Array becomes an Integer after applying unshift on

问题:

I have an Object like this:

{
  "index": 0,
  "name": "b1a042ff6-0c75-4af2-a9da-1a16f333baee_p0",
  "category": "others",
  "rawUrl": "https://firebasestorage.googleapis.com/v0/b/vrcam-dev-5a815.appspot.com/o/5ab4f2a0-88e9-4bf5-86b5-61b528be707f/panoramas/panorama_XTsagLoxbA.png?alt=media&token=68ef261e-0c5e-4bf0-aebc-ab845fcec01a",
  "isTopLogo": false,
  "origin": "https://firebasestorage.googleapis.com/v0/b/vrcam-dev-5a815.appspot.com/o/5ab4f2a0-88e9-4bf5-86b5-61b528be707f/panoramas/panorama_XTsagLoxbA.png?alt=media&token=68ef261e-0c5e-4bf0-aebc-ab845fcec01a",
  "position": {
    "x": 0,
    "y": 0
  },
  "panoramaRotation": {
    "x": 0,
    "y": 0,
    "z": 0
  }
}

Which I want to unshift to an Array (it's cloned version) like this:

[{
  "adjustedRawUrl": "",
  "category": "others",
  "createdAt": 1514432540000,
  "cubemapReady": false,
  "desktopUrl": "https://im.ages.io/BGOLielt?cors&w=513",
  "floorplanRotation": 0,
  "index": 0,
  "is720": false,
  "isTopLogo": false,
  "mobileUrl": "https://im.ages.io/BGOLielt?cors&w=4096",
  "name": "b1a042ff6-0c75-4af2-a9da-1a16f333baee_p0",
  "objectId": "3c312986-0ef1-42fc-9068-46fc13a04b8f",
  "panoramaRotation": {
    "x": 0,
    "y": 0,
    "z": 0
  },
  "position": {
    "x": 0,
    "y": 0
  },
  "rawUrl": "https://firebasestorage.googleapis.com/v0/b/vrcam-dev-5a815.appspot.com/o/5ab4f2a0-88e9-4bf5-86b5-61b528be707f/panoramas/panorama_lPea0mIc6H.png?alt=media&token=9e1494ff-2525-42a6-a058-12c26560349a",
  "stereoUrl": "",
  "thumbnail": "https://im.ages.io/BGOLielt?cors&w=400",
  "updatedAt": 1514432619000
}, {
  "adjustedRawUrl": "",
  "category": "others",
  "createdAt": 1514432231000,
  "cubemapReady": false,
  "desktopUrl": "https://im.ages.io/FK9uiel2?cors&w=251",
  "floorplanRotation": 0,
  "index": 1,
  "is720": false,
  "isTopLogo": false,
  "mobileUrl": "https://im.ages.io/FK9uiel2?cors&w=4096",
  "name": "b1a042ff6-0c75-4af2-a9da-1a16f333baee_p0",
  "objectId": "08e9197c-ab27-48d8-a48d-b33452fcfd11",
  "panoramaRotation": {
    "x": 0,
    "y": 0,
    "z": 0
  },
  "position": {
    "x": 0,
    "y": 0
  },
  "rawUrl": "https://firebasestorage.googleapis.com/v0/b/vrcam-dev-5a815.appspot.com/o/5ab4f2a0-88e9-4bf5-86b5-61b528be707f/panoramas/panorama_pMUnnhYmpH.png?alt=media&token=e422e4f1-389b-43b7-927b-022b31e37d61",
  "stereoUrl": "",
  "thumbnail": "https://im.ages.io/FK9uiel2?cors&w=400",
  "updatedAt": 1514432619000
}]

This is how I'm doing it:

const newClonedArray = JSON.parse(JSON.stringify(array)).unshift(object)

To my shock the result wasn't an Array but an Integer: 3.

Why is this? And how to fix it?

回答1:

array.unshift() returns length of the modified array. It does not create a new array instead modifies the existing array. Thus you do not need to use any assignment here.

var object ={
  "index": 0,
  "name": "b1a042ff6-0c75-4af2-a9da-1a16f333baee_p0",
  "category": "others",
  "rawUrl": "https://firebasestorage.googleapis.com/v0/b/vrcam-dev-5a815.appspot.com/o/5ab4f2a0-88e9-4bf5-86b5-61b528be707f/panoramas/panorama_XTsagLoxbA.png?alt=media&token=68ef261e-0c5e-4bf0-aebc-ab845fcec01a",
  "isTopLogo": false,
  "origin": "https://firebasestorage.googleapis.com/v0/b/vrcam-dev-5a815.appspot.com/o/5ab4f2a0-88e9-4bf5-86b5-61b528be707f/panoramas/panorama_XTsagLoxbA.png?alt=media&token=68ef261e-0c5e-4bf0-aebc-ab845fcec01a",
  "position": {
    "x": 0,
    "y": 0
  },
  "panoramaRotation": {
    "x": 0,
    "y": 0,
    "z": 0
  }
}

var array = [];

array.unshift(object);

console.log(array);

回答2:

The docs explain

The unshift() method adds one or more elements to the beginning of an array and returns the new length of the array.

Maybe instead of assigning the result of unshift, do

const newClonedArray = JSON.parse(JSON.stringify(array));
newClonedArray.unshift(object);