Wrap an immediate function invocation in parentheses

When do I get this error?

The "Wrap an immediate function invocation in parentheses" error is thrown when JSLint, JSHint and ESLint encounter an immediately invoked function expression that is not wrapped in parentheses. JSHint will only raise this warning if the immed option is set to true. In the following example we assign the return value of the anonymous function the variable x:

/*jshint immed: true */
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. Since function declarations cannot be immediately invoked, and function expressions can be, a common technique to create an immediately-invoked function expression is to simply wrap a function statement in parentheses. The opening parenthesis causes the contained function to be parsed as an expression, rather than a declaration:

var x;
(function () {
    "use strict";
    x = 10;
}());

If you remove the wrapping parentheses from the above example, you will end up with a syntax error. For that reason, when immediately invoking a function expression that doesn't require any special treatment to turn it into an expression (as in the first example above), convention dictates that you should wrap it in parentheses anyway, for consistency and to make it clearer that the resulting value of the overall expression is the return value of the function, rather than a reference to the function itself:

/*jshint immed: true */
var x = (function () {
    "use strict";
    return {
        y: 1
    };
}());

In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax. The identifier of this warning is W062. This means you can tell JSHint to not issue this warning with the /*jshint -W062 */ directive. You can also set the immed option to false.

In ESLint the rule that generates this warning is named wrap-iife. You can disable it by setting it to 0, or enable it by setting it to 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.