How to set variable types properly in a raw dataframe?

I wonder if someone can explain me how can i set type for each variable in my dataframe.

I made up a function to call quotes from yahoo API as JSON and i've rbind each of them into a dataframe called portfolio. Unfortunantely data are very raw and all comes as characters.

I'm asking you if there is a simple way in which r can automatically store values as numeric or character without setting each variable through the dataframe.

Here I paste you my script :

library(jsonlite)
# function that sets all NULL values to NA

nullToNA <- function(x) {
    x[sapply(x, is.null)] <- NA
    return(x)
}

# function that gets Quotes from Yahoo API

getQuote <- function(ticker) {
    path <- paste("https://query.yahooapis.com/v1/public/yqlq=select%20*%20from%20yahoo.finance.quotes%20where%20symbol%20in%20(%22",ticker,"%22)&format=json&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys&callback=", sep="")
    stock <- fromJSON(path, flatten = TRUE)
    q <- stock$query$results$quote
    q <- nullToNA(q)
}

# define the tickers to include in portfolio

tickers <- c("AAPL","GOOG", "AMZN") 

# portfolio construction

portfolio <- c()

for (ticker in tickers){
    portfolio <- rbind(portfolio, getQuote(ticker))
}

portfolio < unlist(portfolio)
portfolio <- data.frame(portfolio, stringsAsFactors = FALSE)

This leads to a dataframe of 3 obs and 83 variables but as i run str() on portfolio i see that each of them is a list instead of character or numeric. Is there a way to fix it?