Counter for After Effects

counter.js

// Initial and last frames of an animation. 
var startKeyFrame = 30;
var endKeyFrame = 50;
// Initial and final values of a text layer.
var startValue = 500;
var endValue = 1000;

// Util vars for a calc function
var currentFrame = time/thisComp.frameDuration, 
    deltaKeyFrames = endKeyFrame - startKeyFrame,
    deltaValues = endValue - startValue,
    unit = deltaValues / deltaKeyFrames,
    deltaFrames = currentFrame - startKeyFrame;

// Create the calc function.
function counterFunc () {
	if (currentFrame <= startKeyFrame) {
		return text.sourceText = startValue;
	} else if (currentFrame > startKeyFrame && currentFrame < endKeyFrame) {
		return text.sourceText = Math.round(startValue + unit * deltaFrames);
	} else if (currentFrame >= endKeyFrame) {
		return text.sourceText = endValue;
	};
};

// Call the calc function.
counterFunc();