We Keep Coding

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

Difference between obj.key=value and obj.set(key, value)?

So while trying to update a document in mongoose, I realized that when I do obj.key=value to a document I obtained with Model.findOne(), it doesn'st assign the property to its value. But after trying obj.set(key, value), the property is assigned to its value in the document. So why is that? Usually when i do the first method to an object, the object gets the property. What is the .set() function? Does it have something to do with mongoose?
//this works
async function updateItem(){
let updatedItem = await Item.findOne({name:req.body.itemName});
Object.entries(req.body).forEach(elem=>{
if(elem[0]!=="itemName"){
updatedItem.set(elem[0], elem[1]);
};
});
};
updateItem();
});
//this doesn't work
async function updateItem(){
let updatedItem = await Item.findOne({name:req.body.itemName});
Object.entries(req.body).forEach(elem=>{
if(elem[0]!=="itemName"){
updatedItem.elem[0] = elem[1];
};
});
};
updateItem();
});

It means that updatedItem is not an object, it's a Map, and to add items to a Map you need to use the get method.
Another thing to point out is that when you set updatedItem.elem[0], you're literally trying to add the key "elem[0]" to updatedItem. To fix this, you need to use dynamic property notation with square brackets:
updatedItem[elem[0]] = elem[1];
This makes a new key with the value of elem[0], instead of the key being elem[0].

Why is it returning undefined?

I want to store as the following method, and get the constant value, by querying using key to find value or by value to find the key
function my_reference() {
return {
30:'',
31:'ERR591',
32:'ERR761',
33:'ERR671',
34:'ERR551',
};
}
console.log( my_reference[31],
my_reference.31,
my_reference().31,
my_reference()[31]
);

my_reference[31],
Trying to read a property (which doesn't exist) of a function. The property is on the object that is the return value of calling the function.
my_reference.31,
Trying to use a number as an identifier. This isn't allowed.
my_reference().31,
Trying to use a number as an identifier. This isn't allowed.
my_reference()[31]
This works

You need to execute the function with my_reference() and after that access the property you want to.. but the keys in javascript objects are always strings:
console.log(my_reference()['31']);

You don't need to use a function to store the references:
var reference = {
30:'',
31:'ERR591',
32:'ERR761',
33:'ERR671',
34:''
};
console.log(reference[31]);

You have to call function if you want it to return result, function called by :
my_reference()
So the both first lines will not work because my_reference will return the function it self and not call it :
my_reference[31]
my_reference.31
The third also will not work console.log(my_reference().31); because attribute can't be numeric.
Hope this helps.

Here is the fixed code
function my_reference() {
return {
_30:'',
_31:'ERR591',
_32:'ERR761',
_33:'ERR671',
_34:'ERR551'
};
}
var f = my_reference();
console.log(f["_31"]);
console.log(f._31);
console.log(my_reference()._31);
console.log(my_reference()["_31"]);
Variables can't be named with just numbers
The first two should be the returned object

AngularJs why is object undefined on ng-disabled function?

