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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

removing objects in an object in an array javascript

I've done some research on this issue. I am trying to manipulate an array of calculated values that looks like this in the console:
{nodeVoltages: Array(11), totalPower: Array(1), xlength: Array(11)}
nodeVoltages: Array(11)
0:48
1:47.71306060387108
2:47.250273223993105
3:46.59686907269243
4:45.71876416434013
5:44.53304242029258
6:42.745236969423615
7:Complex {re: 40.38334500994142, im:1.919295696316476, __ember1513267958317: "ember368"}
8:Complex { re:39.55961661806138, im:3.8933604519196416, __ember1513267958317: "ember369"}
This array is created dynamically through some math that I've come up with so there is no input data that I can give you. I'm trying to make the above array look like this:
{nodeVoltages: Array(11), totalPower: Array(1), xlength: Array(11)}
nodeVoltages: Array(11)
0:48
1:47.71306060387108
2:47.250273223993105
3:46.59686907269243
4:45.71876416434013
5:44.53304242029258
6:42.745236969423615
7:40.38334500994142
8:39.55961661806138
Using mathjs, I was able to evaluate my expressions and dynamically add the values into an array with the array.push command and display them. However, my code breaks once the imaginary values pop up in the results of my array.
How can I remove these imaginary numbers from my array? In other words, I need to remove the "im:" parts of the values when they begin to appear before I push them to the displayed array.
I tried to do this with some code I found from a previous answer to someone else's question (How do I remove a particular element from an array in JavaScript?) splice command like this:
var nodeVoltage2 = parser.eval(expression2);
//checks if there are imaginary values and removes them
if ("im" in nodeVoltage2) {
nodeVoltage2.splice(2,1)
}
//adds value to result array for analysis
nodeVoltages.push(nodeVoltage2);
but it returns in the console that "im is not defined".
Any help is greatly appreciated!

You can use the array map function.
Basically, we loop through the array. If the item has a .re property, we take that value only. If there is no .re property, we keep the value as is.
We can either write that in shorthand, as with result using the ternary operator and arrow function, or we can write it in a slightly more verbose but traditional way, as with resultTwo
let data = [
48
,47.71306060387108
,47.250273223993105
,46.59686907269243
,45.71876416434013
,44.53304242029258
,42.745236969423615
,{re: 40.38334500994142, im:1.919295696316476, __ember1513267958317: "ember368"}
,{ re:39.55961661806138, im:3.8933604519196416, __ember1513267958317: "ember369"}
]
let result = data.map((x) => x && x.re ? x.re : x);
let resultTwo = data.map(function(elem) {
// First, we need to check that the array element is not null / undefined
// We then need to check that it has a property called re that is also not null / undefined
if (elem != null && elem.re != null) {
// Just return the property we're interested in
return elem.re;
} else {
// Return the element as is
return elem;
}
});
console.log(result);
console.log(resultTwo);

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

Using Filter Function to Find Values Equal to False and Setting to True

In my Angular app I have a function that drills down to an array, and then uses a filter function to pull out values in a new array where "completed" is "false".
This is working as expected. And the way our data is, there is always one object in the array that has the property for "completed" set to "false", so I can target [0] to get to that. So, from there all I need to do is set it to "true". However, for whatever reason, how to accomplish this last step is eluding me.
This is my whole function, and what I've tried thus far:
private completeLastWorkflowStatus() {
let currentService = this.checkDiscipline();
for (let service of this.client.services) {
if (service.service === currentService) {
let targetWorkflow = service.workflow;
let inCompleteWorkflow = targetWorkflow.filter(workflow => workflow.completed === false);
console.log(inCompleteWorkflow);
if (inCompleteWorkflow[0].completed === false) {
inCompleteWorkflow[0].completed === true;
console.log(inCompleteWorkflow[0].completed);
}
}
}
}
For the last console.log listed above, I still get "false" as the value. What am I missing here? How can I set the value of "completed" to "true" for this one object in the array?

