mine/node_modules/three/examples/js/interactive/SelectionHelper.js

function SelectionHelper ( selectionBox, renderer, cssClassName ) {

	this.element = document.createElement( "div" );
	this.element.classList.add( cssClassName );
	this.element.style.pointerEvents = "none";

	this.renderer = renderer;

	this.startPoint = { x: 0, y: 0 };
	this.pointTopLeft = { x: 0, y: 0 };
	this.pointBottomRight = { x: 0, y: 0 };

	this.isDown = false;

	this.renderer.domElement.addEventListener( "mousedown", function ( event ) {
		
		this.isDown = true;
		this.onSelectStart( event );

	}.bind( this ), false );

	this.renderer.domElement.addEventListener( "mousemove", function ( event ) {

		if ( this.isDown ) {

			this.onSelectMove( event );

		}

	}.bind( this ), false );

	this.renderer.domElement.addEventListener( "mouseup", function ( event ) {

		this.isDown = false;
		this.onSelectOver( event );

	}.bind( this ), false );
	
}

SelectionHelper.prototype.onSelectStart = function ( event ) {
	
	this.renderer.domElement.parentElement.appendChild( this.element );

	this.element.style.left = event.clientX + "px";
	this.element.style.top = event.clientY + "px";
	this.element.style.width = "0px";
	this.element.style.height = "0px";

	this.startPoint.x = event.clientX;
	this.startPoint.y = event.clientY;

}

SelectionHelper.prototype.onSelectMove = function ( event ) {

	this.pointBottomRight.x = Math.max( this.startPoint.x, event.clientX );
	this.pointBottomRight.y = Math.max( this.startPoint.y, event.clientY );
	this.pointTopLeft.x = Math.min( this.startPoint.x, event.clientX );
	this.pointTopLeft.y = Math.min( this.startPoint.y, event.clientY );

	this.element.style.left = this.pointTopLeft.x + "px";
	this.element.style.top = this.pointTopLeft.y + "px";
	this.element.style.width = ( this.pointBottomRight.x - this.pointTopLeft.x ) + "px";
	this.element.style.height = ( this.pointBottomRight.y - this.pointTopLeft.y ) + "px";
	
}

SelectionHelper.prototype.onSelectOver = function ( event ) {

	this.element.parentElement.removeChild( this.element );
	
}