Lodash _.method: How Deep-Path Callbacks Work

The Lodash _.method utility creates a reusable function that invokes a method located at a specified property path on a target object. This article explains the internal mechanics of _.method, detailing how Lodash decomposes deep property paths, preserves the correct this execution context, applies predetermined arguments, and safely handles non-existent or non-callable properties.

The Purpose of _.method

In functional programming workflows, developers frequently need to invoke a method across collections of objects. The syntax for _.method is:

_.method(path, [args])

Instead of executing immediately, _.method returns a new function (a higher-order callback). When this returned function is passed an object, it navigates to the property specified by path and calls it as a function, forwarding any provided args.

Path Parsing and Navigation

The path argument can be passed as a dot-delimited string (e.g., 'profile.contact.getEmail') or as an array of property keys (e.g., ['profile', 'contact', 'getEmail']).

Internally, Lodash normalizes string paths into an array of keys using an internal path-parsing mechanism (similar to _.toPath). To invoke a method correctly, Lodash needs two components:

  1. The method itself: The function located at the final segment of the path.
  2. The parent object: The object directly containing that function, which serves as the receiver (this context) for the invocation.

Lodash traverses the target object sequentially through each key in the path up to the penultimate key. This traversal isolates the parent object holding the target method.

Context Preservation and Execution

A critical aspect of invoking methods on JavaScript objects is maintaining the correct this binding. Invoking a nested method without its parent context often results in this resolving to undefined or the global object.

When _.method resolves the parent context and the target function, it invokes the target using Function.prototype.apply or direct method invocation:

parentObject[methodKey].apply(parentObject, args);

By ensuring that parentObject acts as the context, any internal references to this within the resolved method function correctly.

Argument Application

Arguments supplied to _.method during its creation are partially applied. If you provide arguments when calling _.method(path, ...args), those arguments are stored in the returned closure. When the callback is subsequently executed on an object, it applies those predetermined arguments to the resolved method.

const objects = [
  { calculate: (factor) => 10 * factor },
  { calculate: (factor) => 20 * factor }
];

const multiplyByTwo = _.method('calculate', 2);
const results = objects.map(multiplyByTwo); // [20, 40]

Safe Invocation and Error Prevention

Standard JavaScript throws a TypeError if an intermediate property in a path is null or undefined, or if the resolved property is not a callable function.

Lodash safeguards against these runtime exceptions. If the path traversal encounters a non-object value before reaching the final key, or if the target property is not a function, the invocation is aborted, and the callback safely returns undefined.

Summary of Operation

When _.method(path, [args]) is called:

  1. It returns a closure that accepts a target object.
  2. Upon receiving the object, it parses the path and traverses the object tree to locate the method and its parent.
  3. It validates that the property exists and is a function.
  4. It calls the function with the parent object bound as this and passes along the predefined arguments.
  5. If any stage of the path is missing or non-callable, it safely returns undefined.