angularjs - Typescript user defined type guard stopped working when assigned to a service object -

i'm using typescript angular , defined type guard in service this:

type types = typea | typeb | typec;  function istypea(data: types): data typea {     return (<typea>data).a !== undefined; }  function istypeb(data: types): data typeb {     return (<typeb>data).b !== undefined; }  function istypec(data: types): data typec {     return (<typec>data).c !== undefined; }  let svc = this; svc.istypea = istypea; svc.istypeb = istypeb; svc.istypec = istypec; 

and in service function use this:

if (istypea(data)) {   // use data.a } else if (istypeb(data)) {   // use data.b } else if (istypec(data)) {   // use data.c } 

it works fine in service, once assign guards service's this object , use them in controller, got following error:

error ts2339: property 'a' not exist on type 'types'. 

the guards used this:

// svc injected controller service if (svc.istypea(data)) {   // use data.a } else if (svc.istypeb(data)) {   // use data.b } else if (svc.istypec(data)) {   // use data.c } 

if redefine guards in controller, works fine again, suggestion have done wrong here?