How Lodash _.result Handles Thrown Exceptions

This article examines how Lodash’s _.result method behaves when a resolved function throws an exception during execution. While developers frequently rely on _.result to safely traverse object paths and return fallback values for missing properties, its internal design does not intercept runtime errors thrown by invoked functions, meaning no fallback mechanism is triggered when an exception occurs.

The Internal Mechanics of _.result

The _.result utility is designed to resolve the value at a specified object path. If the resolved value is a function, Lodash automatically invokes it with the this binding set to its parent object and returns the resulting value. If the resolved value is undefined, Lodash returns the provided defaultValue instead.

Internally, Lodash processes the path segment by segment:

var value = object == null ? undefined : object[toKey(path[index])];
if (value === undefined) {
  index = length;
  value = defaultValue;
}
object = isFunction(value) ? value.call(object) : value;

The Fallback Behavior During Exceptions

When the targeted method deeply throws an exception, _.result executes no fallback mechanism.

Lodash does not wrap the function invocation (value.call(object)) in a try...catch block. Consequently:

  1. The default value is ignored: The defaultValue parameter only applies when a property key does not exist or strictly evaluates to undefined before invocation.
  2. The exception propagates immediately: Any error thrown inside the method escapes _.result entirely and bubbles up the call stack to the surrounding environment.
  3. Execution halts: If the calling code does not explicitly handle the error with an external try...catch block, the process or runtime script will terminate.

Handling Method Exceptions Safely

Because _.result does not catch errors thrown from within invoked properties, developers must pair it with explicit error-handling strategies if a function might fail during evaluation.

Using Lodash's _.attempt

To intercept exceptions and provide an actual fallback when functions throw, Lodash provides _.attempt:

const result = _.attempt(() => _.result(object, 'nested.method'));
const finalValue = _.isError(result) ? fallbackValue : result;

Native try...catch Blocks

Wrapping the _.result invocation in a standard try...catch statement ensures predictable recovery:

let value;
try {
  value = _.result(object, 'deeply.nested.failingMethod', fallbackValue);
} catch (error) {
  value = fallbackValue;
}

Relying on _.result provides protection solely against missing object properties, null values, and non-callable properties, but it leaves runtime exceptions thrown inside resolved methods completely unhandled by design.