If you have been working with JavaScript, you have probably seen a lot of usage with the + operator and discovered that the output is not necessarily what you expected.

The language supports using the + operator between (almost) all of the different data types - the results might therefore be confusing. In JavaScript, there is no compiler to hold your hand on your “addition journey”.

The addition operator either performs string concatenation or numeric addition.

With the rule above you should be able to understand all additions in JavaScript, but some of us need to see examples and explanations to fully understand the different scenarios.

First of all, remember that JavaScript performs the addition from left to right - evaluating parentheses first. This should be familiar with what you have learned in middle-grade mathematics.

Step-by-step example
1 + 2 + (3 + 4)
1 + 2 + 7
3 + 7
10

As the rule states - with two numeric elements, the output is a numeric value:

1 + 1 = 2

String concatenation is well known between different programming languages and is a reasonable usage of the + operator:

"Santa" + " " + "Claus" = "Santa Claus"

So far, so good!

The most important “rules” with addition in JavaScript

1. Adding a non-numeric value to a numeric value will try to convert the non-numeric value to a numeric value, if possible. If not, both are converted to strings.
2. Adding a non-numeric value to some value will convert both values to strings before adding them.

So, what does that actually mean?

1 + "2" = "12";
1 + 1 + "2" = "22";
1 + "1" + "2" = "112";

Confused? At least I am…

In the first line, a numeric and non-numeric value is added. Both are converted to strings before they are added

"1" + "2" = "12"

In the second line, there are both numeric addition and string concatenation. Remember, addition goes from left to right.

1 + 1 = 2;
2 + "2" = "2" + "2" = "22";

The third line only contains string concatenation.

"1" + "1" + "2" => "112"

Unfortunately, it doesn’t end here. Some other data types in JavaScript behave in a rather non-typical way when used for addition.

Boolean

What happens when you add something with a boolean value?

1 + true = 2;
1 + false = 1;
true + true = 2;
false + false = 0;
"1" + true = "1true";
"1" + false = "1false";

Boolean values can be converted to numeric values in JavaScript, and the result of the conversion is either 1 or 0.

true => 1
false => 0

Combining a boolean and a string value converts true to “true” and false to “false”.

Objects, arrays and functions

When one of the values in an addition is an object, array or function it will try to convert it to a primitive value. 1) Execute the valueOf()-function and use that value if it is a primitive 2) Execute the toString()-function

const me = { name: "Charlie", age: 34 };
me + 1 = "[object Object]1";

The default valueOf()-implementation is an empty object {} - which is not a primitive and it will try to convert it using toString(). The default toString()-value for an object is "[object Object]" - not very useful for addition. Unfortunately, I have seen this output many times during my time as a developer - it usually happens when I forget to specify the property of the object that should be part of the addition.

Sometimes, it might be handy to override the valueOf() or toString()-function and make it return something useful.

const me = { name: "Charlie", age: 34 };
me.valueOf = function () {
  return this.age;
};
me + 1 = 35;

The return type is now a numeric value and it will add 34 with 1 - which is 35.

The same can be done for a function or a class - for both valueOf and toString.

function BekkEmployee(name, age) {
  this.name = name;
  this.age = age;
  this.toString = function () {
    return `${this.name} is ${this.age} years old`;
  };
}

new BekkEmployee("Charlie", 34) + 100 = "Charlie is 34 years old100";

For arrays, valueOf returns the array - which is not a primitive value. The output of its toString() is the content joined with a comma. An empty array is the same as an empty string.

[] + 1 = "1"[(1, 2, 3)] + [4, 5, 6] = "1,2,3" + "3,4,5" = "1,2,34,5,6";

Since you have been reading all along - here is a quiz for you:

PS: try to solve it before you check the browser console

const me = { name: "Charlie", age: 34 };
me.toString = function () {
  return this.age;
};

true + me + 65 + [2] + false + me + "merry christmas";

Date

All rules have some exceptions...

All native ECMAScript objects except Date objects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if the hint String were given.

Date.valueOf returns a numeric type, but a date will be transformed into a string type when using the + operator.

new Date(2020, 11, 1) +
12345 = "Tue Dec 01 2020 00:00:00 GMT+0100 (Central European Standard Time)12345";
new Date(2020, 11, 1) +
"12345" = "Tue Dec 01 2020 00:00:00 GMT+0100 (Central European Standard Time)12345";

new Date(
  2020,
  11,
  1
).toString() = "Tue Dec 01 2020 00:00:00 GMT+0100 (Central European Standard Time)";
new Date(2020, 11, 1).valueOf() = 1606777200000;

Weirdos

Now that you know the important rules of addition, there are some data types you also should be aware of:

undefined + 1 = NaN; // undefined is converted to numeric => NaN
undefined + "1" = "undefined1";
null + 1 = 1; // null is converted to numeric => 0
null + "1" = "null1";
NaN + 1 = NaN;
NaN + "1" = "NaN1";
Symbol(1) + 1; // TypeError: Cannot convert a Symbol value to a number
BigInt(1) + 1; // TypeError: Cannot mix BigInt and other types, use explicit conversions
BigInt(1) + "2" = "12";

As you have read, addition is not as easy as we know from mathematics. However, if you remember some of the conversion rules above you might save yourself from some headaches in the future!