9 Commonly Amusing Mistakes in Javascript Programming Every Developer Faces

There are several problems that Javascript developers often encounter, which sometimes seem amusing because they happen so frequently or are caused by unexpected little mistakes. Here are some examples

1. Forgetting to Declare Variables with var, let, or const

   x = 10;

   When you forget to declare a variable using var, let, or const, the variable automatically becomes a global variable, which can lead to unexpected issues.

2. The Result of typeof NaN

   console.log(typeof NaN); // "number"

   Even though NaN (Not a Number) means it's not a number, typeof NaN returns "number," which confuses many developers.

3. Using == Instead of ===

   console.log(0 == false); // true
   console.log(0 === false); // false

   Using == can lead to unexpected comparisons due to automatic type conversion. Using === avoids this issue by not converting data types before comparing.

4. Forgetting the break in a switch Statement

   switch(day) {
       case 1:
           console.log("Monday");
       case 2:
           console.log("Tuesday");
   }

   Forgetting to include break after a case will cause the program to run through all remaining cases, which may result in incorrect outcomes.

5. Functions Without a return Statement Return undefined

   function doSomething() {
       // no return statement
   }
   console.log(doSomething()); // undefined

   If you forget to include a return statement in a function, it will return undefined, which can confuse developers.

6. Forgetting That this Changes Context

   const obj = {
       name: "John",
       sayHello: function() {
           setTimeout(function() {
               console.log(this.name);
           }, 1000);
       }
   };
   obj.sayHello(); // undefined

   In this example, the this within the function passed to setTimeout no longer refers to obj, which may prevent you from accessing the properties of obj as expected.

7. Accessing an array Element That Doesn't Exist Returns undefined

   const arr = [1, 2, 3];
   console.log(arr[5]); // undefined

   When trying to access an array element that doesn’t exist, it returns undefined instead of an error, which might lead to confusion when debugging.

8. Forgetting to Use await with Asynchronous Functions

   async function fetchData() {
       const result = someAsyncFunction();
       console.log(result); // Promise {<pending>}
   }

   Forgetting to include await will return a Promise instead of the desired result from the asynchronous function.

9. Forgetting to Add ; at the End of Statements

   let a = 5
   let b = 10
   let c = a + b
   console.log(c) // 15

   While JavaScript automatically adds semicolons in most cases (automatic semicolon insertion or ASI), forgetting to manually add ; can sometimes lead to unexpected behavior, especially when chaining multiple statements on the same line or using certain patterns that ASI doesn't handle well.

These issues occur frequently, and while they can be amusing when looking back, they often stem from negligence or small misunderstandings on the developer's part.