random expression executed once

I have the following expression on a text source:

var myArray = ["beans","rice","tacos","bbq"];
var i = Math.floor(random(myArray.length));
myArray[i]


I need a expression to execute one time and not every frame when the playhead plays through the comp. How can I run an expression to execute only at the beginning of a timeline and not on every frame? I don't see any triggers/events for detecting the first run of the timeline, for example an onEnterFrame handler function that actionscript utilizes.

thnx in advance

var myArray = ["beans","rice","tacos","bbq"];
var i = Math.floor(random(myArray.length));
myArray[i]