The Entity Query API binding is a javascript library for creating entity query querystrings. The interface tries to mimic the Drupal QueryInterface with few exceptions. It is meant to be used in tandem with the Entity Query API module.
var EntityQuery = require("drupal-api").entityQuery;
var query = new EntityQuery("node");
query
.condition("field_color", "red", "CONTAINS")
.orConditionGroup()
.condition("field_color", "blue", "CONTAINS")
.condition("field_color", "green", "CONTAINS");
query.sort("created", "DESC");
queryString = query.getQueryString();
// Outputs:
// condition_0[field]=field_color&condition_0[value]=red&condition_0[operator]=CONTAINS \
// &group_0[conjunction]=OR \
// &condition_00[field]=field_color&condition_00[value]=blue&condition_00[operator]=CONTAINS&condition_00[group]=group_0 \
// &condition_01[field]=field_color&condition_01[value]=green&condition_01[operator]=CONTAINS&condition_01[group]=group_0 \
// &sort_0[field]=created&sort_0[direction]=DESC