Typeof comparison expression should check right hand side value

[tinganho]

As I understood typeof operator can only return undefined, object, boolean, number, string, symbol, function.

If the RHS is a string literal, I think it should check against the above values and throw an error on:

const str = 'str';
if (typeof str === 'd') {// error right hand side can only be `undefined`, `object`, ...
}

[mhegazy]

There are two parts here, first type of the return type of typeof operators to be "undefined" | "object" | "boolean" | "number" | "string" | "symbol" | "function" instead of string which sounds safe.

and two is to make a comparison between two a string literal type and a string literal an error, which would be a breaking change and prevent defensive coding patterns.

[DanielRosenwasser]

There are two parts here, first type of the return type of typeof operators to be "undefined" | "object" | "boolean" | "number" | "string" | "symbol" | "function" instead of string which sounds safe.

For the record, in the spec's section on The typeof Operator:

Type of val	Result
Undefined	"undefined"
Null	"object"
Boolean	"boolean"
Number	"number"
String	"string"
Symbol	"symbol"
Object (ordinary and does not implement [[Call]])	"object"
Object (standard exotic and does not implement [[Call]])	"object"
Object (implements [[Call]])	"function"
Object (non-standard exotic and does not implement [[Call]])	Implementation-defined. Must not be "undefined", "boolean", "function", "number", "symbol", or "string".

NOTE
Implementations are discouraged from defining new typeof result values for non-standard exotic objects. If possible "object" should be used for such objects.

[RyanCavanaugh]

Good comments from Brian in #10594

[RyanCavanaugh]

Until this is normatively specified we don't want to encode this into the type system. A TSLint rule would be very easy to write.

[RyanCavanaugh]

We did do this! 🎉