如何对包含 function 的字符串执行“评估”

Question

In the following example, I would expect b and c = eval(a) to contain the same function, one that always returns true. However, a does not get evaluated, it rather throws a syntax error: Uncaught SyntaxError: Function statements require a function name . However, if if name the function, c contains undefined .

var a = "function() { return true }";

var b = function() { return true };

var c = eval(a);

console.log(b,c);

Answer 1

The problem is that the code you're feeding into eval is being evaluated where a statement is expected, but the code you have after var b = is being evaluated as an expression. When the parser is expecting a statement, the function keyword starts a function declaration , which requires a name. But where an expression is expected, the function keyword starts a function expression , in which a name is optional.

To make your eval work on an expression instead, surround the code with ( and ) :

 var a = "function() { return true }"; var b = function() { return true }; var c = eval("(" + a + ")"); // −−−−−−−−−−^^^^^−−−^^^^^ console.log(b,c);

---

Warning : Be very careful using eval and avoid using it if possible. Its purpose is to execute arbitrary code from strings. If you use it, you must trust that the source of that string is trustworthy. Never take user input from user A and then eval it in a session for user B unless user B is fully aware of what's going on (for instance, as with the Stack Snippet above — I'm user A, you're user B; presumably if you click the Run button, you realize code will get run). In fact, when dealing with non-programmers, don't even eval code from user A in user A's session. :-)