Fabric.js inner shadow (improvise)

Demo/sample:

Explanation

// -----
const createObjectInnerShadow = () => {
let offset = 10;
let object = canvas.getActiveObject();
// set first object
    if( object === undefined || object === null ){
      window.alert('add and select a object');
      return;
    }
const setAngle = ( elements, angle, multiplier ) => {
      if( angle > 0 ){
        const group = new fabric.Group(elements);
        group.rotate( angle === undefined ? 0 : angle * multiplier ); // 1|-1
        group.destroy()
        canvas.renderAll();
      }
    }
// step 1
const { angle, scaleX, scaleY } = object;
    // set to angle = 0 (reset)
    setAngle( [object], angle, -1 );
    // set to scale 1 (reset)
    if( scaleX != 1 || scaleY != 1 ){
      offset = 0;        
      object.set({
        scaleX: 1,
        scaleY: 1,
      });
    }
// step 2
const { width, height, left, top } = object;
    const rect = new fabric.Rect({
      typeName: 'Rect',
      width: width + ( offset * 2 ), 
      height: height + ( offset * 2 ), 
      left: left - offset,
      top: top - offset,
      fill: "black",
      shadow: {
        color: "rgba(0,0,0,1)",
        blur: 10,
        offsetX: 0,
        offsetY: 0,
      },
    });
// step 3 - apply inverted mask
var clipOfObject = clone( object );
    rect.clipPath = clipOfObject;
    rect.clipPath.set({
      typeName: 'ClipPath',
      inverted: true, // THE MAGIC
      left: ( rect.clipPath.width / 2 ) * -1,
      top: ( rect.clipPath.height / 2 ) * -1,
    });
  
    // step 4 - apply another mask to maker inner shadow
var clipOfObjectFinal = clone( object );
    const clipFinal = new fabric.Group([rect]);
    clipFinal.typeName = 'GroupShadow';
    clipFinal.clipPath = clipOfObjectFinal;
    clipFinal.clipPath.set({
      typeName: 'ClipPath',
      left: ( rect.clipPath.width / 2 ) * -1,
      top: ( rect.clipPath.height / 2 ) * -1,
    });
// add
    canvas.add(clipFinal);
// set back scale
    if( scaleX != 1 || scaleY != 1 ){
      [object,clipFinal].map( (el) => {
        el.set({
          scaleX: scaleX,
          scaleY: scaleY,
        });
      });
    }
    // set back angle
    setAngle( [object,clipFinal], angle, 1 );
// render all
    canvas.renderAll();
}
const clone = (object) => {
if (object === undefined || object === null || typeof object !== "object")
      return object
    else
    return Object.assign(Object.create(Object.getPrototypeOf(object)), object);
}

See demo: https://github.com/natancabral/fabric-js-inner-shadow