When do I get this error?

The "Invalid typeof value '{a}'" error is thrown when JSHint encounters a comparison with a typeof expression on one side and an invalid string literal on the other. In the following example we have a function that will return true if the argument is of type "bool":

function demo(a) {
    return typeof a === "bool";
}

This functionality was introduced in JSHint 2.3.0. Prior versions do not raise any error in this situation. Neither JSLint nor ESLint raises any error in this situation.

Why do I get this error?

This error is raised to highlight code that is unlikely to work as you expect. The typeof operator returns one of a few possible values that are given by the ECMAScript specification (although in some situations in certain browsers it will return one of two extra possibilities). The possible values are as follows ([ES5 §11.4.3]):

• "undefined" - when the operand has the undefined value

• "boolean" - when the operand has a literal boolean value

• "string" - when the operand has a literal string value

• "number" - when the operand has a literal numeric value

• "function" - when the operand is an object and implements the internal [[Call]] property

• "object" - when the operand has the null value or is an object that does not implement the internal [[Call]] property

In certain situations the operator could also return "xml" or "unknown" but you're unlikely to come across these. JSHint requires that any comparison with a typeof expression uses one of these strings. If a different string is used the condition will always evaluate to false since it is not possible for the typeof operator to ever return the value you're using. To solve this issue simply use a valid value:

function demo(a) {
    return typeof a === "boolean";
}

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 W122. This means you can tell JSHint to not issue this warning with the /*jshint -W122 */ directive. You can also set the sub option to true.