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Six Common Mistakes For JavaScript Programmers To Avoid

Sunday 16 February 2020, by Frank Hamilton

Six Common Mistakes For JavaScript Programmers To Avoid

Since JavaScript made its first appearance in 1995, it has gradually evolved into a scripting language that is fast, reliable, secure, and used by almost every web application. Today, there are so many people out there trying to learn Java coding. Sadly, most of them decide to give up halfway due to common errors that make programming seem "too hard" or "too confusing."

Although JavaScript is similar to other scripting languages, there are some common coding mistakes, both older and new programmers make. These common mistakes lead to inefficient, buggy, and disorganized codes. In this article, I'll cover six of these common mistakes and how best programmers can avoid them.

1. The "+" Symbol Is For Addition and Concatenation

In most code languages, the "+" symbol for both addition and concatenation differs, the opposite is the case when it comes to JavaScript. It is essential to take extra caution when writing statements with the "+" sign.


When using JavaScript, you combine strings and numbers because of user input. If you write out the code: var x = 10 + "7"; it goes up to "107," because 7 is a string, and the plus sign translates to a concatenation symbol whenever you're working with strings on JavaScript. This error is bound to occur when submission values are taken as imputed, and it becomes easy to forget that the user input can be evaluated as a string.

To avoid this common mistake, you need to convert the string value to an integer. I.e. var x = 10 + parseInt("7", 10); here the string value is parsed to the integer value 7, and addition is used in place of concatenation.

2. Incorrect Use Of "If" Statement Comparison Operator

The incorrect use of the "if" statement comparison operator is a prevalent mistake that professional JavaScript programmers are prone to making. Even when it occurs as a slight typographical error in your code, it can create a major logic bug when you're done coding. So you must be very mindful of avoiding this mistake.

The operators in Java Scripting are "==" and "=". The "==" operator is for making a comparison while the "=" operator is used to assign a value to a variable. In other programming languages using such operators will give you an error response, but with JavaScript, it gets evaluated, and you receive a statement result reading true or false.


If you impute the statement: var x = 0; if (x == 5), JavaScript will read this as true, thus it evaluates if x equals 5 correctly.

However, in an instance where you have an incorrect imputation, e.g., var x = 0; if (x = 5), here there is a typographical error in the 'if' statement (the assigned operator was used instead of the comparison operator). While in other programming languages, it will read as an error, JavaScript will not. Though such errors can be detected with testing, it is best to always use the comparison operator with your 'if' statements.

3. There Are No Block-Level Scope on JavaScript

In many languages, you can define a variable just for your loop structure. The loop will be destroyed if you loop through a variable using JavaScript.


Where (var j = 0; j < 11; j++) { j = j + 1; } console.log ( j ); generally when you have a code like this even the most experienced developers would assume that “j” would be null. However, If it's a different programming language, the j variable is often used in the "for" loop but gets destroyed once the loop is completed.

JavaScript, when showing the result for the above code, shows the output for the j variable as 10. This result could lead to severe bugs in your code, and as a fresh JavaScript developer, it is quite easy to miss this language quirk that isn't found in other programming languages.

4. Named Object Indexes Shouldn't be Used as Arrays

JavaScript arrays make use of numeric integer indexes. In like manner, objects could be used just like arrays, although the indexes will have to be named. But in an instance where a named index is used in an array, the result for the written code will be wrong.

For example:

var colour = [];

colour[0] = “grey”;

colour[1] = “black”;

colour[2] = “white”;

var x = colour.length;

var y = colour [0];

In the code above, the array is in “colour”. The colours have been assigned to the first three; the length has been evaluated and assigned the first colour to the variable y. The variable x is then evaluated to 3 and y contains the value “grey.” This is a normal pattern of an array.

For the second example, we have created an object using a phone

var phone = [];

phone ["colour"] = "silver";

phone ["make"] = "samsung";

phone [“model”] = “galaxy”;

var x = phone.length;

var y = phone [0];

This second code defines an object. From the named indexes, you are aware that it's an object rather than an array. Three named indexes define the phone. The error here is with variables x and y. Unlike the code that has the array, the values in the second code are undefined and produce invalid results. The length property evaluates to 0, and y is assigned "undefined."

When working with these two data types, it is essential to know which of them you're working with so you know the exact calculations and properties to use.

5. Reference To "This" Correctly

The self-referencing scopes within callbacks and closures have been on the increase. And they are a common source of "this/that confusion" alongside the influence of context. This mistake makes you prone to getting the error message "Uncaught TypeError: undefined is not a function."

The standard solution to this mistake would be to save your reference "this" in a variable that is easily inherited by the closure; this solution works well with older browsers. While for the newer browsers, "bind()" method to achieve a proper reference.

6. Don't Mix-up Undefined and Null

For other programming languages, it is possible to set null to variables; on the flip side, JavaScript only uses undefined and null. JavaScript also defaults objects to null and variables to undefined. There are two instances when these values are assigned:

  • An incorrect calculation was made.
  • A null reference was assigned to an object.

When comparing objects, you need to do it carefully because an object needs to be defined for it to be null.

For example:

if (object !== null && type of object !== “undefined”)

Where the object isn't identified, you get an error result. So you need to check whether the object is undefined or not, to enable you to identify if it's null:

I.e. if (type of object !== “undefined” && object!== null)

Being attentive is the key to getting the right results.


Avoiding these mistakes goes a long way in preventing your codes will help you prevent your system from getting disorganized. Also, these tips are not only applicable to JavaScript or web development but can be used when developing other software.

About The Author

Frank Hamilton has been working as an editor at cheap writing service reviews and author best paper writing services reviews. He is a professional writing expert in such topics as blogging, digital marketing and IT. He also loves traveling and speaks Spanish, French, French and English.

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