use same signature for model function & ts type

declare function signature only once

Author: Meir017

src/models/User.ts

@@ -19,10 +19,12 @@ export type UserModel = mongoose.Document & {
     picture: string
   },
 
-  comparePassword: (candidatePassword: string, cb: (err: any, isMatch: any) => {}) => void,
+  comparePassword: comparePasswordFunction,
   gravatar: (size: number) => string
 };
 
+type comparePasswordFunction = (candidatePassword: string, cb: (err: any, isMatch: any) => {}) => void;
+
 export type AuthToken = {
   accessToken: string,
   kind: string
@@ -64,12 +66,13 @@ userSchema.pre("save", function save(next) {
   });
 });
 
-userSchema.methods.comparePassword = function (candidatePassword: string, cb: (err: any, isMatch: any) => {}) {
+const comparePassword: comparePasswordFunction = function (candidatePassword, cb) {
   bcrypt.compare(candidatePassword, this.password, (err: mongoose.Error, isMatch: boolean) => {
     cb(err, isMatch);
   });
 };
 
+userSchema.methods.comparePassword = comparePassword;
 
 /**
  * Helper method for getting user's gravatar.
@@ -87,4 +90,4 @@ userSchema.methods.gravatar = function (size: number) {
 
 // export const User: UserType = mongoose.model<UserType>('User', userSchema);
 const User = mongoose.model("User", userSchema);
-export default User;
+export default User;