JSLint will throw the "Do not wrap function literals in parens unless they are to be immediately invoked" error when it encounters a function expression wrapped in parentheses with no following invoking parentheses. JSHint (before version 1.0.0) will throw this error in the same situation, but only if the immed option is set to true. In the following example we assign a function expression to a variable x:
This error is raised to highlight a potentially confusing piece of code. It is common in JavaScript to see immediately invoked function expressions. Here's the above snippet again, this time with the invoking parentheses. Notice how subtle the difference is:
While the difference may not look like much, the two snippets are completely different in their behaviour. The first example (which does not pass JSLint) will result in a function expression assigned to x. The second snippet will result in the return value of that function assigned to x.
When you see a function expression wrapped in parentheses, you would usually expect it to be an immediately invoked function expression. When it isn't, there is greater potential for confusion and misunderstanding of your code.
In JSHint 1.0.0 and above this warning has changed to "Wrapping non-IIFE function literals in parens is unnecessary". More detail can be found the page dedicated to that message.