Move the invocation into the parens that contain the function

When do I get this error?

The "Move the invocation into the parens that contain the function" error is thrown when JSLint and ESLint encounter an immediately invoked function expression in which the invoking parentheses appear outside the wrapping parentheses. In the following example we assign the return value of the anonymous function to the variable x:

var x = (function () {
    "use strict";
    return {
        y: 1
    };
})();

Why do I get this error?

This error is raised to highlight a lack of convention. Your code will run fine if you do not fix this error, but it may be confusing to others. However, this particular error gives rise to debate over which position for the invoking parentheses should actually be the convention. If you are using JSLint, then your convention should be the one suggested.

The argument for moving the parentheses is that it makes the code easier to understand. It may not be immediately obvious that the parentheses are intended to invoke the function if they are outside the wrapping pair. By moving them inside, they immediately follow the function statement and may therefore be more easily associated with it. To fix this error, simply move the pair of invoking parentheses inside the pair that wrap the function itself:

var x = (function () {
    "use strict";
    return {
        y: 1
    };
}());

If you're using ESLint you are able to configure the convention that you prefer. The wrap-iife option will accept a string, "inside" or "outside", that will enforce a particular position (for some strange reason the "inside" option appears to force the invoking parentheses to appear outside of the wrapping ones, and vice versa):

/*eslint wrap-iife: [1, "outside"] */
var x = (function () {
    "use strict";
    return {
        y: 1
    };
}());

About the author

James Allardice

This article was written by James Allardice, JavaScript engineer at Mammal in London. Passionate about AngularJS, Node and writing clean and maintainable JavaScript and uses JSHint every day to help achieve this.