static multiMerge(keyValuePairs, callback?)
Merges existing values with input values, assuming they are stringified json. Returns a Promise object.
Not supported by all native implementations.
Example:
// first user, initial values
let UID234_object = {
name: 'Chris',
age: 30,
traits: {hair: 'brown', eyes: 'brown'},
};
// first user, delta values
let UID234_delta = {
age: 31,
traits: {eyes: 'blue', shoe_size: 10},
};
// second user, initial values
let UID345_object = {
name: 'Marge',
