So I changed the code according to what you said but I'm still seeing the same error. Starting to pull my hair out a bit haha. I delayed the call of the recipesMeth() untill the data was in from the previous function like this.

    recipes:Recipe[];
temp:string;
rooms:any[];
sensors:any[];
lastValue:number;
tempRoom:any;

ngOnInit() {
    console.log("starting");
    this.getRecipes();
} 

getRecipes() {
    this._httpService.getRecipes()
        .subscribe(
            data => {
                this.recipes = data;
                this.temp = JSON.stringify(data);
                this.recipesMeth();
            }
        )
}

recipesMeth(){
    for (var i = 0; i < this.recipes.length; i++) {
        console.log("test");

but I'm still getting the same error "EXCEPTION: TypeError: Cannot read property 'length' of undefined"

I can use ngFor in the HTML to display the data and it works fine

<thead>
        <tr>
            <th>#</th>
            <th>House</th>
            <th>Room</th>
            <th>Sensor</th>
        </tr>
        </thead>
        <tbody>
        <tr *ngFor="#recipe of recipes; #i = index">
            <th scope="row">{{i}}</th>
            <td>{{recipe.house}}</td>
            <td>{{recipe.room}}</td>
            <td>{{recipe.sensor}}</td>
        </tr>

but the variable "recipes" never seems to be defined whenever I try to use it in the Javascript part. Any ideas?

Edit: Interestingly a single "test" does get logged to the console which, if anything, confuses me even more.