In my most recent post, I mentioned the various objects that exist in JavaScript, and the operators most commonly responsible for coercing one object type into another.
In this post, I’ll be focusing on the Unary Operator (+) and the Negation Operator (–)
+"5" == +(5) == 5
In the above example we see the unary operator acting upon the “5” string resulting in said string being coerced into a number, naturally, the number 5.
Let’s take a look at the process as it happens:
- The unary operator upon first encountering the string, checks the nature of the string. In this case, it’s a string which is numeric by nature.
- Next, the unary operator coerces the string into its respective number. Note in this case, I’ve placed the 5 in parentheses to show that the 5 has still yet to be further acted upon. This will come into importance in later examples.
- Finally the unary operator acts upon the numeric 5 and coerces further with its secondary function, acting as a positive on the numeric 5 we have already coerced from the string. The final result is an unsigned (and thus naturally positive) 5.
Let’s go a step further, now this time with the Negation Operator, the – sign.
-"5" == -(5) == -5
In this example we see the negation operator acting upon the “5” string resulting in said string being coerced into a number, in this case, the number -5.
So let’s see what happens this time around:
- The negation operator upon first encountering the string, checks the nature of the string. As with the previous example, it’s a string which is numeric by nature.
- Next, the negation operator coerces the string into its respective number. Note as with the previous example, I’ve also placed the 5 in parentheses to show that the 5 has still yet to be further acted upon.
- Finally the negation operator acts upon the numeric 5 and coerces further with its secondary function, acting as a negator on the numeric 5 we have already coerced from the string. The final result is -5.
If we were to have used the string “-5” instead in these two examples we would get -5 and 5 respectively. Why? Let’s take a quick look.
+"-5" == +(-5) == -5
-“-5” == -(-5) == +5 == 5
So, once the -5 is “cast out” (the supposed technical term for the conversion of one object type into another) from the string, the unary or negation operator performs the respective mathematical operation on the -5 to convert it to -5 or 5 respectively.
So, where does that leave us? Sure, we can cast out numbers until we’re blue in the face. But what about other kinds of strings, what about arrays, objects, dates, booleans? Could we cast out a number from a function?
The answer is yes… technically. What if we had a string that had a combination of letters and numbers, or starting with a number, or numbers at the end, or no numbers at all? Would an empty string cast out?
I’ve taken the liberty of doing these for you. I’ve shown the result after the “//” in each example. Let’s start with strings:
+"7 monkeys" // NaN
NaN means “Not a Number”, even though typeof(+”7 monkeys”) yields “number”, so apparently “Not a Number” is still a number…? Generally +x should for all intensive purposes yield “number”, but because “7 monkeys” or any similar string cannot be completely coerced into a number, Not a Number results. Strangely though, parseInt(“7 monkeys”) and parseFloat(“7 monkeys”) both yield 7. I will talk about these functions in a later post.
-"7 Monkeys" // also yields NaN
+"3+3" // NaN
The above example threw me at first, after all, with +”3+3″ one would think the unary operator would evaluate the string and return 6… Not so. Unary and Negation operators DO NOT evaluate the contents of a string, in fact they don’t evaluate anything. Unary and Negation operators will only do the following:
- A unary or negation operator will first coerce the object used against the operator to a number based on the content of the object.
- If the unary operator is used, the system will then work out the resulting number based on standard mathematical rules based on placing a positive sign in front of it.
- If the negation operator is used, the system will then work out the resulting number based on standard mathematical rules based on placing a negative sign in front of it.
- Unary and Negation operators will work only on strings that are formatted as recognized numbers within JavaScript.
Note that last point… For a string to be correctly coerced to a number, the initial string has to be formatted like a JavaScript number.
Generally a JavaScript number is formatted in one of three ways:
- ±A (where A represents a series of integers (containing 1 or more digits)
- ±A.B (where A and B represent two series of integers (containing 1 or more digits) on either side of the decimal point
- ±A.Be±C (where A, B and C represent three series of integers (containing 1 or more digits), and e (or E) represents the 10C exponential function. Technically this will be the same as ±A.B × 10±C. So, for example, 7.56e3 = 7.56 × 103 = 7.56 × 1000 = 7560, and 4.2e-2 = 4.2 × 10-2 = 4.2 × 0.01 = 0.042.
One exception to the rule is that a number in JavaScript can be represented with a decimal point but nothing following it. So, +”7.” will coerce into 7, and +”7.e2″ will coerce to 700. but +”7.e2.3″ will render out as a Syntax Error due to the fact that the exponential needs to be an integer, and not in this case, 2.3.
+”foo” // NaN
In the conversion of a string to a number, if the number does not match the conventional JavaScript number formats I mentioned earlier, the only other alternative is for it to render out as Not a Number (NaN).
I will provide further insight into these operators and their interactions with other JavaScript object types in my next post.
JavaScript Quirks 