Environments & versions
We will also consider that the developer will follow good practices as a Style Guide - like airbnb, google, etc - or any other standard.
Now, the context is defined, we can start to talk about the subject.
1 2 3  == 0; // true "0" == 0; // true  == "0"; // false
There is many cases of comparisons where some are pretty “logical”, but transitivity isn’t satisfied. Fortunately, it’s easy to avoid that problem:
1 2 3  === 0; // false "0" === 0; // false  === "0"; // false
The difference between == and === is that the first one check equality between values (it can cast type to compare), and the second one check the type and the value.
typeof and instanceof
Those two keywords can have unexpected behavior, for example with string type:
1 2 typeof "chaine de caractere"; // "string" "chaine de caractere" instanceof String; // false
This is not really a bug, but a wrongly defined behavior, because you can write:
1 (new String("chaine de caractere")) instanceof String; // true
We can note that the “primitive variable” string isn’t considered as its object equivalent.
Another example of typeof issue, often used to mock the language is:
1 typeof NaN; // "number"
Yes, you read right, NaN - Not a Number - is considered as a number when using typeof operator. And that same category, null is considered of type object. But as for String, null is non instanceof Object, same for NaN and Number.
While doing my research to write that article, I noticed there were issues that were listed and that are not - by mistake or bad faith - real issues, I’ll talk about some of them.
Order of numbers
1 Math.min() < Math.max() // false
When I saw that example for the first time, I doubted of the language and it’s conception. Then, I made som search about those two functions. Math.min() and Math.max() are not giving minimum and maximum number - as we can think - but minimum and maximum of their arguments. The algorithm for Math.max() have as a default value Infinity and compare with each parameter.
1 2 Math.min() // Infinity Math.max() // -Infinity
The real issue of those functions, is, that they shouldn’t accept when the developer giving no argument.
Immutability and variable hoisting
A very often reported problem in the past was about the variable hoisting and its immutability. This was fixed in ES6 when the standard added the keywords let and const. We can also talk about Object.freeze() that is going to apply even more immutability restrictions than const.
There were few issues that can be considered as “offside” because not about the language itself. One of this problem is the floating numbers representation.
1 0.1+0.2==0.3; // false, because equal to 0.30000000000000004 when using standard float representation
Main solution to those issues
After reading those list of negative and postives points, we can see that there are plenty of easy to make (and to solve) problems. Fortunalty, there’s plenty of code checker that we call “Linter” that can help you on that. There’s for example: eslint, jshint or jslint that you can add (or are already added) to you text editor or your IDE.
I could also talk (and I will probably do in a future article) about code quality tool - working with any language like Sonarqube or Codacy.
It’s about being able to push the language to its limits using some (good or bad) behavior of the engine to be able to run the script.
If you wanna read more about how it works, you can go on their official website: http://www.jsfuck.com