Incorrect Array Length

Question

console.log(data) reveals the following:

(4) […]

0: "a19870e84cc92abeb5904fccd1f74329"

1: "12793d4229b32a0b1fc387e041ed7be8"

2: "12793d4229b32a0b1fc387e041ed7be8"

3: "5fec6cd9e7c78a4a985d1f5b82f05280"

length: 4

Yet, console.log(data.length) says the following:

..and indeed, I'm unable to loop through the array with a for loop because data.length = 0.

How can my array have 4 elements but have zero length all at once? I'm confused.

ps. the array gets produced via an async' redux function, here:

export const getFiles = () => {
  return async (dispatch: AppDispatch, getState: Function) => {

    const state = getState()
    const scriptAddress = state.chainInfo.data.scriptAddress

    var fileData: string[] = []

    api.cmd("coins;", function(respJSON: any) {

      if( api.util.checkAllResponses(respJSON) ) {

        //console.log(respJSON)
        const coins = respJSON[0].response.coins
        for ( let i = 0; i < coins.length; i++ ) {
          if (coins[i].data.coin.address == scriptAddress) {
            fileData.push(coins[i].data.prevstate[0].data)
          }
        }
      }
    })
  
    dispatch(write({data: fileData})(GetActionTypes.GET_SUCCESS))
  }
}

And it updates a react component via mapStateToProps - so it only gets the data after the async operation has updated the store via a reducer. So I don't 'think' it's an async problem.

And gosh - I should've mentioned all the react, redux, mapState stuff. Sorry!

Can you replicate this in a snippet here? It would be nice to see more of the code — Tasos, Oct 15 '20 at 10:48
Possibly related: [Is Chrome's JavaScript console lazy about evaluating arrays?](https://stackoverflow.com/questions/4057440/is-chromes-javascript-console-lazy-about-evaluating-arrays) — DBS, Oct 15 '20 at 10:53
@TasosBu - not easy to reproduce as the code's spread out amongst redux store stuff... — glowkeeper, Oct 15 '20 at 10:54
Just create a snippet and paste the contents of the data in it and print its length — Tasos, Oct 15 '20 at 10:55
@DBS - I'm experiencing the 'zero' behaviour in code, too (i.e. I'm unable to loop through the array). So I don't think it's a console thing... — glowkeeper, Oct 15 '20 at 10:57
Im guessing that the value of data changes after the console.log(data) — Tasos, Oct 15 '20 at 10:57
Asynchronous could be tricky, probably you are trying to make some operations before promise is resolved. More code will help with the investigation — Krzysztof Safjanowski, Oct 15 '20 at 10:58
Dispatch i called to early, I guess that api.cmd is asynchronous (with callback) but you are not waiting till results will be processed. Please move `dispatch` after for loop. — Krzysztof Safjanowski, Oct 15 '20 at 11:20
@KrzysztofSafjanowski dispatch IS called after the for loop and AFTER api.cmd has returned. — glowkeeper, Oct 15 '20 at 11:24
@glowkeeper mayby `await` is missing in above example. Current code tries to handle asynchronous in not proper way. — Krzysztof Safjanowski, Oct 15 '20 at 11:29
api.cmd is a promise - no need for await, so async' code is 'probably' correct. — glowkeeper, Oct 15 '20 at 11:32
But actually, @KrzysztofSafjanowski - you gave me an idea - and it's fixed! — glowkeeper, Oct 15 '20 at 11:39

score 1 · Answer 1 · answered Oct 15 '20 at 10:49

1

length is just an property, you are able to overwrite by mistake for example:

const a = [1, 2, 3, 4];
console.log('before: ', a.length)
a.length = 0;
console.log('after: ', a.length)

answered Oct 15 '20 at 10:49

Krzysztof Safjanowski

7,292
3
35
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Yes - and I'm accessing that property above - you'll see it says '4' when I log the data, but '0' when I log data.length. I promise you I am not overwriting it in between calls (at least, not intentionally). – glowkeeper Oct 15 '20 at 10:53
Can you verify that .length works as expected by running `console.log([1].length)` ? – Tasos Oct 15 '20 at 10:54
Also try and print data and data.length in the same console.log like this: `console.log(data, "length: ", data.length)` – Tasos Oct 15 '20 at 10:57
@TasosBu - I do that (`console.log(data, data.length)` - I just simplified for the example. It produces this: Array(4) [ {…}, {…}, {…}, {…} ] 0 – glowkeeper Oct 15 '20 at 11:14

score 0 · Answer 2 · answered Oct 15 '20 at 11:41

FIXED!

I moved the dispatch inside api.cmd, and data.length behaves correctly:

export const getFiles = () => {
  return async (dispatch: AppDispatch, getState: Function) => {

    const state = getState()
    const scriptAddress = state.chainInfo.data.scriptAddress

    var fileData: string[] = []

    api.cmd("coins;", function(respJSON: any) {

      if( api.util.checkAllResponses(respJSON) ) {

        //console.log(respJSON)
        const coins = respJSON[0].response.coins
        for ( let i = 0; i < coins.length; i++ ) {
          if (coins[i].data.coin.address == scriptAddress) {
            fileData.push(coins[i].data.prevstate[0].data)
          }
        }
      }
  
      dispatch(write({data: fileData})(GetActionTypes.GET_SUCCESS))
    })
  }
}

Boom! Thanks for all the comments...

Glad you solved your problem - we never really had a chance of helping you with the information you originally provided. Next time include the code causing the problem and we'll be more successful :) — Jamiec, Oct 15 '20 at 11:52

Incorrect Array Length

2 Answers2