Skip to content

Operators That Add and Subtract

These seem simple at first but there are a few surprises.

The Additive Operators

In math, additive refers to both addition and subtraction. You can think of subtraction as just adding with negative numbers.

The plus sign + is used for math addition and string concatenation.
The minus sign - is used for math subtraction.

Using the Additive Operators

Adding and Subtracting Directly into Variables.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
// Additive operators into variables

let totalCount;

totalCount = 123 + 543;

document.write("totalCount: " + totalCount);
document.write("<br />");

totalCount = totalCount - 100;

document.write("New totalCount: " + totalCount);

Adding and Subtracting with Variables

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
// Additive operators with variables
let totalPrice;
let lowPrice;
let highPrice;
let mediumPrice;

lowPrice = 0.99;
highPrice = 1000.99;
mediumPrice = 199.95;

totalPrice = lowPrice + highPrice;

document.write("totalPrice: " + totalPrice);
document.write("<br />");

totalPrice = highPrice - mediumPrice;

document.write("New totalPrice: " + totalPrice);

Concatenation of strings.

The plus sign + is also used for concatenation

Concatenation, or combining string to form a new longer string. Notice that the <br /> tags are being concatenated too. 

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
// Concatenation with +
let firstName;
let lastName;
let fullName;

firstName = "Fred";
lastName = "Flintstone";

fullName = firstName + " " + lastName;

document.write("firstName: " + firstName + "<br />");
document.write("lastName: " + lastName + "<br />");
document.write("The fullName: " + fullName);

Concatenation of Strings and Numbers.

When the plus sign has one string to its left OR right and a number on the other side, the number will be converted to a string and concatenated to the string. 

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
// Concatenation with numbers

let petName;
let petWeight;

petName = "Fido";
petWeight = 10;

document.write("My pet, " + petName + ", weighs "
        + petWeight + " lbs.");

Concatenation when you don't expect it!.

When you have a string that contains just numbers and you try to add it to a true number JavaScript will concatenate it and not do addition. 

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
//Whoops!

let accountTotal;
let invoiceOne;
let invoiceTwo;

invoiceOne = 1000;
invoiceTwo = "599";

accountTotal = invoiceOne + invoiceTwo;

document.write("Account Total: " + accountTotal + " <-- ???");

Converting from a string to a number.

When you have a string that contains a valid number, you can convert it to a number. We use the Number() function for this.

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
// Converting a string to a number
let numberString;
let number;
let total;

numberString = "56.999";
number = 67.884;

total = number + numberString;

document.write("total: " + total + " <-- String concatenation");
document.write("<br />");

numberString = Number(numberString);

total = number + numberString;

document.write("total: " + total + " <-- That's better!");

New Coding Standard

Calculations

Refer to the Coding standards regarding specifics

document.write()

Note standards regarding caclulations and the use of document.write

Labs

  1. Lab04: Additive Operators
    • unit02/labs/lab-04-additiveOperators.html

Exercises

  1. Exercise for lab04
    • unit02/exercises/exercise-04.html