Tutorials

Lodash _.assignInWith() Object Method

Posted in lodash Tutorial

Updated on Apr 07, 2024

By Mari Selvan

👁️ 21 - Views

⏳ 4 mins

💬 1 Comment

Photo Credit to CodeToFun

🙋 Introduction

Effective manipulation and merging of objects are essential tasks in JavaScript development. Lodash, a powerful utility library, provides a variety of functions to streamline these operations. Among them, the _.assignInWith() method stands out as a versatile tool for merging multiple objects with customizable behavior.

This method enables developers to precisely control the merging process, making it a valuable asset in scenarios where fine-grained control over object assignment is crucial.

🧠 Understanding _.assignInWith() Method

The _.assignInWith() method in Lodash is designed for merging multiple source objects into a target object. What sets it apart is the ability to customize the assignment logic using a customizer function. This allows developers to define how conflicts are resolved and how values are assigned during the merging process.

💡 Syntax

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

syntax.js

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_.assignInWith(object, ...sources, customizer)

object: The target object.
...sources: The source objects to merge.
customizer: The function to customize the assignment behavior.

📝 Example

Let's dive into a simple example to illustrate the usage of the _.assignInWith() 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 customizer = (objValue, srcValue, key, object, source) => {
  // Custom logic for assignment
  return key === 'b' ? objValue * srcValue : undefined;
};

const mergedObject = _.assignInWith(targetObject, source1, source2, customizer);

console.log(mergedObject);
// Output: { a: 1, b: 6, c: 4, d: 5 }

In this example, the targetObject is merged with source1 and source2 using a customizer function that multiplies values when the key is 'b'.

🏆 Best Practices

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

Understand Customizer Function:

When using _.assignInWith(), thoroughly understand the customizer function as it determines how values are assigned during the merging process.

example.js

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

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

const customizer = (objValue, srcValue, key, object, source) => {
  // Custom logic for assignment
  return key === 'b' ? objValue * srcValue : undefined;
};

const mergedObject = _.assignInWith(targetObject, source, customizer);

console.log(mergedObject);
// Output: { a: 1, b: 6, c: 4 }

Handle Conflicts Carefully:

When merging objects with overlapping keys, be cautious about potential conflicts. The customizer function can help resolve conflicts based on your specific needs.

example.js

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const targetObject = {
  a: 1,
  b: 2,
  c: {
    d: 3
  }
};

const source = {
  b: 3,
  c: {
    e: 4
  }
};

const customizer = (objValue, srcValue, key) => {
  if(key === 'c') {
    // Resolve conflicts within nested objects
    return _.mergeWith({}, objValue, srcValue, customizer);
  }
};

const mergedObject = _.assignInWith(targetObject, source, customizer);

console.log(mergedObject);
// Output: { a: 1, b: 3, c: { d: 3, e: 4 } }

Use _.mergeWith() for Deep Merging:

For deep merging of nested objects, consider using _.mergeWith() within the customizer function.

example.js

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const targetObject = {
  a: 1,
  b: {
    c: 2
  }
};

const source = {
  b: {
    c: 3,
    d: 4
  }
};

const customizer = (objValue, srcValue) => {
  if(_.isObject(objValue)) {
    // Deep merge nested objects
    return _.mergeWith({}, objValue, srcValue, customizer);
  }
};

const mergedObject = _.assignInWith(targetObject, source, customizer);

console.log(mergedObject);
// Output: { a: 1, b: { c: 3, d: 4 } }

📚 Use Cases

Configurable Object Merging:

_.assignInWith() is excellent for scenarios where you need fine-grained control over how objects are merged. This is particularly useful when dealing with configuration objects.

example.js

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const defaultConfig = {
  timeout: 1000,
  retries: 3
};

const userConfig = {
  timeout: 500,
  delay: 200
};

const customizer = (objValue, srcValue, key) => {
  if(key === 'timeout') {
    // Use user-configured timeout if available
    return srcValue;
  }
};

const mergedConfig = _.assignInWith(defaultConfig, userConfig, customizer);

console.log(mergedConfig);
// Output: { timeout: 500, retries: 3, delay: 200 }

Dynamic Object Composition:

When dynamically composing objects based on various sources, _.assignInWith() allows you to control the composition logic.

example.js

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

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

const computedProps = {
  d: 5
};

const customizer = (objValue, srcValue, key) => {
  // Combine dynamic and computed properties
  return key === 'b' ? objValue + srcValue : undefined;
};

const composedObject = _.assignInWith(baseObject, dynamicProps, computedProps, customizer);

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

Merging Objects with Special Rules:

In scenarios where standard merging is insufficient and special rules apply, _.assignInWith() empowers you to define custom logic.

example.js

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

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

const customizer = (objValue, srcValue, key) => {
  // Special rule: If the key is 'b', concatenate values as strings
  return key === 'b' ? `${objValue}${srcValue}` : undefined;
};

const mergedObject = _.assignInWith(baseObject, source, customizer);

console.log(mergedObject);
// Output: { a: 1, b: '23', c: 4 }

🎉 Conclusion

The _.assignInWith() method in Lodash provides a powerful and flexible solution for merging objects in JavaScript. With its customizable assignment logic, it caters to a wide range of scenarios where precise control over object composition is essential.

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