Handling Dynamic Keys in Lodash groupBy

Lodash’s _.groupBy method processes a collection by executing an iteratee function for each element, aggregating the results into an object where each key represents a group. When dealing with dynamic object keys—such as computed property names, varying keys across elements, or keys mutated at runtime—the iteratee resolves these values strictly on an item-by-item basis at the precise moment of execution. This article examines how the iteratee resolves dynamic properties, how JavaScript's evaluation model impacts grouping results, and the best practices for handling mutating or inconsistent object keys.

Synchronous Evaluation and Execution Context

Lodash executes _.groupBy synchronously over the collection. For every item, the iteratee function receives (value). If your data structure relies on dynamic keys, the iteratee must resolve the target property dynamically rather than using Lodash's string property shorthand (like _.groupBy(collection, 'key')).

When passing a custom function as the iteratee, the dynamic key can be read using bracket notation:

const dynamicKey = getActiveKeyName(); // returns a string dynamically, e.g., 'category_2024'

const grouped = _.groupBy(items, (item) => {
  return item[dynamicKey];
});

Because the iteratee is called once per item, any change to the variable holding the key name inside the iteratee or within the outer scope affects subsequent items. The key assigned in the returned output object is determined entirely by the return value of that specific call.

Handling Varying Keys Across Heterogeneous Objects

In datasets where objects do not share uniform schemas, the target key might change from one element to the next. The iteratee can compute or locate the relevant key dynamically per object:

const data = [
  { typeA_id: 'alpha', value: 10 },
  { typeB_id: 'beta', value: 20 },
  { typeA_id: 'alpha', value: 30 }
];

const grouped = _.groupBy(data, (item) => {
  // Dynamically resolve which identifier key exists on the object
  const dynamicKey = Object.keys(item).find(k => k.endsWith('_id'));
  return item[dynamicKey];
});

In this scenario:

1. The iteratee inspects the keys of the current item.
2. It extracts the value associated with the matching dynamic property name.
3. Lodash places the element in the bucket labeled by that resolved value (e.g., 'alpha' or 'beta').

Mutation During Iteration

Lodash does not deep-clone collections before processing. If an asynchronous process, side effect, or the iteratee itself mutates an object's keys while _.groupBy is running, the grouping reflects the object's state at the exact time it was evaluated:

• Pre-iteration changes: If an object’s key is modified before the loop reaches that object, the iteratee reads the updated property.
• Post-iteration changes: Modifying an object after it has been categorized will not move it to a different group. The output object maintains references to the original objects in their already-assigned arrays.
• Missing or deleted keys: If dynamic key resolution yields undefined, Lodash converts undefined to the string key 'undefined' in the output object.

Type Coercion of Group Keys

All return values from the iteratee are coerced to strings to serve as object keys in the final output dictionary. If dynamic logic produces non-string identifiers (such as numbers, booleans, or objects), standard JavaScript string coercion rules apply:

const items = [{ id: 1 }, { id: 2 }];
const grouped = _.groupBy(items, (item) => item.id);
// Resulting keys will be strings: { "1": [...], "2": [...] }

When relying on dynamically changing keys, ensure the iteratee outputs a deterministic primitive value to prevent unintended collisions under generic keys like '[object Object]' or 'undefined'.