JavaScript is one of the most widely used technologies in the engineering world as it is very efficient, and great for building fast, high-performing applications. Despite the awesomeness of JavaScript, the language has some tricky parts, one of which you might have come across.
Understanding these strange parts will help you understand JavaScript better, offer up some insights, increase your debugging speed and much more.
In this article, I’ll discuss some of the strange parts of JavaScript.
Prerequisites
Before going through this article, having a little knowledge of the following will help:
- JavaScript
- ES6
Alright, let's dive straight into the parts:
1. Math.min() is greater than Math.max()
If no argument is passed to both the min() and max() functions, min() returns positive infinity while max() returns negative infinity. Positive infinity is greater than negative infinity meaning min() is greater than max() which is unusual and isn’t the case if number values are passed to the two static functions.
Math.min(1, 2, 3); // returns 1
Math.max(1, 2, 3); // returns 3
Math.min(); //returns Infinity
Math.max(); //returns –Infinity
console.log(Math.min() > Math.max()) // outputs true
2. Addition of two arrays returns a string
If we add two arrays together, a string value is gotten. This is so because JavaScript's interpreter first converts both arrays to strings and then concatenates them.
[1, 2, 3, 4, 5] + [5, 6, 7, 8, 9] // yields ‘1, 2, 3, 45, 6, 7, 8, 9’
3. HTML Comments can be written in JavaScript
HTML Comments (<!-- -->) are ignored by JavaScript’s interpreter. The opening and closing HTML comments if used separately, are also ignored.
console.log(‘html comments’)
<!-- I am a comment and will be ignored -->
// the above outputs ‘html comments’ with no error
In the past, HTML comments were used inside <script> tags so that browsers with no JavaScript support ignored it. How?
<html>
<body>
<script>
<!--
console.log(‘tests’);
//-->
</script>
</body>
</html>
In the above snippet, browsers with no JavaScript support will read the console statement as a comment, while in browsers with JavaScript support, the closing HTML comment(-->) will be commented out making the console statement interpretable.
As most browsers now support JavaScript, using the HTML comments in JavaScript code is now irrelevant not advised. This is one of the reasons a lot of JavaScript syntax highlighters do not display HTML comments as valid comments even though they actually are.
4. Arguments is not defined in arrow functions
The arguments object, holds values of arguments passed to a function.
function func() {
console.log(arguments)
}
func(1, 2, 3); // {0: 1, 1: 2, 2:3}
A ReferenceError will occur if the arguments object is used in an arrow function.
let func = () => arguments;
func(1, 2, 3); // Reference Error: arguments is not defined
Arrow functions are a shorter version of regular functions, and as such they do not provide bindings to the arguments object. Sad? don't be, there’s an alternative for achieving similar result through the rest parameter. See the below snippet:
let func = (...args) => args;
func(1, 2, 3); // [1, 2, 3]
5. Null to Zero Comparison
The result gotten from comparing null to zero is contradictory. Consider the below expressions:
null == 0; // false
null > 0; // false
null < 0; // false
null >= 0; // true
null <= 0; // true
In the above, null is neither equal to nor > nor < 0 but it is >= 0 and also <= 0 which is an odd behavior. Wait!... does this make sense? Well, the behavior is so, because of how the expressions are evaluated.
The equality operator follows the Absolute Equality Comparison Algorithm. It compares the values on both sides, if the values can’t be compared precisely, it converts both to numbers and compare. This conversion does not happen on a side that is null or undefined so it returns false hence, the result in the first expression. In the second and third expression, the < and > operators follow the Absolute Relational Comparison Algorithm and unlike the equality operator, convert the null value to number. So they yield zero.
null > 0
+null > +0
0 > 0 // false (same for the < operator)
In the last two expressions, the <= and >= operators follow these steps and it is evaluated as so:
if null < 0 is false, then null >= 0 is true if null > 0 is false, then null <= 0 is true.
i.e.
null >= 0;
!(null < 0);
!(+null < +0);
!(0 < 0);
!false;
true;
To learn more about null to zero comparison, check this out.
6. Arrow functions return undefined instead of an empty curly-brace object
If you try to return a {} directly in an arrow function you'll get undefined.
let func = () => {};
func(); // undefined.
This is mainly because, {} is part of the syntax of arrow functions so JavaScript’s interpreter reads it as one, making the function behave like any other function that returns undefined by default.
By wrapping the return value in a bracket, it is possible to return the {} object.
let func = () => ({});
func(); // {}
7. The precedence of finally block over try block
The finally block takes precedence over the try block and so also the value of their return statements.
let func = function() {
try {
return 'evil';
} finally {
return 'good';
}
}
func(); // good
The above func() function returns ‘good’ and not ‘evil’.
8. Relational comparison of three numbers
Comparing three numbers together might produce what seems like a misleading result. Take a look at the code snippet below:
console.log(1 < 2 < 3); // true
console.log(3 > 2 > 1); // false
The result??.. Yes, they are different and both correct. Why??? The JavaScript engine first interprets the first two values which produce a boolean and afterwards, converts the boolean value to a number and compares it with the third value.
1 < 2 < 3; // selects first two values
(1 < 2) < 3; //1 < 2 -> true
true < 3; // true is converted to 1
1 < 3; // true
//while
3 > 2 > 1;
(3 > 2) > 1;
true > 1;
1 > 1; // false
9. Addition of object and array
Let’s consider the below snippet:
[] + {} // yields “[object Object]”
{} + [] // yields 0
Now, this is another contradicting situation and it’s also as a result of how the JavaScript engine interprets the expressions. The interpreter first converts the two values to strings and concatenates them like in the below snippet.
[] + {}
[].toString() + {}.toString(); //converting the empty array yields an empty string
‘’ + ‘[object Object]’ // yields ‘[object Object’], which is the result of the first expression
But why isn’t the result of the second expression also “[object Object]”? This is because the engine reads the first value ({}) as a block of code and not an empty object and does no conversion on it so this leaves us with +[] which is a unary operator. Instead of converting +[] to a string, the engine converts it to a number and the conversion returns 0.
10. Unexpected return due to Automatic Semicolon Insertion (ASI)
An unexpected value will be returned, if the value is not on the same line with the return keyword.
let func = (function() {
return
{
a: 1
}
})(); // yields undefined
Automatic Semicolon Insertion is a concept where the JavaScript engine automatically inserts semicolon after some lines e.g. the line with the return keyword. This ASI made the function to terminate and return undefined. To remedy this, the returned expression must be in the same line with the return keyword.
let func = (function() {
return {
a: 1
}
})(); // yields { a: 1 }
Wrap Up
I guess you've been wowed by most, if not all of these weird parts of the JavaScript language. JavaScript is vast, and so, there are more tricky parts which the article doesn't contain. I hope the clues from this article will help you find more and be a better developer.
Thanks for reading :)