@B1ts I am pretty sure in that case the error would say:

TypeError: number.replace() is not a function.

But instead it says "string.replace()" is not a function.

My misstake, you are right, thanks

Nope, there is 1 test where argument is a number. (IDK why, but there is :/)

Try adding console.log(typeof string)

I used this methods on an income function argument...and income argument is a STRING, but when I used string.toString.(split/replace) everything is work, but i dont understand why?

In this function icome argument is a string already...am I right?

Because you're using it on something that is NOT A STRING.

5.replace() doesn't exist, neither does [].replace (IDK what you're actually using it on)

TypeError: string.replace is not a function
at toUnderscore
at /home/codewarrior/index.js:19:20
at /home/codewarrior/index.js:20:5

and if I use string.replace I have simmilar error

You're trying to use .split() on something that isn't a string.

I have this error
TypeError: string.split is not a function
at toUnderscore
at /home/codewarrior/index.js:27:20
at /home/codewarrior/index.js:28:5
at Object.handleError

If i wanted to use string.split('') method in JS