function UVsDebug( geometry, size = 1024 ) {

		// handles wrapping of uv.x > 1 only
		const abc = 'abc';
		const a = new THREE.Vector2();
		const b = new THREE.Vector2();
		const uvs = [ new THREE.Vector2(), new THREE.Vector2(), new THREE.Vector2() ];
		const face = [];
		const canvas = document.createElement( 'canvas' );
		const width = size; // power of 2 required for wrapping

		const height = size;
		canvas.width = width;
		canvas.height = height;
		const ctx = canvas.getContext( '2d' );
		ctx.lineWidth = 1;
		ctx.strokeStyle = 'rgb( 63, 63, 63 )';
		ctx.textAlign = 'center'; // paint background white

		ctx.fillStyle = 'rgb( 255, 255, 255 )';
		ctx.fillRect( 0, 0, width, height );

		if ( geometry.isGeometry ) {

			console.error( 'THREE.UVsDebug no longer supports Geometry. Use THREE.BufferGeometry instead.' );
			return;

		} else {

			const index = geometry.index;
			const uvAttribute = geometry.attributes.uv;

			if ( index ) {

				// indexed geometry
				for ( let i = 0, il = index.count; i < il; i += 3 ) {

					face[ 0 ] = index.getX( i );
					face[ 1 ] = index.getX( i + 1 );
					face[ 2 ] = index.getX( i + 2 );
					uvs[ 0 ].fromBufferAttribute( uvAttribute, face[ 0 ] );
					uvs[ 1 ].fromBufferAttribute( uvAttribute, face[ 1 ] );
					uvs[ 2 ].fromBufferAttribute( uvAttribute, face[ 2 ] );
					processFace( face, uvs, i / 3 );

				}

			} else {

				// non-indexed geometry
				for ( let i = 0, il = uvAttribute.count; i < il; i += 3 ) {

					face[ 0 ] = i;
					face[ 1 ] = i + 1;
					face[ 2 ] = i + 2;
					uvs[ 0 ].fromBufferAttribute( uvAttribute, face[ 0 ] );
					uvs[ 1 ].fromBufferAttribute( uvAttribute, face[ 1 ] );
					uvs[ 2 ].fromBufferAttribute( uvAttribute, face[ 2 ] );
					processFace( face, uvs, i / 3 );

				}

			}

		}

		return canvas;

		function processFace( face, uvs, index ) {

			// draw contour of face
			ctx.beginPath();
			a.set( 0, 0 );

			for ( let j = 0, jl = uvs.length; j < jl; j ++ ) {

				const uv = uvs[ j ];
				a.x += uv.x;
				a.y += uv.y;

				if ( j === 0 ) {

					ctx.moveTo( uv.x * ( width - 2 ) + 0.5, ( 1 - uv.y ) * ( height - 2 ) + 0.5 );

				} else {

					ctx.lineTo( uv.x * ( width - 2 ) + 0.5, ( 1 - uv.y ) * ( height - 2 ) + 0.5 );

				}

			}

			ctx.closePath();
			ctx.stroke(); // calculate center of face

			a.divideScalar( uvs.length ); // label the face number

			ctx.font = '18px Arial';
			ctx.fillStyle = 'rgb( 63, 63, 63 )';
			ctx.fillText( index, a.x * width, ( 1 - a.y ) * height );

			if ( a.x > 0.95 ) {

				// wrap x // 0.95 is arbitrary
				ctx.fillText( index, a.x % 1 * width, ( 1 - a.y ) * height );

			} //


			ctx.font = '12px Arial';
			ctx.fillStyle = 'rgb( 191, 191, 191 )'; // label uv edge orders

			for ( let j = 0, jl = uvs.length; j < jl; j ++ ) {

				const uv = uvs[ j ];
				b.addVectors( a, uv ).divideScalar( 2 );
				const vnum = face[ j ];
				ctx.fillText( abc[ j ] + vnum, b.x * width, ( 1 - b.y ) * height );

				if ( b.x > 0.95 ) {

					// wrap x
					ctx.fillText( abc[ j ] + vnum, b.x % 1 * width, ( 1 - b.y ) * height );

				}

			}

		}

	}
tool for "unwrapping" and debugging three.js geometries UV mapping

Sample usage:
	document.body.appendChild( UVsDebug( new THREE.SphereGeometry( 10, 10, 10, 10 ) );