Ok, so I have a database with data sets in it. the application performs a base API call to retrieve that base data set containing all other data sets. I will receive a string variable with the name of the key I need to access so let's say const addon = "Book". However, I don't know the key name beforehand. So the following code works but I need to somehow not hard code the key parameter but rather use the string value incoming from the const addon. I am not quite sure how to do this please point me to the right documentation or explain how to achieve the wanted result.
const columns = levelOne.Book && Object.keys(levelOne.Book);
However, as the incoming param might not be "Book" but anything else it will only work for this case. It is guaranteed that there is a key-value pair where the key bears the name stored in the string value of addon.
You can use a variable as the key. For example, levelOne[variable] where variable is the string that you want to use as the key.
Also, you can get the keys through Object.keys(levelOne) and then you can set variable value from the keys array.
Related
I am working with AbsoluteOrientationSensor
I have an object that contains the values of the accelerometer of the phone.
The values that I need are nested inside an Symbol in the Object.
How do I access those values?
The Object Name is "message".
I have already tried this
console.log(message.__sensor__.quaternion);
But I am getting the result as "undefined".
I have never worked with the Symbol data type in JavaScript before.
The Values that I want to access are the quaternion values
This is the screenshot of the Object Structure -
Thank you for your help.
From the documentation:
Properties
OrientationSensor.quaternion: Returns a four element Array whose elements contain the components of the unit quaternion representing the device's orientation.
So:
console.log(message.quaternion);
You can also see in the screenshot that the object itself has a getter quaternion.
I have never worked with the Symbol data type in JavaScript before.
Symbols are used for various reasons as property names, but if they are used, it almost always means that you, as a consumer of the API, are not supposed to access that value directly. Instead you should use the "public" API the object provides to access this data, such as in this case.
function GetSymbol(object, name) {
const string = `Symbol(${name})`
return Object.getOwnPropertySymbols(object).find(symbol => symbol.toString() === string)
}
const __sensor__ = GetSymbol(message, "__sensor__")
console.log(message[__sensor__])
If I have a dictionary like:
x = {"A" : 1, "B" : 2 }
And I pass this dictionary into the pug file, how can I access a specific key value without iterating through all the elements in the dictionary?
For ex. x[A]
The only way I can think of right now is to iterate through the elements:
for key, val in x
And then have an if conditional in there that displays val when key equals A.
Am I missing something obvious here?
If you passes x dictionary to your .pug file you can simply show the key 1 of this dictionary using ${x.A}, same as in normal javascript.
Be sure to add back-tick between the expression.
every Javascript object is an associative array which is the most general sort of array you can invent - sometimes this is called a hash or map structure or a dictionary object.
An associative array is simply a set of key value pairs.
The value is stored in association with its key and if you provide the key the array will return the value.
This is all an associative array is and the name comes from the association between the key and the value. The key is a sort of generalized address that can be used to retrieve the stored value.
For example:
array={key1: 'value1',key2:'value2'};
creates an object called array with two keys and two values which in this case happen to be two constant strings.
Notice that the value stored can be any JavaScript object and in this example it is probably better to think of storing two string objects rather two string literals.
The key can be either an identifier, a string or a number but more about the difference in the key type as we progress.
You can retrieve a value via it key using array notation:
console.log(array['key2']);
Which displays the string value2. If you try and access a key that doesn't exist then you get the result undefined.
As the associative array is used to as the basis of the JavaScript object there is an alternative way to access a value that makes the key look like a property. That is you can access the value using "property syntax" as in:
console.log(array.key2);
So in jade/pug if you pass the x it will show you the similar result like in js. In your case it would be something like that:
${x["A"]} or ${x.A}
I have an IndexedDB of changes. I add an item like this, and then log the result to check the key has been created successfully:
_this._idb.add('steps', step).done(function (items) {
var item = items[0];
_logger.log("ADDED STEP", { id: item.__id__, step: item }, "CT");
});
The output from this is as expected:
...as you can see, the id has been added to the object when it is stored.
However, when I query the db to getback a list of objects, using this code:
this._idb.steps.query('timestamp').bound(start, end).execute().done(function (results) {
_logger.log("Results", results, "CT");
}
I don't get the id as part of the object that is returned:
... and the lack of id makes updating and deleting impossible.
