How to unshift new value with existing array with spread operator?

Question

I am trying to unshift an array value with existing array by using spread operator approach. getting error. any one help me to get correct approach?

here is my code :

uploadedFiles : [...state.uploadedFiles.unshift( (<fileActions.UploadFileSuccess>action).payload ) ]

getting an error as :

The Number is not array type

Thanks in advance.

Two quick notes: 1. I'm going to guess from the name that state is a React state object. If so, you must never directly modify it (which unshift does). 2. ... isn't an operator. :-) — T.J. Crowder
– T.J. Crowder, Commented Apr 5, 2019 at 9:43

Jonas Wilms · Accepted Answer · 2019-04-05 09:42:31Z

5

unshift mutates the array and returns the new length of the array, which is a number, and that can't be spreaded. Your code boils down to:

 [...state.uploadFiles.length]

Now to do that in an immutable and correct way, don't use unshift, just add the element to the array before spreading the previous version:

 [action.payload, ....state.uploadFiles]

answered Apr 5, 2019 at 9:42

Jonas Wilms

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2 Comments

user2024080 Over a year ago

the try i made to add the new object as first member of array. as like your suggestion it puts the new member as last one...

T.J. Crowder Over a year ago

@user2024080 - No, the above definitely makes action.payload the first entry in the resulting array.

Nina Scholz · Accepted Answer · 2019-04-05 09:41:47Z

3

Array#unshift returns the new length of the array.

You need just to take the item without spreading.

uploadedFiles: [(<fileActions.UploadFileSuccess>action).payload, ...state.uploadedFiles]

If you like to update state.uploadedFiles, you could unshift in advance.

state.uploadedFiles.unshift((<fileActions.UploadFileSuccess>action).payload);

uploadedFiles: [...state.uploadedFiles]

answered Apr 5, 2019 at 9:41

Nina Scholz

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