Syntax, Parameter and Return Value
Syntax:
_.assignWith(target, ...sources, customizer)
Parameters:
-
target (Object): The destination object.
-
...sources (Object): The source objects.
- customizer (Function): The function used to customize assigned values.
Return Value:
(Object) - Returns the modified target object.
Examples
Customizing Property Assignment:
JavaScript
var _ = require('lodash');
function customizer(objValue, srcValue) {
if (_.isArray(objValue)) {
return objValue.concat(srcValue);
}
}
var object = { 'a': [1], 'b': [2] };
var other = { 'a': [3], 'b': [4] };
_.assignWith(object, other, customizer);
console.log(object);
You can also try this code with Online Javascript Compiler
Output:
{ 'a': [1, 3], 'b': [2, 4] }
Demonstrates using a customizer function to concatenate array values from source objects.
Handling Property Conflicts:
JavaScript
var target = { 'prop1': 'value1' };
var source = { 'prop1': 'value2', 'prop2': 'value3' };
_.assignWith(target, source, (objVal, srcVal) => {
return objVal ? objVal : srcVal;
});
console.log(target);
You can also try this code with Online Javascript Compiler
Output:
{ 'prop1': 'value1', 'prop2': 'value3' }
Shows how to handle property conflicts by preferring the target object's value.
Transforming Assigned Values:
JavaScript
var target = { 'x': 10, 'y': 20 };
var source = { 'x': 5, 'z': 15 };
_.assignWith(target, source, (objVal, srcVal) => {
return objVal * srcVal;
});
console.log(target);
You can also try this code with Online Javascript Compiler
Output:
{ 'x': 50, 'y': 20, 'z': 15 }
An example of transforming values during assignment by multiplying them.
Merging Objects with Priority:
JavaScript
var base = { 'name': 'John', 'age': 30 };
var changes = { 'name': 'Jane', 'age': 25 };
_.assignWith(base, changes, (baseVal, changeVal) => baseVal);
console.log(base);
You can also try this code with Online Javascript Compiler
Output:
{ 'name': 'John', 'age': 30 }
Demonstrates prioritizing the base object's properties over changes.
Frequently Asked Questions
How does _.assignWith() differ from _.assign()?
While _.assign() copies properties from source objects to the target object, _.assignWith() allows for a custom function to dictate how these values are assigned or merged.
What happens if the customizer returns undefined?
If the customizer function returns undefined, property assignments are handled by the method's default behavior.
Can _.assignWith() be used to deep merge objects?
_.assignWith() performs a shallow merge. For deep merging, a custom implementation or a different method like _.mergeWith() would be required.
Conclusion
Lodash's _.assignWith() method offers an advanced way to merge properties from source objects into a target object, with the flexibility of a customizer function. It's particularly valuable in complex scenarios where standard object merging is not sufficient and specific rules need to be applied for property assignment.
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