inCompleteWorkflow[0].completed === true; is not assignment. Do inCompleteWorkflow[0].completed = true;

javascript compare objects in array

I have a grid of pictures on a page. And, periodically, I want to randomly swap one out for one of 50 I have in an array of Objects- but only if they're not already in the grid. This last part is what my code is failing to do.
I first get all 50 items, and put them into an allmedia array:
// initialize
var allmedia = getAllMedia();
var imagesInGrid = [];
When I place the items in the grid, I add to an array of grid items:
imagesInGrid.push(allmedia [i]); // while looping to fill DOM grid
Then, every 8 seconds I run a getRandomImage() routine that randomly gets an image from the allmedia array and then tests it to see if it's not already in the DOM.
function getRandomImageNotInGrid(){
var randomNumber = Math.floor(Math.random() * allmedia.length);
if (!isInArray(allmedia[randomNumber], imagesInGrid)) {
return allmedia[randomNumber];
} else {
return getRandomImageNotInGrid();
}
}
function isInArray(item, arr) {
if(arr[0]===undefined) return false;
for (var i=arr.length;i--;) {
if (arr[i]===item) {
return true;
}
}
return false;
}
But when I step through the code the (arr[i]===item) test is failing. I can see that the two objects are exactly the same, but the === isn't seeing this as true.
Is this a ByReference / ByValue issue? What am I doing wrong?
console.log:
arr[i]===item
false
arr[i]==item
false
typeof item
"object"
typeof arr[i]
"object"
Edit::
In the output below, I don't understand why arr[0] is not the same as 'item'. I use the exact same object that I put into allmedia as I do when I place the item into the page and, accordingly update imagesInGrid.
console.dir(arr[0]);
Object
caption: Object
comments: Object
created_time: "1305132396"
filter: "Poprocket"
id: "69204668"
images: Object
likes: Object
link: "http://instagr.am/p/EH_q8/"
location: Object
tags: Array[2]
type: "image"
user: Object
__proto__: Object
console.dir(item);
Object
caption: Object
comments: Object
created_time: "1305132396"
filter: "Poprocket"
id: "69204668"
images: Object
likes: Object
link: "http://instagr.am/p/EH_q8/"
location: Object
tags: Array[2]
type: "image"
user: Object
__proto__: Object

Instead of randomly selecting one from allmedia, can you instead remove one from allmedia?
var randomNumber = Math.floor(Math.random() * allmedia.length);
imagesInGrid.push(allmedia.splice(randomNumber,1));

When you use ===, you are comparing the objects by reference. What type of objects are you using to compare? Are you sure they are the same object reference? For example, if you are using strings, you may want to use == instead of ===. If you are using DOM objects, you will want to compare the source, as Alxandr suggested.
Also, your for loop syntax appears to be wrong:
for (var i=arr.length;i--;)
Should be:
for (var i=arr.length - 1; i >= 0; i--)
...if I'm not mistaken.

You don't show any code for how the image "objects" are created or how they are added and removed from the DOM. If you are creating image elements and storing references in an array, then replacing the DOM element with the one from the array, then the comparision should work.
However, if your image object is a bundle of data that is used to create an image element for display, then they will never be equal to each other. Every javascript object is unique, it will only ever be equal to itself.
But I suspect the simplest solution is that suggested by ic3b3rg - remove the object from allMedia when you select it, that way you don't have to test if it's already in imagesInGrid because you can only select each image once. If you want the display to go on forever, then when allmedia is empty, put all the images from imagesInGrid back into it and start again.
Edit
Your problem is the for loop. When you set :
for (var i=arr.length;i--;) {
// On first iteration, i=arr.length
// so arr[i] is undefined
}
i is not decremented until after the first loop, so set i=arr.length-1. It is more common to use while with a decrementing counter:
var i = arr.length;
while (i--) {
// i is decremented after the test
}