Got a framer prototype with this handy function in it, tried writing my own, judging by my use-case it mimics Framer's modulate perfectly, however, I should to run it through some unit tests see if it always does. [feedback welcome]

modulate.js

// map a value from one range onto another, in this case scrolling
var st = _private.getScrollTop(),
    m1 = _private.modulate( st, [0, 500], [0,1] ),
    m2 = _private.modulate( st, [0, 400], [1,0] ),
    m3 = _private.modulate( st, [0, 300], [0,3] );


_private.modulate = function(val, range1, range2){
    var min1 = range1[0], max1 = range1[1],
        min2 = range2[0], max2 = range2[1],
        diff1 = 0, diff2 = 0,
        pct = 0, output = 0, dir = 1;
    
    if(min1 > max1){
        diff1 = min1 - max1;
    }else if(min1 < max1){
        diff1 = max1 - min1;
    }
    if(min2 > max2){
        diff2 = min2 - max2;
        dir = -1;
    }else if(min2 < max2){
        diff2 = max2 - min2;
    }

    pct = val * ( 100 / diff1);
    if(pct > 100){ pct = 100; }
    else if(pct < 0 ){ pct = 0; }

    output = ( pct / 100 ) * diff2;
    output = range2[0] + ( output * dir );

    return output;
};