Hey there! Let's break down why you're hitting that Uncaught SyntaxError and fix it right away.

The Root of the Problem #

When you define var test = 'prompt("Username: Jimbles \n Message:")', the \n in that string gets interpreted as an actual newline character in the string content—not as the escaped newline you want for the prompt() dialog. So when eval() tries to parse this, it sees something like:

prompt("Username: Jimbles 
 Message:")

JavaScript doesn't allow unescaped newlines inside string literals, which is exactly why you get that syntax error. The parser thinks the string ends at the newline, leaving Message:") hanging as invalid code.

Solutions #

Since you need to use eval() with externally sourced code, here are two reliable fixes:

1. Escape Newlines Before Passing to eval()

If the external code comes with actual newlines (like your example), you can pre-process the string to replace those newlines with escaped \\n sequences. This way, eval() will parse them correctly as part of the prompt() string.

// Example: External code with actual newlines
var externalCode = 'prompt("Username: Jimbles \n Message:")';

// Replace all actual newlines with escaped \n
var safeCode = externalCode.replace(/\n/g, '\\n');

// Now eval will work correctly
eval(safeCode);

2. Ensure External Code Uses Properly Escaped Newlines

If you have control over how the external code is generated, make sure it uses double-escaped newlines (\\n) instead of actual newlines. For example, the external code should look like this:

var test = 'prompt("Username: Jimbles \\n Message:")';
eval(test);

Here, the \\n gets parsed by eval() as a single \n, which will show up as a newline in the prompt() dialog.

Why This Works #

By escaping the newline characters, you're telling JavaScript to treat them as part of the string inside prompt() instead of interpreting them as a break in the code syntax. The end result is a valid prompt() call that displays your desired newline in the dialog.

A quick note: Always be cautious with eval()—if the external code is untrusted, it can pose security risks. But since you've stated you must use it, these fixes will get your prompt-based chat project on track.

内容的提问来源于stack exchange，提问作者Gorgamite