Why Top Tech Companies Ban eval(): Security, Performance, and Safer Alternatives

There is a JavaScript feature that is often explicitly prohibited – the eval() function and its variants. This seemingly powerful feature is blacklisted by Microsoft, Google, Facebook, and other top companies.

eval(): A powerful but dangerous double‑edged sword

eval()

can execute a string as JavaScript code, enabling many dynamic capabilities:

const expr = 'var x = 10; x * 2';
eval(expr); // returns 20

However, this dynamic execution brings serious security risks and performance problems.

Security risk: breeding ground for injection attacks

The most severe issue appears when eval() is combined with user input, turning it into a perfect vehicle for code‑injection attacks.

Attackers can use this to:

Steal sensitive data

Execute malicious actions

Hijack user sessions

Modify page content

Performance problem: V8 engine’s nightmare

JavaScript engines cannot optimise code that contains eval() because:

It blocks compile‑time optimisation: eval() functions cannot be pre‑compiled.

It disables inline caches: the scope chain of eval() must stay dynamic.

It forces slower variable look‑ups: the compiler cannot resolve variable references statically.

Chrome V8 internal tests show that code with eval() can run at least ten times slower.

Maintainability: blind spot for code audits

Code that uses eval() suffers from:

Difficult debugging (unclear stack traces)

Increased code complexity

Reduced effectiveness of static analysis tools

Harder code audits

Execution context pollution

eval()

runs code in the current scope, which may unintentionally modify or overwrite variables.

How big companies respond

Google’s JavaScript style guide states:

“Do not use eval(). It makes code vulnerable to injection attacks and prevents the JavaScript engine from optimising.”

Microsoft’s secure coding guidelines say:

“Prohibit the use of eval() and the Function constructor unless a strictly reviewed special need exists.”

Typical measures include:

Strict ESLint rules that disable eval.

Marking eval usage as a severe issue during code review.

Providing safe alternative APIs.

Security scanning tools that automatically detect eval usage.

Alternatives

For most scenarios where eval() is used, better alternatives exist:

1. Use JSON.parse instead of parsing data

2. Use object maps instead of dynamic property access

3. Use new APIs instead of dynamic evaluation

// Do not do this
const result = eval('2 * 3 + 4');

// Use this instead
const result = new Function('return 2 * 3 + 4')();

// Or better, use a dedicated expression‑parsing library