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var calc_distance = function(x1,x2){
    return (x1 > x2) ? x1 - x2 : x2 - x1; 
    }

var p1 = {x:10,y:20};
var p2 = {x:50,y:100};
var p3 = {x:30,y:null};

var d = calc_distance(p1.x, p2.x); // calc whole distance
var d1 = calc_distance(p1.x, p3.x); // calc distance to p3
var blend = ((100/d) * d1) / 100;// procentual value between p1 and p2

//~ var x = p1.x + blend * (p2.x - p1.x);
var y = p1.y + blend * (p2.y - p1.y);
p3.y = y;