Why You Should Not Use eval() in JavaScript?

Just because something works doesn’t mean it’s a good practice. If you’re a developer, then you probably see the eval() function at some point. Maybe you will use it to execute some dynamic code, or you can hear from someone to use it. But it’s important to learn complete information about eval() before using it in code. In this blog, you will get all the information about the eval() function and why you should avoid using it, and what safer alternatives you can use instead of using eval() in JavaScript.

Table of Contents:

• What is eval() in JavaScript?
• Why is eval() Useful?
• Understanding How eval() Works Internally
• Why You Should Avoid Using eval()?
  1. Security
  2. Breaks Lexical Scope
  3. Kills Performance
  4. Hard to Debug and Maintain
• Alternatives to eval() in JavaScript
• eval() vs JSON.parse(): What’s the Difference?
• Real-World Examples of Security Issues Using eval()
• Conclusion

What is eval() in JavaScript?

The eval() in JavaScript is a built-in function that performs a very specific task by taking a string and evaluating it as JavaScript code. Let’s understand it with the help of a simple code:

Example:

Output:

Explanation: In this example, as you can see, when working with the eval() function, you can pass the string, and eval() takes this string as JavaScript code and prints its output as 4.

Why is eval() Useful?

Some developers use the eval() function when they want to:

• Execute dynamically generated code.
• For parsing JSON strings.
• Run code snippets from the user input.

Initially, it was found very useful, but its long-term consequences were painful.

Understanding How eval() Works Internally

The eval() in JavaScript takes a string as input and executes it as if it were regular JavaScript code. Here’s how eval() in JavaScript works internally:

• Takes a string: You pass some code written as a string to eval().
• Reads the string like real code: JavaScript treats the string as if you wrote actual code.
• Runs the code right away: Whatever code is in the string gets executed immediately.
• Uses your current variables: If eval() is inside a function, it can use and even change your variables.
• Return a result: If the code does something like math, eval() gives you back the answer.
• It can be risky: Because it runs any code, it’s dangerous if the string comes from someone else. It can break your app or cause security issues.

Example:

Output:

Explanation: In this code, a and b are two variables, and the string “a + b” is passed into eval(). The eval() reads it like real code and runs it. At last, it calculates 10 + 20 and returns 30 as an output.

Why You Should Avoid Using eval()?

Here are some of the reasons why you should avoid JavaScript eval() function in your code:

1. Security

If a malicious user injects harmful code, the eval() function blindly executes it. This opens the door for code injection, XSS (cross-site scripting) attacks, and other security vulnerabilities. The solution is simple. Never run untrusted code with eval().

2. Breaks Lexical Scope

Variables and functions in JavaScript follow a structure called scope, which defines the place up to which you can access other variables and functions. But when you use eval(), it can mess with that structure by creating or modifying variables unexpectedly. This makes your code harder to read and understand.

3. Kills Performance

JavaScript engines like V8 (used in Chrome) are very fast and optimized. They compile code just-in-time (JIT) to machine code very fast. But when you use eval(), all of that optimization goes out of window. Because that browser has no idea about what the string inside eval() contains until runtime. And this slows down the performance significantly.

4. Hard to Debug and Maintain

If you’re writing all JavaScript code in the form of a string and putting that string into the eval() function, then it’s difficult to find errors or bugs in your code. Writing code in the form of strings stops syntax highlighting, code completion, and static analysis.

Alternatives to eval() in JavaScript

Now, using eval() sometimes produces bugs in code, so what developers use in place of the eval() function? Let’s discuss all the safer alternatives to eval() in JavaScript for different applications:

• Use JSON.parse() instead of eval() to parse JSON strings
• Use object maps or switch statements for producing dynamic outputs.
• Use templating libraries or regex for string parsing.
• Use the Function() constructor to evaluate simple expressions more safely.

By using these alternative methods, you can learn how to parse dynamic code without using eval() in JavaScript.

eval() vs JSON.parse(): What’s the Difference?

Real-World Examples of Security Issues Using eval()

Here are some real-world examples of security issues caused by using eval() in JavaScript:

1. Cross-Site Scripting (XSS): If a website uses eval() to run user input, an attacker can add unsafe code or code with bugs, which could steal cookies, session tokens, or redirect users.

Example:

// BAD: eval on user input
let userCode = getUserInput();  // e.g., "<script>alert('Hacked!')</script>"
eval(userCode); // Executes malicious script

2. Remote Code Execution (RCE): If an API or form allows input that reaches eval(), an attacker can run any JavaScript they want on your page.

Example:

let userData = '{"data": "console.log(document.cookie)"}';
let obj = JSON.parse(userData);

// Dangerous misuse
eval(obj.data);  // This prints (or leaks) the user's cookie

3. Backdoor Access via Code Injection: Some developers use eval() to dynamically run commands, like in an admin tool that works as a hidden backdoor for attackers to hack the system.

Example:

// Example: Admin tool for executing commands
function runCommand(cmd) {
  eval(cmd);
}

runCommand("deleteAllUsers()"); // Expected use

// But an attacker can do:
runCommand("stealData(); sendToHacker();");

4. Bypassing Validation Logic: If eval() is used to check data or conditions, then attackers can trick the system into accepting bad or insecure input.

Example:

let isValid = eval(userInput); // e.g., "true || alert('Hacked!')"

Conclusion

Using eval() in JavaScript may be a shortcut for writing complex code, but in reality, it’s not and produces errors, increases the risk of security, slows down your application, and makes your code hard to read and maintain.

These articles offer a thorough overview of JavaScript’s essential concepts and foundational knowledge.-

How to iterate over a JavaScript object – Demonstrates use cases in real-world object traversal.

How do I check if an array includes a value in JavaScript – Highlights the difference between includes() and find().

How to clone a JavaScript object – Includes comparisons of cloning speed and accuracy.

How to compare dates in JavaScript – Demonstrates working with time zones and formats.

Event loop with JavaScript – Visualizes how events are queued and processed.

Why You Should Not Use eval() in JavaScript – FAQs