'invalid typeof value' article

Author: James Allardice

articles.md

@@ -310,7 +310,7 @@ the filename of the new article next to it.
  - '{a}' is only available in ES6 (use esnext option)
  - `bad-declaration.md` - ~~You might be leaking a variable ({a}) here~~
  - Extending prototype of native object
- - Invalid typeof value
+ - `invalid-typeof-value.md` - ~~Invalid typeof value~~
  - '{a}' is already defined in outer scope
  - A generator function shall contain a yield statement
  - This line contains non-breaking spaces

articles/invalid-typeof-value.md

@@ -0,0 +1,86 @@
+<!---
+{
+    "titles": [
+        "Invalid typeof value '{a}'",
+        "W122"
+    ],
+    "slugs": [
+        "invalid-typeof-value-a",
+        "w122"
+    ],
+    "linters": [
+        "jshint"
+    ],
+    "author": "jallardice"
+}
+-->
+
+### 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"`:
+
+<!---
+{
+    "linter": "jshint"
+}
+-->
+```javascript
+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 &sect;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:
+
+<!---
+{
+    "linter": "jshint"
+}
+-->
+```javascript
+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][jshintopts]. 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`.
+
+[es5-11.4.3]: https://es5.github.io/#11.4.3
+[jshintopts]: http://jshint.com/docs/#options