Primitive type guards require `typeof` to be on the left hand side of the comparison

[dargoner]

Typecript method, the parameter does not support the function and the character of the joint type, prompt TS2345 error

[yortus]

The error goes away for me after re-arranging the type guards in on as follows:

    ...
    if (typeof fn == "string") {

    }
    if (typeof fn != "function" || !type) {
        return false;
    }
    ...

[mindplay-dk]

I'm running into something similar.

I'm getting the error about parentElement here even though the statement is wrapped in isHTMLElement which returns an assertion that should make parentElement known as HTMLElement inside that block.

[yortus]

Note the following compiler behaviour (with typescript@next at least):

function foo(x: string|number) {
    if (typeof x === "string") {
        x // x is narrowed to string here
    }

    if ("string" === typeof x) {
        x // x is NOT narrowed here, still string|number
    }
}

Seems weird that the two equivalent forms are treated differently. But that's what it does at present.

[yortus]

@mindplay-dk I think yours is a different problem - on the error line you linked to, the parentElement reference is inside the body of a nested function expression. Narrowing doesn't cross function boundaries except in very specific cases (see #8849).

[sandersn]

The spec provides only a few fixed forms for narrowing primitives. typeof x === 'string' narrows, but 'string' === typeof x does not.

As a workaround, you can write a type predicate:

function isString(x: any): x is string {
  return "string" == typeof x;
}
// later...
if (isString(fn) || !type) {
  return false;
}

[sandersn]

We discussed this offline and decided to change the spec in this case.