= 6;
//=> 4

counter **= 3;
//=> 64

JavaScript tries to return a value for every operation, but sometimes we'll ask it to calculate the incalculable. For example, imagine that one of the lines of code in our program increments the value of a counter by 1. However, something broke in a different part of the program, and counter is currently undefined. When the JavaScript engine reaches the incrementing line, what happens?

counter++;
//=> NaN

The JavaScript engine can't add 1 to undefined, so it tells us the result is Not a Number — NaN.

Top Tip: Much like undefined, you should never assign NaN as the value of a variable and instead let it be a signal that some weird maths are happening in your code.

To satisfy most of our math needs, JavaScript provides several built-in objects that we can reference anywhere in JavaScript code, including Number and Math. With these objects, we can perform complex tasks like generating random numbers.

The Number object comes with a collection of handy methods that we can use for checking and converting numbers in JavaScript.

Checks whether the provided argument is an integer:

Number.isInteger(42);
//=> true

Number.isInteger(0.42);
//=> false

Checks whether the provided argument is finite:

Number.isFinite(9001);
//=> true

Number.isFinite(Infinity);
//=> false

Checks whether the provided argument is NaN:

Number.isNaN(10);
//=> false

Number.isNaN(undefined);
//=> false

Number.isNaN(NaN);
//=> true

Accepts a string as its first argument and parses it as an integer. The second argument is the base that should be used in parsing (e.g., 2 for binary or 10 for decimal). For example, 100 is 100 in decimal but 4 in binary:

Number.parseInt('100', 10);
//=> 100

Number.parseInt('100', 2);
//=> 4

The second argument is optional, but you should always provide it to avoid confusion.

Number.parseFloat() only accepts a single argument, the string that should be parsed into a floating-point number:

Number.parseFloat('3.14159');
//=> 3.14159

The Math object contains some properties representing common mathematical values, such as Math.PI and Math.E, as well as a number of methods for performing useful calculations.

JavaScript provides three methods for rounding numbers. Math.ceil() rounds the number up, Math.floor() rounds the number down, and Math.round() rounds the number either up or down, whichever is nearest:

Math.ceil(0.5);
//=> 1

Math.floor(0.5);
//=> 0

Math.round(0.5);
//=> 1

Math.round(0.49);
//=> 0

These two methods accept a number of arguments and return the lowest and highest constituent, respectively:

Math.max(1, 2, 3, 4, 5);
//=> 5

Math.min(1, 2, 3, 4, 5);
//=> 1

This method generates a random number between 0 (inclusive) and 1 (exclusive):

Math.random();
//=> 0.4495507082209371

In combination with some simple arithmetic and one of the rounding methods, we can generate random integers within a specific range. For example, to generate a random integer between 1 and 10:

Math.floor(Math.random() * 10) + 1;
//=> 8

Math.floor(Math.random() * 10) + 1;
//=> 1

Math.floor(Math.random() * 10) + 1;
//=> 6

Math.random() returns a number between 0 and 0.999..., which we multiply by 10 to give us a number between 0 and 9.999.... We then pass that number to Math.floor(), which returns an integer between 0 and 9. That's one less than the desired range (1 to 10), so we add one at the end of the equation. Try it out in the JS console!

There are four challenges we need you to solve. Code your solution in index.js. We'll provide some brief instructions here, but you should really rely on the test failure messages to guide your code.

1. Create a variable called multiply set to an equation that will multiply the variables num1 and num2; the result of the multiplication should be 62.
2. Create a variable called random that will generate a random integer greater than 0.
3. Create a variable called mod set to an equation that will calculate the remainder of dividing variable num3 by num4; the remainder should be 4.
4. Create a variable called max that finds the highest number in a set; the value returned should be 20.

• MDN
  • Basic math in JavaScript
  • Arithmetic operators
  • Operator precedence
  • Assignment operators
  • NaN
  • Number
  • Math
• 2ality — How numbers are encoded in JavaScript
• Order of Operations