30 Second Countdown Timer

Hi, I am trying to create a countdown that counts down from 30 seconds for my game.

This is my code:

var secondsLeft = 30;
this.countdown_txt.text = secondsLeft;
var timerId = setInterval(countdown, 1000);
function countdown(){
if(secondsLeft == 0){
     //doStuff
}
else{
     secondsLeft--;
     this.countdown_txt.text = secondsLeft;
     }
}

When I run this I get an error that occurs on line 11: Uncaught TypeError: Cannot set property 'text' of undefined.

I'm not sure why this is the case as I have a similar setup for counting the score when clicking on a movieclip and that works fine. Any help would be appreciated.