JavaScript is a dynamic language, which means you can write code that creates and runs other code on the fly. The main tool for this is the eval() function.

But while eval() sounds powerful, it comes with big risks. Using it is like giving a stranger your car keys — yes, they can drive, but what if they crash it?

In this lesson, we’ll explore:

  • What eval() does
  • Why it’s risky
  • Safer alternatives to use
  • Some practical scenarios

🔎 What is eval()?

The eval() function takes a string and executes it as JavaScript code.

Example:

let result = eval("2 + 3 * 5");
console.log(result); // 17

Here, eval() reads the string "2 + 3 * 5" and treats it like real code.

You can even declare variables:

eval("var x = 42;");
console.log(x); // 42

👉 Powerful, right? But also very dangerous if the string comes from an untrusted source.

⚠️ Why is eval() Dangerous?

  1. Security Risks If you evaluate user input, a malicious user could inject harmful code.

    let userInput = "alert('Hacked!')";
eval(userInput); // Pops up a malicious alert

    Imagine if instead of alert, the attacker inserted code to steal cookies or redirect pages.

  2. Performance Issues eval() forces the JavaScript engine to pause and recompile new code on the fly. This slows down execution compared to normal functions.

  3. Readability and Debugging Code that relies on eval() is harder to read, understand, and maintain.

Because of these problems, modern best practices say: 👉 Avoid eval() unless you absolutely must.

✅ Safer Alternatives to eval()

  1. JSON.parse() Often people use eval() to parse JSON strings. But the correct, safe way is:

    let data = '{"name":"Ada","age":16}';
let obj = JSON.parse(data);
console.log(obj.name); // Ada

  2. Function Constructor If you must evaluate code dynamically, you can use:

    let sum = new Function("a", "b", "return a + b;");
console.log(sum(2, 3)); // 5

    This is slightly safer because it runs in local scope isolation.

  3. Lookup Tables Instead of eval Instead of evaluating strings like "add" or "subtract", use an object map:

    const operations = {
  add: (a, b) => a + b,
  subtract: (a, b) => a - b,
};
let op = "add";
console.log(operations[op](5, 3)); // 8

🛠 Practical Scenarios

  • Bad Use Case → A form that lets users type math equations, and you directly eval() their input.
  • Better Use Case → Use a math parser library (like math.js) that safely evaluates expressions without eval().
  • Acceptable Use Case → Writing quick debugging code in your own environment where input is trusted.

✅ Summary

  • eval() lets JavaScript execute code strings dynamically.
  • It’s powerful but also risky because of security, performance, and readability concerns.
  • Always prefer safer options: JSON.parse(), Function constructors, or lookup tables.
  • Use eval() only as a last resort, in trusted environments.

📝 Review – Fill in the Gaps

  1. The eval() function executes a __ as JavaScript code.
  2. Using eval() with user input can lead to __ risks.
  3. Code that uses eval() is harder to read and __.
  4. Instead of using eval() to parse JSON, use __.
  5. The Function constructor creates a new function in __ scope.
  6. Using objects as lookup tables is a safer alternative to __.
  7. One problem with eval() is that it slows down program __.
  8. A bad use of eval() would be evaluating user-submitted math __.
  9. A better choice than eval() for math expressions is using a __ library.
  10. In modern best practices, eval() should be used only as a __ resort.
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