How can I add custom ranking attributes using the API Client?

To add custom ranking attributes via the API Client, 

1. First, you would need to specify the custom ranking attributes using the setSettings method. For example,

index.setSettings({
  customRanking:[
    'desc(popularity)',
    'asc(price)'
  ]
}).then(()=>{
  // done
});

You can see code examples here.

2. Then, you can pass the ‘custom’ criterion at the bottom in ranking.

ranking: [
  // the `asc` and `desc` modifiers must be placed at the top // if you are configuring an index for sorting purposes only
  'asc(attribute1)',
  'desc(attribute2)',
  'typo',
  'geo',
  'words',
  'filters',
  'proximity',
  'attribute',
  'exact',
  'custom'
]