Calculations with booleans

Sometimes I like to take a look at the quirkiness of the language I code in and compare it to other languages. This time, I thought I'd share my findings with you, in case you also find them entertaining and interesting. On this occasion I looked at a quirk featured in WTFJS, namely doing Math with booleans. Let's dive in.

true + true = ?

In Javascript, we use the + operator to concatenate strings as well as to add up numbers, but what happens when we through some booleans into the mix? Let's try!

'Hi' + true;    // output: "Hitrue"
true + 'Hi';    // output: "trueHi"

Okay, so far, so good, Javascript notices that we use a string with a + operator and treats the boolean like a String and concatenates the two. Now, let's take a look at the numbers.

5 + true;    // output: 6
true + 5;    // output: 6

Wait, what? Let's take a closer look at what's happening here.

the Number-function

The boolean value is passed to the Number-function, which converts it to a Number; true is converted to 1 and false to 0.

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Note

The Javascript Number type has limits to what it can store and also limits on how high the numbers stored in can be. All Numbers are stored as floating-point values.

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If it fails to convert the value into a number, it will return NaN. It can convert true, false, null, as well as a decimal and hexadecimal number (in strings).

Number(true);        // output: 1
Number(false);       // output: 0
Number(null);        // output: 0
Number(undefined);   // output: NaN
Number(0x7E5);       // output: 2021
Number('0x7E5');     // output: 2021
Number('12.5');      // output: 12.5

calculation with booleans only

When we only add boolean operands, Javascript will still try and convert them into Numbers, rather than interpreting both as being strings.

true + true;     // output: 2
true + false;    // output: 1
false + false;   // output: 0

Beyond the addition

The same effect, of course, happens with subtractions, multiplications and divisions, as well as when using unary operators (positive and negative - although NaN will not be signed).

+true       // output: 1
+false      // output: 0
+null       // output: 0
+undefined  // output: NaN
+0x75E      // output: 2021
+'0x75E'    // output: 2021
+'12.5'     // output: 12.5

Is it just JS

You might know all of this already, so let's compare it to a few other languages.

PHP and Python will also allow calculations with booleans. They both also convert true to 1 and false to 0.

Java and Rust on the other hand, will both refuse calculations with boolean values and throw an error during compilation.

error: bad operand types for binary operator '+'
Java

error [E0277]: cannot add bool to {integer}
Rust

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I hope you had fun reading about calculations with booleans. Thanks for reading!