I had this pretty easy method in Typescript (strict) that was giving me the following TSLint error:

TSLint: expected call-signature: 'toggleText' to have a typedef(typedef)

 The function looked like this:

  toggleText(usernotification: Usernotification) {
    usernotification.showText = !usernotification.showText;
  }

Solution

The solution is first to set up the argument to = (argument). Second, tell the method what you expect as a result (like a promise, a boolean, ...). In this case, it is nothing, so we add :void =>

  toggleText = (usernotification: Usernotification): void => {
    usernotification.showText = !usernotification.showText;
  }