shopify-api-question.js

function checkStatus(response) {
    if (response.status >= 200 && response.status < 300) {
      return response;
    } else {
      var error = new Error(response.statusText);
      error.response = response;
      throw error;
    }
  };

  function parseJSON(response) {
    return response.json();
  };
  
  var variant_id = 100;

fetch('/cart/add.js', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json'
    },
    credentials: 'same-origin',
    body: JSON.stringify({"id":variant_id,"quantity":1})
  })
  .then(checkStatus)
  .then(parseJSON)
  .then(function(json) {
    console.log(json);
  })
  .catch(function(error) {
    console.log('request failed', error);
  });