stateMock.js

angular.module('stateMock',[]);
angular.module('stateMock').service("$state", function($q){
    this.expectedTransitions = [];
    this.transitionTo = function(stateName){
        if(this.expectedTransitions.length > 0){
            var expectedState = this.expectedTransitions.shift();
            if(expectedState !== stateName){
                throw Error("Expected transition to state: " + expectedState + " but transitioned to " + stateName );
            }
        }else{
            throw Error("No more transitions were expected! Tried to transition to "+ stateName );
        }
        console.log("Mock transition to: " + stateName);
        return $q.when();
    }
    this.go = this.transitionTo;
    this.expectTransitionTo = function(stateName){
        this.expectedTransitions.push(stateName);
    }


    this.ensureAllTransitionsHappened = function(){
        if(this.expectedTransitions.length > 0){
            throw Error("Not all transitions happened!");
        }
    }
});

the same function don't work with es6
angular.module('stateMock',[])
angular.module('stateMock')
.service("$state", ($q) => {
this.expectedTransitions = []
this.transitionTo = (stateName) => {
if(this.expectedTransitions.length > 0){
let expectedState = this.expectedTransitions.shift()
if(expectedState !== stateName){
throw Error("Expected transition to state: " + expectedState + " but transitioned to " + stateName )
}
}else{
throw Error("No more transitions were expected! Tried to transition to "+ stateName )
}
let deferred = $q.defer()
let promise = deferred.promise
deferred.resolve()
return promise
}
this.go = this.transitionTo

    this.expectTransitionTo = (stateName) => {
        this.expectedTransitions.push(stateName)
    }


    this.ensureAllTransitionsHappened = () => {
        if(this.expectedTransitions.length > 0){
            throw Error("Not all transitions happened!")
        }
    }
})

Any idea please ??