Tutorials

Lodash _.assignWith() Object Method

Posted in lodash Tutorial

Updated on Mar 13, 2024

By Mari Selvan

👁️ 18 - Views

⏳ 4 mins

💬 1 Comment

Photo Credit to CodeToFun

🙋 Introduction

In the vast landscape of JavaScript, efficient handling of objects is crucial for developers. Lodash, a feature-rich utility library, provides a plethora of functions to simplify object manipulation. One such indispensable tool is the _.assignWith() method. This method allows developers to merge multiple source objects into a target object while providing a customizer function to control the merging process.

Let's delve into the details of _.assignWith() and explore its applications in JavaScript development.

🧠 Understanding _.assignWith() Method

The _.assignWith() method in Lodash is designed for merging multiple source objects into a target object. What sets it apart is the ability to customize the merging behavior using a customizer function. This function gives developers fine-grained control over how conflicts in key-value pairs are resolved during the merge.

💡 Syntax

The syntax for the _.assignWith() method is straightforward:

syntax.js

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_.assignWith(object, sources, customizer)

object: The target object.
sources: The source objects to merge.
customizer: The function to customize assigned values.

📝 Example

Let's dive into a simple example to illustrate the usage of the _.assignWith() method:

example.js

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const _ = require('lodash');

const targetObject = { a: 1, b: 2 };
const source1 = { b: 3, c: 4 };
const source2 = { d: 5 };

const mergedObject = _.assignWith(targetObject, source1, source2, (objValue, srcValue) => objValue + srcValue);

console.log(mergedObject);
// Output: { a: 1, b: 5, c: 4, d: 5 }

In this example, targetObject is merged with source1 and source2. The customizer function is used to sum the values for common keys, resulting in a merged object.

🏆 Best Practices

When working with the _.assignWith() method, consider the following best practices:

Understand Customizer Function:

Comprehend the role of the customizer function in resolving conflicts during the merge. The customizer function takes four arguments: (objValue, srcValue, key, object).

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const customMerge = (objValue, srcValue, key) => {
  if(key === 'priority') {
    return Math.max(objValue, srcValue);
  }
  return undefined; // Default behavior for non-'priority' keys
};

const mergedWithCustomizer = _.assignWith({
  priority: 2
}, {
  priority: 5,
  status: 'active'
}, customMerge);

console.log(mergedWithCustomizer);
// Output: { priority: 5, status: 'active' }

Handle Nested Objects:

When dealing with nested objects, ensure that the customizer function appropriately handles the merging process for nested structures.

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const targetNested = {
  a: {
    b: 2
  }
};

const sourceNested = {
  a: {
    c: 3
  }
};

const mergedNested = _.assignWith(targetNested, sourceNested, (objValue, srcValue) => {
  if(_.isObject(objValue)) {
    return _.assignWith(objValue, srcValue, (nestedObj, nestedSrc) => nestedObj + nestedSrc);
  }
  return undefined;
});

console.log(mergedNested);
// Output: { a: { b: 2, c: 3 } }

Maintain Immutability:

If maintaining immutability is a concern, consider using the spread operator or other techniques to create a new object instead of modifying the target object in-place.
example.js
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```
const targetImmutable = { a: 1 };
const sourceImmutable = { b: 2 };

const mergedImmutable = { ...targetImmutable, ...sourceImmutable };

console.log(mergedImmutable);
// Output: { a: 1, b: 2 }
```

📚 Use Cases

Conflict Resolution:

_.assignWith() is particularly useful when merging objects with conflicting keys. The customizer function allows you to define rules for resolving conflicts.

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const targetConflicts = {
  a: 1,
  b: 2,
  priority: 3
};

const sourceConflicts = {
  b: 4,
  priority: 5,
  status: 'active'
};

const resolvedConflicts = _.assignWith(targetConflicts, sourceConflicts, (objValue, srcValue, key) => {
  if(key === 'priority') {
    return Math.max(objValue, srcValue);
  }
  return undefined; // Default behavior for non-'priority' keys
});

console.log(resolvedConflicts);
// Output: { a: 1, b: 4, priority: 5, status: 'active' }

Custom Merging Logic:

When standard merging logic is insufficient, the customizer function empowers you to implement complex merging strategies tailored to your specific use case.

example.js

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const targetCustom = {
  a: 1,
  data: {
    x: 2
  }
};

const sourceCustom = {
  b: 3,
  data: {
    y: 4
  }
};

const customMergeLogic = (objValue, srcValue, key) => {
  if(key === 'data') {
    return {
      ...objValue,
      ...srcValue
    }; // Merge nested objects with spread operator
  }
  return undefined; // Default behavior for other keys
};

const mergedWithCustomLogic = _.assignWith(targetCustom, sourceCustom, customMergeLogic);

console.log(mergedWithCustomLogic);
// Output: { a: 1, b: 3, data: { x: 2, y: 4 } }

Configuring Default Values:

_.assignWith() can be used to merge objects while providing default values for missing keys.

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const targetDefaults = {
  a: 1,
  b: 2
};

const sourceDefaults = {
  b: 3,
  c: 4
};

const mergedWithDefaults = _.assignWith(targetDefaults, sourceDefaults, (objValue, srcValue) => {
  if(_.isUndefined(objValue)) {
    return srcValue;
  }
  return objValue;
});

console.log(mergedWithDefaults);
// Output: { a: 1, b: 2, c: 4 }

🎉 Conclusion

The _.assignWith() method in Lodash is a versatile tool for merging objects with the added flexibility of a customizer function. Whether you're resolving conflicts, implementing custom merging logic, or configuring default values, this method provides a powerful solution for object manipulation in JavaScript.

By adhering to best practices and exploring diverse use cases, you can harness the full potential of the _.assignWith() method in your Lodash projects.