eval

Eval is a built-in function name used in many programming languages to execute code contained in a string at runtime. The general idea is to convert text into executable instructions, allowing dynamic code generation or interpretation. Semantics and safety vary by language, but common concerns apply across implementations.

In JavaScript, eval executes the given string as code in the current scope, and can read and modify local variables and state. Its return value is the result of the last expression, or undefined if the string contains only statements. In strict mode, eval interacts with scope in more restricted ways. In Python, eval evaluates a string as a Python expression and returns its value; it cannot run statements. For statements, Python provides exec. Other languages such as PHP, Ruby, and MATLAB also offer eval-like facilities with language-specific rules about scope and side effects.

Risks and limitations are central to eval. Evaluating untrusted input can lead to arbitrary code execution, security breaches, and data leakage. Performance may suffer, as code generated at runtime is typically not optimizable by ahead-of-time compilers. Debugging can be harder when code appears only as text. Therefore, many environments discourage or disallow eval in favor of safer alternatives.

Best practices include avoiding eval when possible, especially with user-supplied data. Use safer data formats and parsers (for example, JSON parsing instead of evaluating a JSON string), or dedicated evaluation tools with restricted capabilities. In Python, ast.literal_eval offers a safe subset for literals; in JavaScript, consider using a sandboxed interpreter or a restricted API when dynamic code is unavoidable. When using eval, restrict inputs rigorously and confine execution to a controlled environment.