How to deep merge instead of shallow merge?
Both Object.assign and Object spread only do a shallow merge.
An example of the problem:
// No object nesting
const x = { a: 1 }
const y = { b: 1 }
const z = { ...x, ...y } // { a: 1, b: 1 }
The output is what you'd expect. However if I try this:
// Object nesting
const x = { a: { a: 1 } }
const y = { a: { b: 1 } }
const z = { ...x, ...y } // { a: { b: 1 } }
Instead of
{ a: { a: 1, b: 1 } }
you get
{ a: { b: 1 } }
x is completely overwritten because the spread operator only goes one level deep. This is the same with Object.assign()
.
Is there a way to do this?
Does anybody know if deep merging exists in the ES6/ES7 spec?
No, it does not.
I know this is a bit of an old issue but the easiest solution in ES2015/ES6 I could come up with was actually quite simple, using Object.assign(),
Hopefully this helps:
/**
* Simple object check.
* @param item
* @returns {boolean}
*/
export function isObject(item) {
return (item && typeof item === 'object' && !Array.isArray(item));
}
/**
* Deep merge two objects.
* @param target
* @param ...sources
*/
export function mergeDeep(target, ...sources) {
if (!sources.length) return target;
const source = sources.shift();
if (isObject(target) && isObject(source)) {
for (const key in source) {
if (isObject(source[key])) {
if (!target[key]) Object.assign(target, { [key]: {} });
mergeDeep(target[key], source[key]);
} else {
Object.assign(target, { [key]: source[key] });
}
}
}
return mergeDeep(target, ...sources);
}
Example usage:
mergeDeep(this, { a: { b: { c: 123 } } });
// or
const merged = mergeDeep({a: 1}, { b : { c: { d: { e: 12345}}}});
console.dir(merged); // { a: 1, b: { c: { d: [Object] } } }
You'll find an immutable version of this in the answer below.
Note that this will lead to infinite recursion on circular references. There's some great answers on here on how to detect circular references if you think you'd face this issue.
The problem is non-trivial when it comes to host objects or any kind of object that's more complex than a bag of values
Another thing to keep in mind: Object graphs that contain cycles. It's usually not difficult to deal with - simply keep a Set
of already-visited source objects - but often forgotten.
You probably should write a deep-merge function that only expects primitive values and simple objects - at most those types that the structured clone algorithm can handle - as merge sources. Throw if it encounters anything it cannot handle or just assign by reference instead of deep merging.
In other words, there is no one-size-fits-all algorithm, you either have to roll your own or look for a library method that happens to cover your use-cases.
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