When used with a value the typeof operator returns one of the following strings:

• "undefined"
• "object"
• "boolean"
• "number"
• "string"
• "function"
• "symbol"
• "bigint"

This rule disallows comparison with anything other than one of these string literals when using the typeof operator, as this likely represents a typing mistake in the string. The rule also disallows comparing the result of a typeof operation with any non-string literal value, such as undefined, which can represent an inadvertent use of a keyword instead of a string. This includes comparing against string variables even if they contain one of the above values as this cannot be guaranteed. An exception to this is comparing the results of two typeof operations as these are both guaranteed to return on of the above strings.

Invalid:

// typo
typeof foo === "strnig";
typeof foo == "undefimed";
typeof bar != "nunber";
typeof bar !== "fucntion";

// compare with non-string literals
typeof foo === undefined;
typeof bar == Object;
typeof baz === anotherVariable;
typeof foo == 5;

Valid:

typeof foo === "undefined";
typeof bar == "object";
typeof baz === "string";
typeof bar === typeof qux;