When is JavaScript's eval() not evil?


The eval() function in JavaScript can be considered “evil” or dangerous because it can execute arbitrary code and potentially introduce security vulnerabilities or unexpected behavior into an application. However, there are some cases where the use of eval() is appropriate and can be useful.

  1. Dynamic code generation: When generating dynamic code based on user input or other external factors, eval() can be used to evaluate the generated code. This can be useful in situations where you need to generate code on the fly, such as in a code editor or a compiler.

  2. JSON parsing: In some cases, eval() can be used to parse JSON strings into JavaScript objects. However, it is important to note that this approach should only be used if you trust the source of the JSON data and there are no security concerns.

For example:

let jsonString = '{"name": "John", "age": 30}';
let obj = eval('(' + jsonString + ')');
console.log(obj.name); // Output: John
  1. Polyfills: In some cases, eval() can be used to create polyfills for older browsers that do not support certain JavaScript features. This can be useful when you need to support older browsers and there is no other alternative.

However, it is important to note that in all cases, the use of eval() should be avoided whenever possible, as there are usually safer and more secure alternatives available. If you must use eval(), you should always validate user input and sanitize any dynamic code to avoid security vulnerabilities.