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String Evaluation

Miscellaneous: String Evaluation

What are the potential security risks associated with using the eval function?

View Answer:
Interview Response: The main risk is the potential for code injection, where malicious code can be executed. It's important to validate and sanitize any input used with `eval`.

Technical Details:

The eval() function in JavaScript is a powerful feature that evaluates a string of JavaScript code and executes it. Because it has the ability to execute arbitrary code, it presents several security risks:

1. Code Injection: If eval() is used on strings that can be manipulated by users (for example, form inputs or URL parameters), it opens up the potential for malicious code injection. An attacker could input a string that contains harmful JavaScript code, which would then be executed.

2. Scope Access: Code executed by eval() runs in the same scope as the call to eval(), meaning it has access to local variables, functions, and potentially sensitive data within that scope.

3. Performance: While not a security risk per se, it's worth noting that eval() can lead to performance issues. Modern JavaScript engines optimize code by compiling it into a more efficient format, but they can't do this as effectively with eval(), because the code within eval() is dynamically parsed and executed.

Given these security and performance considerations, it's generally recommended to avoid eval() if possible. Alternatives include using JSON.parse() for converting JSON strings into objects, using functions like setTimeout and setInterval with function arguments instead of string arguments, or using the Function constructor to create functions from strings in a more controlled manner.


Remember, always sanitize and validate any user-provided data if you absolutely must use eval(), though in general it's best to avoid it if at all possible.

View Answer:
Interview Response: In general, it is discouraged to use `eval` in production code due to security risks and potential performance implications. There are usually alternative approaches to achieve the desired functionality.

What is the purpose of the eval() function in JavaScript?

View Answer:
Interview Response: The eval() function is used to evaluate a string as a JavaScript expression or code snippet, allowing dynamic execution of code. However, it's generally discouraged due to security risks.

Technical Response: According to the MDN, the honest answer is that you should never use eval in your applications. However, the purpose of eval is to evaluate JavaScript code represented as a string value. One string parameter evaluates in the built-in eval function, and returns the completed value of the evaluated code. If the completion value is empty, undefined is returned.

Code Example:

let code = 'console.log("Hello")';
eval(code); // logs Hello

Should you use the eval built-in JavaScript function property? Why or why not?

View Answer:
Interview Response: We should never use eval in modern applications. Many problems include security, optimization, speed, and minification compatibility issues. The MDN does not recommend its use in modern programming.

Technical Response: In modern programming, developers use eval very sparingly. Developers often say that “eval is evil”. The reason is simple: a long, long time ago, JavaScript was a weaker language; many things only worked with eval. But that time passed a decade ago. There is no reason to use eval because the JavaScript engine does not optimize it. If someone is using it, there is a good chance they can replace it with a modern language construct or a JavaScript Module. Evil, I mean eval has several issues, including speed and code minification compatibility issues. The fundamental problem is that it poses a security risk to the user, and the MDN does not recommend its use in modern programming.

Using external local variables inside eval is also considered a bad programming practice, as it makes maintaining the code more complex. If your code needs some data from the outer scope, use “new” Function and pass it as arguments.

Executing JavaScript from a string is an enormous security risk. It is far too easy for a bad actor to run arbitrary code when using eval().

Is there a better option or built-in JavaScript object than eval that you can use?

View Answer:
Interview Response: Yes, safer alternatives to eval() include the Function constructor for creating functions dynamically or JSON.parse() for parsing JSON strings without executing arbitrary code.

Code Example: Converting from eval to a Function() object.

// Bad code with eval():
function looseJsonParse(obj) {
return eval('(' + obj + ')');

console.log(looseJsonParse('{a:(4-1), b:function(){}, c:new Date()}'));

// Better code without eval():
function looseJsonParse(obj) {
return Function('"use strict";return (' + obj + ')')();

console.log(looseJsonParse('{a:(4-1), b:function(){}, c:new Date()}'));