angular - startOf(string) issue in Angular2

i'm trying use moment.js in angular2 date of first day of week in typescript.

but when try use method startof(), got weird error :

moment('2017-07-26').startof('isoweek');

argument of type '"isoweek"' not assignable parameter of type 'startof'.

but in moment doc, , other thread signature correct :

moment().startof(string);

where mistake ?

p.s. : sorry bad english , mistakes did. edit welcomed, if needed !

RT