TypeScript isNan only accepts a number

I advise you to implement your code differently.
The reasons:

You better try to convert the value into a number and check if that's NaN or not (as @smnbbrv wrote):

if (typeof expectedValue === "string" && !Number.isNaN(Number(expectedValue))) {    expectedValue = Number(expectedValue);}

You can pass your value as any:

isNaN(ctualValue as any)

To bypass the compiler check.

javascript node.js typescript webstorm

You should not solve it because this is how JavaScript works.

Just cast the input to number first

Number("10") // 10Number("abc") // NaN

and then check the result with the isNan function:

isNaN(Number("abc"))

javascript node.js typescript webstorm

As ironically only numbers can be NaN, you need to transform strings into numbers first.

A very simple way to do this is the unary plus operator.

So you can do a simple isNaN(+"10").

Keep in mind that thing like +"", +" " and +"\n" are 0!

CodeHunter