intersect-distance.js

var poly = {
      "type": "Feature",
      "properties": {},
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [
              51.17431640625,
              47.025206001585396
            ],
            [
              45.17578125,
              43.13306116240612
            ],
            [
              54.5361328125,
              41.85319643776675
            ],
            [
              51.17431640625,
              47.025206001585396
            ]
          ]
        ]
      }
    }
var line = {
      "type": "Feature",
      "properties": {},
      "geometry": {
        "type": "LineString",
        "coordinates": [
          [
            42.78076171875,
            46.01222384063236
          ],
          [
            56.865234375,
            44.26093725039923
          ]
        ]
      }
    }


var intersection = turf.lineDistance(turf.intersect(poly, line), 'miles');

//=intersection