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What does the syntax x:y mean in JavaScript?

I am following a course on blockchain which has the following piece of code.
What does " index:this.chain.length+1 " mean? Is index a variable in the object newBlock? Or is it a key value pair? If it is a variable, why don't we simply use index=this.chain.length+1? Also what is the type of the object newBlock?
function Blockchain()
{
this.chain=[];
this.newTranscations=[];
}
Blockchain.prototype.createNeBlock = function(nonce,previousBlockHash,hash)
{
const newBlock ={
index:this.chain.length+1,
timestamp:Date.now(),
// all of the transactions in this block will be the transactions that waiting to be put in a block
transactions:this.newTranscations,
// nonce is hust a number giving proof of the transaction
nonce:nonce,
hash:hash,
previousBlockHash: previousBlockHash
}
// As we move all the pending transactions to the new block, we clear this array
this.newTranscations=[];
this.chain.push(newBlock);
return newBlock;
}

var Box = {
"playdoh":{"playdoh":["none", "some", "none", "none", "some"]}
};
Box of playdoh upon playdoh, you're getting into the study of Objects/Arrays/Maps.
To call the above out, it'd be
console.log(Box["playdoh"]["playdoh"][0]);
= none
console.log(Box["playdoh"]["playdoh"][4]);
= some
console.log(Box["playdoh"]["playdoh"][5]);
= null (undefined)
is the same as
console.log(Box.playdoh.playdoh[0]);
= none
console.log(Box.playdoh.playdoh[4]);
= some
console.log(Box.playdoh.playdoh[5]);
= null (undefined)

It is one of several ways to initialize an object called newBlock in javascript. Take a look at this documentation on MDN
The index property is of type number in this case, and it is set to equal chain[].length + 1

Javascript's method forEach() creates array with undefined keys

I am building a simple todo app, and I'm trying to get the assigned users for each task. But let's say that in my database, for some reason, the tasks id starts at 80, instead of starting at 1, and I have 5 tasks in total.
I wrote the following code to get the relationship between user and task, so I would expect that at the end it should return an array containing 5 keys, each key containing an array with the assigned users id to the specific task.
Problem is that I get an array with 85 keys in total, and the first 80 keys are undefined.
I've tried using .map() instead of .forEach() but I get the same result.
let assignedUsers = new Array();
this.taskLists.forEach(taskList => {
taskList.tasks.forEach(task => {
let taskId = task.id;
assignedUsers[taskId] = [];
task.users.forEach(user => {
if(taskId == user.pivot.task_id) {
assignedUsers[taskId].push(user.pivot.user_id);
}
});
});
});
return assignedUsers;
I assume the issue is at this line, but I don't understand why...
assignedUsers[taskId] = [];
I managed to filter and remove the empty keys from the array using the line below:
assignedUsers = assignedUsers.filter(e => e);
Still, I want to understand why this is happening and if there's any way I could avoid it from happening.
Looking forward to your comments!

If your taskId is not a Number or autoconvertable to a Number, you have to use a Object. assignedUsers = {};

This should work as you want it to. It also uses more of JS features for the sake of readability.
return this.taskLists.reduce((acc, taskList) => {
taskList.tasks.forEach(task => {
const taskId = task.id;
acc[taskId] = task.users.filter(user => taskId == user.pivot.task_id);
});
return acc;
}, []);
But you would probably want to use an object as the array would have "holes" between 0 and all unused indexes.

Your keys are task.id, so if there are undefined keys they must be from an undefined task id. Just skip if task id is falsey. If you expect the task id to possibly be 0, you can make a more specific check for typeof taskId === undefined
this.taskLists.forEach(taskList => {
taskList.tasks.forEach(task => {
let taskId = task.id;
// Skip this task if it doesn't have a defined id
if(!taskId) return;
assignedUsers[taskId] = [];
task.users.forEach(user => {
if(taskId == user.pivot.task_id) {
assignedUsers[taskId].push(user.pivot.user_id);
}
});
});
});

Function to conditionally assign variable member data to nested array in object?

