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() function.

Table of Contents:

• What is eval()?
• Why is eval() Useful?
• Why You Should Avoid Using eval()?
  • 1. Security
  • 2. Breaks Lexical Scope
  • 3. Kills Performance
  • 4. Hard to Debug and Maintain
• Alternatives to eval()
• Conclusion

What is eval()?

The eval() function is a built-in function in JavaScript, and it performs a very specific job – it takes a string and evaluates it as JavaScript code. Let’s understand it with the help of a simple code:

Example:

const result = eval("2 + 2");

console.log(result);

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.

Why You Should Avoid Using eval()?

Here are some of the reasons why you should avoid using eval() in 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()

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 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.

Conclusion

Using eval() 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.

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