Function interpolate

esm/gsap-core.js:888–945

(start, end, progress, mutate)

  });
},
    interpolate = function interpolate(start, end, progress, mutate) {
  var func = isNaN(start + end) ? 0 : function (p) {
    return (1 - p) * start + p * end;
  };

  if (!func) {
    var isString = _isString(start),
        master = {},
        p,
        i,
        interpolators,
        l,
        il;

    progress === true && (mutate = 1) && (progress = null);

    if (isString) {
      start = {
        p: start
      };
      end = {
        p: end
      };
    } else if (_isArray(start) && !_isArray(end)) {
      interpolators = [];
      l = start.length;
      il = l - 2;

      for (i = 1; i < l; i++) {
        interpolators.push(interpolate(start[i - 1], start[i])); //build the interpolators up front as a performance optimization so that when the function is called many times, it can just reuse them.
      }

      l--;

      func = function func(p) {
        p *= l;
        var i = Math.min(il, ~~p);
        return interpolators[i](p - i);
      };

      progress = end;
    } else if (!mutate) {
      start = _merge(_isArray(start) ? [] : {}, start);
    }

    if (!interpolators) {
      for (p in end) {
        _addPropTween.call(master, start, p, "get", end[p]);
      }

      func = function func(p) {
        return _renderPropTweens(p, master) || (isString ? start.p : start);
      };
    }
  }

  return _conditionalReturn(progress, func);
},