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javascriptnode.jsarrays
WK123
WK123

Reputation: 630

NodeJS Array.length unusual behavior

I noticed something unusual today and wondered of anyone had any ideas of explanantion. I am testing the length of an array in NodeJS which is populated within a for loop. Something like the code snippet below.

// Set up an array to hold the IDS
var ids = []
// Iterate through each of the devices 
for (let i = 0; i < devices.length; i++) {
  let id = devices[i].deviceManufacturerCode
  if (id == qubino_id){
    ids[devices[i].label] = id
   }
}
console.log(ids,ids.length)

The output from the console.log after the for loop is iterated through is:

[ 'Qubino Energy Monitor': '0159-0007-0052' ] 0

The elements in the array are what I expected, a key value pair of the device.label and the id of the device. The length of 0 is not expected though as there is an entry in the array.

I changed the loop to append values instead:

// Set up an array to hold the IDS
var ids = []
// Iterate through each of the devices 
for (let i = 0; i < devices.length; i++) {
  let id = devices[i].deviceManufacturerCode
  if (id == qubino_id){
    ids.push(id) // CHANGE HERE 
   }
}
console.log(ids,ids.length)

The output from the console.log after the for loop is iterated through is now:

[ '0159-0007-0052' ] 1

The output of length has now increased to 1 which is expected and I have dropped the key from the array entry so everything is correct.

If I want to get the key, value object in the array and the length it increase I have to create the object first then push it to the array.

// Set up an array to hold the IDS
var ids = []
// Iterate through each of the devices 
for (let i = 0; i < devices.length; i++) {
  let id = devices[i].deviceManufacturerCode
  if (id == qubino_id){
    let name = devices[i].label
    let obj = {}
    obj[name] =  id  
    ids.push(obj)
   }
}
console.log(ids,ids.length)

The console.log now returns:

[ { 'Qubino Energy Monitor': '0159-0007-0052' } ] 1

As expected an object at index 0, making the array length of 1.

Why in the first case am I getting a length of 0 for the array?

Upvotes: 0

Views: 60

Answers (3)

MatthiasV
MatthiasV

Reputation: 71

That is a perfectly usual behaviour. Because in the first example you are not adding items to an array but adding a property to the "ids" object. If you would for example do something like this you would correctly index the array:

var arr = []
arr[1] = "foo"
console.log(arr.length)
// output: 2

Also see: Why does a string index in an array not increase the 'length'?

Upvotes: 0

Robert Moskal
Robert Moskal

Reputation: 22553

You are treating an array like a dictionary when you

ids[devices[i].label] = id

Check out the simplified snippet:

const devices = [{deviceManufacturerCode:'100-01', label: 'foo'}]

// Set up an array to hold the IDS
var ids = []
// Iterate through each of the devices 
for (let i = 0; i < devices.length; i++) {
  let id = devices[i].deviceManufacturerCode
  ids[devices[i].label] = id
}
console.log(ids,ids.length)

That console.log statement seems wrong, I don't see how you got that.

[ 'Qubino Energy Monitor': '0159-0007-0052' ]

Upvotes: 1

Anatoly
Anatoly

Reputation: 22758

Is label prop contains numbers? An array supposed to work with numbers internally converted to strings (just like keys in ordinary JS-object). Compare:

let array = []
arr[3] = 1 // arr.length = 4

let array = []
arr['abc'] = 2 // arr.length = 0

Upvotes: 2

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