The react native documentation for AsyncStorage.mergeItem() is a bit vague and provides no example how it works or how it’s used. Today, I figured it out.

Here’s their current description

Merges existing value with input value, assuming they are stringified json. Returns a Promise object. Not supported by all native implementations.

If you’re familiar with the Object.assign(), this is similar except it does a deep merge. Let me explain with an example.

 let KEY1 = 'test_merge'
 let KEY2 = 'test_assign' // I'll use this in the next example
 let ORIG = {
   'simple_value': 1,
   'dual_obj': { 'dual_obj_1': 1, 'dual_obj_2': 1 },
   'single_obj': {'single_obj_1': 1}

 let DELTA = {
   'dual_obj': { 'dual_obj_1': 2, 'dual_obj_new': 2 },
   'simple_value': 2,

 AsyncStorage.setItem(KEY1, JSON.stringify(ORIG), () => {
   AsyncStorage.mergeItem(KEY1, JSON.stringify(DELTA), () => {
     AsyncStorage.getItem(KEY1, (err, result) => {
       console.log('KEY1 result of merged object: %O', JSON.parse(result))

I’ve created two objects ORIG and DELTA. After setting the first item in local storage using AsyncStorage.setItem(), I then merge the DELTA object in. And here’s the result..


Notice that in “dual_obj” we’ve succeeded in modifying the “dual_obj_1” value. As well, we were able to add the “dual_obj_new” key.  We also updated the “simple_value”. That’s a deep merge.

I thought I’d get the same thing if I were to do an Object.assign(ORIG, DELTA), but I was wrong. I used KEY2 with the following code…

 AsyncStorage.setItem(KEY2, JSON.stringify(ORIG), () => {
   AsyncStorage.setItem(KEY2, JSON.stringify(Object.assign(ORIG,DELTA)), () => {
     AsyncStorage.getItem(KEY2, (err, result) => {
       console.log('KEY2 result of assigned object: %O', JSON.parse(result))

And ended up with the following output…


Notice the missing object.

If you found this helpful, give me a shout out on twitter (@geirmanc)


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