[Vuejs]-How to use variables of data function on VueJS?

Use toggle(elementId = 0, type = 'item') {} instead of toggle: (elementId = 0, type = 'item') => {}.
arrow function assigns this to its parent’s this of the scope.
See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions#Arrow_functions_used_as_methods