Common Lodash Type Errors in JavaScript Strict Mode

Using the Lodash utility library within JavaScript's strict mode ("use strict";) often uncovers latent bugs that execute silently in standard execution contexts. This article explores the most common TypeError exceptions encountered when combining Lodash with strict mode, including read-only property mutations, improper execution contexts (this), chained wrapper mismanagement, and module import errors, providing clear explanations and targeted solutions for each.

1. Assignment to Read-Only Properties (_.set, _.assign, _.merge)

In non-strict mode, attempting to modify frozen objects, sealed objects, or properties with { writable: false } fails silently. In strict mode, JavaScript immediately throws a TypeError: Cannot assign to read only property.

"use strict";
const user = Object.freeze({ name: "Alex" });

// Throws TypeError: Cannot assign to read only property 'name' of object '#<Object>'
_.set(user, "name", "Jordan");

Lodash methods like _.set, _.assign, and _.merge attempt direct assignment on the target object. When working in strict mode with immutable or frozen data structures, create a shallow or deep clone prior to modification:

"use strict";
const user = Object.freeze({ name: "Alex" });
const updatedUser = _.set(_.cloneDeep(user), "name", "Jordan");

2. Undefined Context in Callback Functions (this)

Strict mode prevents the automatic binding of this to the global object (window or global). If a custom callback passed to a Lodash iterator relies on this without explicit binding, this defaults to undefined, frequently causing TypeError: Cannot read properties of undefined.

"use strict";
const processor = {
  multiplier: 2,
  process(numbers) {
    return _.map(numbers, function(n) {
      // In strict mode, 'this' is undefined here unless explicitly bound
      return n * this.multiplier; 
    });
  }
};

// Throws TypeError: Cannot read properties of undefined (reading 'multiplier')
processor.process([1, 2, 3]);

To resolve this, pass an arrow function (which lexically preserves this) or explicitly bind the method using _.bind:

"use strict";
processor.process = function(numbers) {
  return _.map(numbers, (n) => n * this.multiplier);
};

3. Calling Non-Functions Due to Improper Chaining

A frequent runtime issue occurs when developers mistake an explicit Lodash chain for a resolved value. If an operation expects an array or object but receives a Lodash wrapper object, downstream calls or property accesses fail with TypeError: [value] is not a function or unexpected undefined reads.

"use strict";
const data = [1, 2, 3, 4];

// Chained sequence without .value()
const result = _(data).filter(n => n % 2 === 0).map(n => n * 2);

// Throws TypeError: result.forEach is not a function
result.forEach(console.log);

Explicit chaining wraps the data inside a Lodash container. Always terminate the sequence with .value() to extract the native JavaScript type before passing it to native methods:

"use strict";
const result = _(data)
  .filter(n => n % 2 === 0)
  .map(n => n * 2)
  .value();

result.forEach(console.log); // Works as expected

4. ESM Named Import Destructuring (TypeError: (0, _lodash.xxx) is not a function)

Because strict mode is the default in ECMAScript modules (ESM), importing Lodash incorrectly surfaces during compilation or runtime as a TypeError. Lodash's primary distribution (lodash) uses CommonJS exports, which does not always support named destructuring depending on the bundler or Node.js environment.

// May result in: TypeError: (0 , _lodash.get) is not a function
import { get } from 'lodash'; 

To prevent this issue:

• Import from lodash-es if your environment natively supports ES Modules:

  import { get } from 'lodash-es';

• Alternatively, import the default object or specific submodule path directly:

  import _ from 'lodash';
  import get from 'lodash/get';

5. Attempting to Invoke Methods on Non-Object Targets (_.curry, _.debounce)

Passing an invalid argument to higher-order Lodash functions like _.debounce, _.throttle, or _.curry triggers a strict-mode TypeError: Expected a function. This occurs frequently when passing a reference that evaluates to undefined because of module loading orders, misspelled identifiers, or missing object keys.

"use strict";
const handlers = {};

// Throws TypeError: Expected a function
const debouncedHandler = _.debounce(handlers.onClick, 300);

Ensure the target function is defined prior to wrapping:

"use strict";
const handlers = {
  onClick: () => console.log("Clicked")
};

const debouncedHandler = _.debounce(handlers.onClick, 300);