I want to add data into an object, and my object contains nested data. Example data:
pageviewEvent {
client: "clientName",
page {
document_referrer: 'some http refer header data goes here',
window_height: 1920,
window_width: 1200
}
}
Some data is undefined or null and I do not want to add this undefined/null data into the object.
I made a function that works to add data to the object conditionally (if not undefined) but I can't figure out how to add data to nested objects in the function?
I could just make a bunch of if statements, but figure it's better to put the condition test into a function to reduce code size.
Example code with comment showing thinking of what I am trying but doesn't work:
//function to check if variable undefined or null, if not -> add to pageviewEvent arrayKey, variableName
function isUndefinedNull(arrayKey, variableName) {
var evalVariableName = eval(variableName);
if (evalVariableName !== undefined && evalVariableName !== null && evalVariableName !== "") {
console.log(arrayKey);
console.log(variableName);
pageviewEvent[arrayKey] = evalVariableName;
//array key could be nested, for instance pageview[page][title] or pageview.page.tile
}
}
//make event array
const pageviewEvent = { }
//add static data
pageviewEvent.client = 'neguse';
//if not null or undefined add data
isUndefinedNull('referrer.full', 'document.referrer');
//want to put data into object pageviewEvent.referrer.full or pageviewEvent[referrer][full]
Thanks for any help. I feel like this answer can help but I can't figure it out.

I recommend using the lodash function _.set(), documentation can be found here: https://lodash.com/docs/4.17.4#set
_.set( pageviewEvent, "referrer.full", "some-value" );
If you want to customise the behaviour of how nesting is handled when there's an undefined value, you can instead use _.setWith() - see https://lodash.com/docs/4.17.4#setWith

immutable.js confused when use get() method

I'm using the immutable.js and redux in project, and I found an quite strange issue.
here is the code used in selector:
{
dealDetail : dealDetails.get(id.toString()).toJS(),
dealTrackLog : dealTrackLogs.get(id).toJS()
}
First, the id is Number, in detail, I must pass string of id, and in trackLogs, on the contrary, it must be Number, otherwise will cause error, "cannot read property toJS() of undefined"
and I think the problem maybe in reducer, here is the code:
// dealDetailReducer
// const initialStateOfDealDetail = fromJS({})
let details = {}
action.data.details.map((detail) => {
details[detail.id] = detail
})
return state.merge(fromJS(details))
...
// dealTrackLogsReducer
// initialStateOfDealTrackLogs = fromJS({})
if (state.get(action.data.id)) {
// has id in state, update
return state.withMutations(s =>
s.update(
action.data.id,
trackLog => trackLog.merge(fromJS(action.data.trackLogs))
)
)
}
// no id in state, just set, id : data
return state.set(action.data.id, fromJS(action.data)
so, I'm hard to understand why and when to pass a Number/String ?

First line
let details = {}
You are using regular object for details state. Objects coerce to string keys.
The second case you are using immutablejs operation that preserve the key type.

Javascript ERROR, undefined, not an object

I am new in JavaScript, And I am trying to map my controller's buttons and leds for mixxx application. Is that an object, an array? var is missing.
BehringerCMDMM1.leds = [
// Master
{ "shiftButton" : 0x12 },
// Deck 1
{ "sync" : 0x30 },
// Deck 2
{ "sync" : 0x33 }
];
I have an error here,
BehringerCMDMM1.shiftButton = function (channel, control, value, status, group) {
// Note that there is no 'if (value)' here so this executes both when the shift button is pressed and when it is released.
// Therefore, BehringerCMDMM1.shift will only be true while the shift button is held down
var deck = BehringerCMDMM1.groupToDeck(group);
BehringerCMDMM1.shift = !BehringerCMDMM1.shift // '!' inverts the value of a boolean (true/false) variable
BehringerCMDMM1.setLED(BehringerCMDMM1.leds[deck]["shiftButton"], BehringerCMDMM1.shift);
}
about "shiftButton" as undefined.
also I have this function
BehringerCMDMM1.setLED = function(value, status) {
status = status ? 0x7F : 0x00;
midi.sendShortMsg(0x94, value, status);
}
This is from a javascript file I found on the internet created for a different controller. So, I am trying things to understand how can I configure mine.

BehringerCMDMM1.leds is an array of objects. Within that array, the element at index 0 is an object that has a shiftButton property. Thus, the only way to get the 0x12 value in your example is to do this:
BehringerCMDMM1.leds[0]['shiftButton']
So when this code executes...
var deck = BehringerCMDMM1.groupToDeck(group);
...the value of deck is probably something other than 0, and you're accessing one of the sync objects in the BehringerCMDMM1.leds array. For example, if the value of deck was 1, then this...
BehringerCMDMM1.leds[deck]['shiftButton']
...will be undefined because you're effectively doing this:
BehringerCMDMM1.leds[1]['shiftButton']

Ok,
I am new in JavaScript, And I am trying to map my controller's buttons and leds for mixxx application. Is that an object, an array?
You have a array of objects.
var is missing.
You should test what is inside yout deck variable. Try this:
console.log(deck);
if (deck in BehringerCMDMM1.leds) {
BehringerCMDMM1.setLED(BehringerCMDMM1.leds[deck]["shiftButton"], BehringerCMDMM1.shift);
} else {
console.log("Index: "+deck+" doesn't exist");
}