Syntax, Parameter and Return Value
Syntax:
_.extendWith(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] };
_.extendWith(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' };
_.extendWith(target, source, (objVal, srcVal) => {
return objVal ? objVal : srcVal;
});
console.log(target);
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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 };
_.extendWith(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 };
_.extendWith(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 _.extendWith() differ from _.assignWith()?
While _.assignWith() only copies an object's own properties, _.extendWith() (or _.assignInWith()) copies both own and inherited properties from the prototype chain.
Is _.extendWith() suitable for deep merging of objects?
_.extendWith() performs a shallow merge. For deep merging, a method like _.mergeWith() should be used.
What happens in case of property name conflicts?
If multiple source objects have properties with the same name, _.extendWith() will overwrite the property in the target object with the last source's value for that property, unless the customizer function specifies otherwise.
Conclusion
Lodash's _.extendWith() method is a powerful tool for merging properties from source objects into a target object, including inherited properties, with the added 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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