VIEW:
I have a rows repeating , with a save button on each row to save each object individually. I want this button to be disabled if no changes have been made.
<tr ng-repeat="option in options | filter:search">
<a ng-click="save(option)" ng-disabled="isUnchanged(option)">Save</a>
</tr>
CONTOLLER:
So I pass the option object to the function, I get its index position in the array. Then compare this 'option' object to its original self in apiKeyOptions[index] which is injected as a service.
angular.module('PartOfApp')
.controller('PartOfAppCtrl', function( $scope, ... apiKeyOptions) {
$scope.options = apiKeyOptions;
$scope.isUnchanged = function(option) {
var index = $scope.options.indexOf(option);
//compare object to the original
if(option.value == apiKeyOptions[index].value && apiKeyOptions[index].setting == option.setting){
//then no changes have been made to this
return true;
}else{
return false;
}
For some reason I get a console of 100's of errors when any data is changed, saying that the apiKeyOptions[index].value and apiKeyOptions[index].setting are undefind.
The app works perfectly as it should returning true if they are the same but still throws a
TypeError: Cannot read property 'value' of undefined
on apiKeyOptions[index]
if I console.log(apiKeyOptions[index].value) I get no undefined values and all log correctly.
Im guessing Im breaking some angular rules, if anyone could help that would be great.
apiKeyOptions overview:
apiKeyOptions is an array of up to 50 objects
each object is in the form
{
defaultValue: boolean,
description: null,
name: String,
setting: "Default" or Boolean,
value: Boolean
}
Added after comment below:
If I add-
console.log(index);
console.log(apiKeyOptions[index]);
to the function $scope.isUnchanged, I get the expected results
example :
13
Object {name: "LOREM IPSUM", description: null, defaultValue: false, setting: "default", value: false…}
So index is not always -1. The reason I pass the object to the function and not $index is because of the filter | search so the index will change depending on the search.
FIXED
As shown in the answer below . I was getting a index = -1 error but its was buried in 100's of CORRECT log outputs.
Oddly this did not stop the app from working and I will need to have a deeper look into how ng-disabled is bound to a value. To fix I simply replaced the indexOf with
for (var i = 0; i< $scope.options.length; i++ ){
if($scope.options[i].name == option.name){
var index = i;
}
}

The problem seems to be with the parameter passed to $scope.isUnchanged = function(option) {
Since ng-repeat creates a new scope for each loop, i suspect that the 'option' available to each loop would be a new object and will not have a reference to 'options' array.
<tr ng-repeat="option in options | filter:search">
Therefore your isUnchanged function will receive parameter as a new object and hence below code always returns -1. Because indexOf matches the given argument in the array and since the argument 'option' is an object and doesn't refer(reference comparison) the same element of array hence no match will found. i.e var a = {id:1};var b = [a]; b.indexOf({id:1}) === -1; b.indexOf(a) === 0;
var index = $scope.options.indexOf(option);//always be -1 in your case
// therefore apiKeyOptions[index] will always be undefined
As a workaround you should pass $index to isUnchanged from the view.

Need to get the value of a variable which is sent as a string?

I'm sending a string as parameter to a function, but i already have a global variable in that name, i want to get the value of that variable but its sending as undefined..
My example code
i have a array as reg[0][0],reg[0][1],reg[1][0],reg[1][0],reg[2][0],reg[2][1]
and i have some global variables as tick1, tick2, tick3...
it will either have the values as 0,1 or 2
and in a function i called
calc_score(id) //id will return as either tick1,tick2,tick3
{
alert(eval("reg[id][1]")); // it should return the value of reg[0][1] if id is 0
}
But its not working.
The id wont be a numeral it will be string .. So how can i do this?

You shouldn't use eval for things like this. If you need to convert id to a number, use the unary + operator:
calc_score(id) //id will return as either tick1,tick2,tick3
{
alert(reg[+id][1]); // it should return the value of reg[0][1] if id is 0
}
or parseInt()
calc_score(id) //id will return as either tick1,tick2,tick3
{
alert(reg[parseInt(id, 10)][1]); // it should return the value of reg[0][1] if id is 0
}
If you need to parse a string like "tick1, tick2" then you have a few options. If the first part will always be "tick", you can slice the end off the string like so:
calc_score(id)
{
id = +id.slice(4); // or +id.substring(4) if you prefer
alert(reg[id][1]);
}
If tick1, tick2, tick3 are global variables, then instead of using eval(), you should reference them via the window object like so:
calc_score(id) //id will return as either "tick1","tick2","tick3"
{
alert(window[id]);
}

Use this:
alert(reg[Number(id)][1]);
of course you should check that id can be cast to a number before you do it. I don't really think you need the eval, unless you are trying to do something else that you haven't mentioned.

Oh! You change the code like the following:
calc_score(id) //id will return as either tick1,tick2,tick3
{
alert(eval("reg[" + id + "][1]")); // it should return the value of reg[0][1] if id is 0
}