angular 4 - read property from an array of objects

I want to read every rate property of my array. I don't know is there any method to call only $values from that rate record ??

I did this to call this array:

async ngOnInit() {
    this.product$ = await this.reviewService.getReview(this.product.$key);
    this.key = Object.values(this.product.reviews);
}

So you can use the map method to get an array of rates and then use reduce, I mean:

ngOnInit():void{
    // get an array of rates 
    const rates = yourArray.map(item => item.rate) // rates=[2,3,5,1,..]

    // use reduce function to calculate the sum
    const sum = rates.reduce(this.total)
} 



private total(total,num){
   return total + num
}