Top iggest Bugs in JavaScript (And How to Avoid Them)

JavaScript is a very strong and adaptable language, but it also has the potential for problems that can be difficult to detect. In this blog article, we'll look at five of the most common flaws that developers find while working with JavaScript, as well as the reasons and solutions to these issues. Whether you're a seasoned developer or just starting out, knowing these common hazards will save you hours of troubleshooting.

This blog article was authored by Rupesh Sharma, also known as @hackyrupesh.

1. Unintended Global Variables

The Problem

JavaScript allows variables to be defined without explicitly declaring them, which can lead to unintended global variables. This is particularly problematic in large codebases or when working with multiple developers, as it can lead to conflicts and difficult-to-debug errors.

Example

function setUserName() {
    userName = "Alice"; // userName is now a global variable
}

setUserName();
console.log(userName); // Outputs: "Alice"

In the example above, userName is declared without var, let, or const, so it automatically becomes a global variable. This can lead to unexpected behavior, especially if userName is later used elsewhere in the code.

The Solution

Always declare variables using let, const, or var. This makes it clear whether a variable is local or global and prevents accidental global variables.

function setUserName() {
    let userName = "Alice"; // userName is now a local variable
}

setUserName();
console.log(userName); // ReferenceError: userName is not defined

References

• Mozilla Developer Network (MDN): JavaScript Variables

---

2. Misusing this Keyword

The Problem

The value of this in JavaScript can change depending on the context in which a function is called. This can lead to unexpected behavior, especially when using callbacks or event handlers.

Example

const user = {
    name: "Alice",
    greet: function() {
        console.log(`Hello, my name is ${this.name}`);
    }
};

setTimeout(user.greet, 1000); // Outputs: "Hello, my name is undefined"

In this example, the this keyword inside greet refers to the global object (or undefined in strict mode) when passed as a callback to setTimeout, rather than the user object.

The Solution

Use arrow functions or bind() to ensure that this remains bound to the correct object.

setTimeout(user.greet.bind(user), 1000); // Outputs: "Hello, my name is Alice"

Alternatively, using arrow functions can also solve the issue since they do not have their own this context.

const user = {
    name: "Alice",
    greet: function() {
        setTimeout(() => console.log(`Hello, my name is ${this.name}`), 1000);
    }
};

user.greet(); // Outputs: "Hello, my name is Alice"

References

• MDN: this in JavaScript

---

3. Undefined and Null Confusion

The Problem

JavaScript has both undefined and null, which can lead to confusion and bugs when they are used interchangeably or not checked properly.

Example

let user = {
    name: "Alice",
    age: null
};

if (user.age) {
    console.log(`User's age is ${user.age}`);
} else {
    console.log("Age is not provided");
}
// Outputs: "Age is not provided"

Here, user.age is null, but the if condition treats it as falsy. This can cause issues if null is intended to be a valid state.

The Solution

Always check for undefined and null explicitly if both are valid values in your application.

if (user.age !== null && user.age !== undefined) {
    console.log(`User's age is ${user.age}`);
} else {
    console.log("Age is not provided");
}

Using strict equality (===) can also help distinguish between undefined and null.

References

• MDN: Null
• MDN: Undefined

---

4. Callback Hell

The Problem

Callback functions are a common way to handle asynchronous operations in JavaScript. However, when they are nested within each other, they can create deeply nested structures, often referred to as "callback hell." This makes code difficult to read, maintain, and debug.

Example

doSomething(function(result1) {
    doSomethingElse(result1, function(result2) {
        doAnotherThing(result2, function(result3) {
            doFinalThing(result3, function(finalResult) {
                console.log(finalResult);
            });
        });
    });
});

This deeply nested structure is hard to follow and even harder to debug.

The Solution

Use Promises or async/await to flatten the structure and make the code more readable.

doSomething()
    .then(result1 => doSomethingElse(result1))
    .then(result2 => doAnotherThing(result2))
    .then(result3 => doFinalThing(result3))
    .then(finalResult => console.log(finalResult))
    .catch(error => console.error(error));

Or, using async/await:

async function executeTasks() {
    try {
        const result1 = await doSomething();
        const result2 = await doSomethingElse(result1);
        const result3 = await doAnotherThing(result2);
        const finalResult = await doFinalThing(result3);
        console.log(finalResult);
    } catch (error) {
        console.error(error);
    }
}

executeTasks();

References

• MDN: Promises
• MDN: async/await

---

5. Floating Point Precision Issues

The Problem

JavaScript uses the IEEE 754 standard for representing numbers, which can lead to precision issues, especially with floating-point arithmetic. This can cause unexpected results in calculations.

Example

console.log(0.1 + 0.2); // Outputs: 0.30000000000000004
console.log(0.1 + 0.2 === 0.3); // Outputs: false

The result of 0.1 + 0.2 is not exactly 0.3 due to floating-point precision errors.

The Solution

To avoid this, you can round the result to a fixed number of decimal places.

function isEqual(a, b) {
    return Math.abs(a - b) < Number.EPSILON;
}

console.log(isEqual(0.1 + 0.2, 0.3)); // Outputs: true

Alternatively, work with integers by scaling the numbers before performing operations and then scaling them back down.

console.log((0.1 * 10 + 0.2 * 10) / 10); // Outputs: 0.3

References

• MDN: Number.EPSILON
• The Problem with Floating Point Arithmetic

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Conclusion

JavaScript is a language full of idiosyncrasies and hidden risks, but knowing the most frequent flaws and how to avoid them allows you to develop cleaner, more dependable code. From unwanted global variables to floating-point accuracy concerns, each of these flaws can create major difficulties if not addressed. However, with proper coding methods and the correct tools, you can reduce these concerns and make your JavaScript code more resilient.

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