How can I get the id of the item when I query indexed db using db.js - or am I approaching this in the wrong way, and is there something else I should be doing?
(Note: I'm using TypeScript to compile the JS, but I don't think that's especially relevant to this question)
This is expected behaviour, you're only going to get the __id__ property if you don't define a keyPath in your db schema.
Because there's no keyPath defined the value is not associated with it in indexeddb, it's only added to the resulting object after it has been added, because at that point in time we know the auto-incremented value that IndexedDB has assigned to it.
Since the value isn't really part of the object I don't have any way to assign it to the object when it comes out during a query, maybe I could use the position in the array but that's more likely to be wrong than right.
If you want the ID to be persisted against the object then you need to define a keyPath as part of the object store schema and the property will be added to the resulting object and available and it will be on the object returned from a query.
Disclaimer - I wrote db.js
Looking at the source, __id__ is only defined when your keyPath is null in the add() method. From what I'm seeing, you'll never see this in a query() response.
In IDB null keyPaths are allowed only when using auto-incrementing ("out-of-line") keys. So if you're getting the object back, it should have an auto-incrementing key on it or some other keyPath.
The __ prefix in JavaScript usually means the developer intended it to be a "private" property. I'm guessing this is for internal use and you shouldn't be counting on this in your application code.
Consider using explicit, so-called "in-line" keys on your object store.
The goal of db.js is easy and simple to use. Your is advanced use case.
As shown in this example
javascript-use-variable-as-object-name
I am using eval to use a DOM attribute to select an element from an array. Though there is no direct way for the user to change the input, I want to be as secure as possible and make sure that the variable is indeed an integer before I evaluated it.
Which of the following would be the best, most secure, way?
$(".listitem").click(function(){
var id = $(this).attr("record-id");
if(!isNaN(new Number(id))){
Storage.search.nearby.currec = rowsHolder[eval(id)];
}else{
// send email to admin, shut down
}
});
or
$(".listitem").click(function(){
var id = $(this).attr("record-id");
if(parseInt(id)){
Storage.search.nearby.currec = rowsHolder[eval(id)];
}else{
// send email to admin, shut down
}
});
More, but not required info:
Basically I am pulling down a large JSON string from online, containing an array of records. Upon building a table from the info using a for statement ( for(i in array) ), I push each row into an array called rowsHolder and give the tr an attribute of record-id="i". Then when the user clicks the row, I call the method you see above. I am using PhoneGap with JQuery Mobile.
As always, thanks for the input
-D
There is absolutely no reason to use eval here.
If your id is kind of a number, use parseFloat(id) to get it. Unnecessary as it would be converted back to a string when used as a property name, though.
If your id is an integer, use parseInt(id, 10) to get it. Unnecessary as it would be converted back to a string when used as a property name, though.
If your id is a string, just let it be a string. The property name you use it for would be one anyway.
What is the difference between key and value in html cookies?
i saw that a cookie looks like this in html request:
not like this:
Cookie: name1=key1; name2=key2;
but rather like this:
Cookie: key1=value1; key2=value2;
and if a want to make a unique personal id for each cookie, say "UUID",
should i write key="UUID"? or value="UUID"?
im sorry for the "silly" question but im really confused here...
thank you!
a key-value pair is a way of storing information in an easily readable manner. You designate each piece of data (value) with a key, and reference the data with that key. So in your case, the string "UUID" would be the key, and the actual unique ID would be the value.
uuid=4dh26532gf564836fgf597g36
In your example, name1 would actually be the key, where key1 would be the value.
In layman's terms, the key is what comes before the equal sign (=), and the value is what comes after it.
Keys are the names of the variables and values are, well the actual value of them. So to use your example, the key/value pair would be UUID=12345
Key value pair is a convenient way of storing structured data: the key designates the kind of information (e.g. be it a name, an identifier, a URL, a path, a hash of some data etc) and value designates a piece of data of the designated kind (e.g. "John", "1247", "http://example.com/", "/data/file1.txt", "4858200518452f9b374549459d644042" etc).
Thus a key is metainformation, i.e. information about information since it describes the kind of information that follows it.
Structured data is easier to analyse and use by programs since it is immediately obvious which part of the data has what meaning.
Note that in your example syntax is unconventional: it should be key=value, i.e. your keys are name1 and name2 and values are key1 and key2.