I have a design annoyance with some existing code in JS. The code is working, so I have no desperate hurry to change it, but the duplication shown below does annoy me. What is the usual/recommended/official way of avoiding this situation?
The actual system is a large/complex financial system, so I have simplified it to the most basic example which demonstrates the problem:
var colours={
red:{id:"red", vals:[1,0,0]},
green:{id:"green", vals:[0,1,0]},
grey:{id:"grey", vals:[0.5,0.5,0.5]}
// ...etc
};
// id needs to be known internally within the object - thus it is defined as a property.
// e.g:
colour.prototype.identify(console.log(this.id));
// id also needs to be used externally to find an object quickly.
// e.g:
function getcolour(s){return colours[s];}
// Although this works. It does mean duplicating data, with the theoretical possibility of a mismatch:
var colours={//...
blue:{id:"green", // oh dear...
How would this normally be handled by the experts?
This question is somewhat subjective.
When creating my applications I typically try do do the following:
never define same data in multiple places. source should always be unambiguous
if I need to create any indices for faster/easier access, I use utility methods to do it. Those methods should be properly unit-tested, so that I would have little doubts on them doing the wrong thing
use third party libraries as much as possible (such as already suggested lodash or underscore) to minimize the amount of code to be written/maintained.
If your algorithms and utilities are properly unit-tested you should not worry (too much) about getting the data into inconsistent state. However, if those are critically important systems/interfaces, you may add some validation on output. And it is generally a good practice to have data validation and marshaling on input.
Explanation on the utility methods:
if you have data array, say
var data = [{"id":"i_1", ...}, {"id":"i_2", ...},{"id":"i_3",....}];
Then and you have to create an index out of that or create more data sets based on the original array, then you create yourself a library of utility methods that do the modification on the array, create derivative data sets, or iterate on the array and create a resulting item on the fly. For example:
var createIndex = function( arr ){
// do something that converts the data array with expected structure to object
// {
// i_1: {"id":"i_1", ...},
// i_2: {"id":"i_2", ...},
// i_3: {"id":"i_3", ...}
return newObj;
}
This method will create a hash-map to access your data, which is faster then to iterate over the original array all the time. But now, this method you can easily unit-test and be sure that when you use it on the source data to get your intended dataset, there will be no inconsistency.
I wouldn't change the colours[key] direct access with other method to avoid duplication.
Any other attempt will lead to processing and you have mentioned that you have a large amount of data.
I assume that the duplication is over the incoming data that is a waste.
An example of processing over the network data consuming could be, going over the map object and set the id dynamically according to the key. (processing vs traffic)
colours[key].id = key
You can filter your object converting it to an array of objects and then filtering unique values. Converting it to an array would allow you to perform a lot of operations quicker and easier.
So you can map your object to an array:
var coloursArray = myObj.map(function(value, index) {
return [value];
});
Remove duplicates:
function removeDuplicates() {
return coloursArray.filter((obj, pos, arr) => {
return arr.map(mapObj => mapObj[id]).indexOf(obj[id]) === pos;
});
}
You can remove duplicates from an array using for example underscore.js through the .uniq method:
var uniqueColoursArray = _.uniq(coloursArray , function(c){ return c.id; });
Moreover, this function is pretty useless because you can access your element directly:
function getcolour(s){return colours[s];}
Calling colours[s] it is also shorter than getcolour(s). Your function would make sense if you pass also the array because it is not accessible in some other scope.
Then I can't understand why you do pass a console.log as parameter here:
colour.prototype.identify(console.log(this.id));
maybe you would like to pass just the this.id
Related
To preface, I have found a solution that works for me in the situations I have tried it, but I am fairly new to javascript and RXJS and don't fully understand what the solution is doing so I can't be sure if it will work in every instance.
I am using reactive forms and have arrays of checkboxes, what I would like to do is get an array of all of the keys used to generate the checkboxes, but without any of the unchecked boxes. Ideally, I could use this to return additional values as well such as a user-readable string, but that is far from important as I can do that in a separate step fairly easily. I have come up with a few methods of my own, but they don't seem to be very robust or performant.
I would have replied on this thread, but I lack the reputation.
This is the best solution I have found, which I would be totally happy to use, but I don't have much experience with RXJS or maps in javascript so I am not totally sure what it is doing:
this.controlNames = Object.keys(this.checkboxGroup.controls).map(_=>_); //This seems to just produce an object of control keys as properties and their value
this.selectedNames = this.checkboxGroup.valueChanges.pipe(map(v => Object.keys(v).filter(k => v[k]))); //Some sort of magic happens and an array is generated and contains only the keys whose values are 'true'
I have tried breaking that snippet apart and using console.log to test what it is doing in each step, but it really didn't give me much useful information. Any advice or or better ideas would be thoroughly appreciated, there seem to be a lot of conventions in javascript that people adhere to and it can be hard to sort through what is a convention and what is actually doing something.
I think I found a way to break it down and get a grip on it and want to post my explanation for anyone who comes looking.
In this part, it is just creating an iterable map of the 'checkboxGroup.controls' object. This could have been used to loop over in the template and make all of the checkboxes. Since my form structure is already generated from arrays of objects with known properties, I don't need this. The underscores aren't doing anything special here, people just like to use them for private variables.
this.controlNames = Object.keys(this.checkboxGroup.controls).map(_=>_);
For those who are new to arrow functions or some of the conventions of javascript, the code above is not quite, but essentially shorthand for this:
this.controlNames = [];
Object.keys(this.checkboxGroup.controls).forEach(function(key) {
this.controlNames.push(key);
}
I have changed the short variables to longer ones to make them easier to understand in this second part. This maps the value changes observable as an iterable 'changesObj', retrieves the keys, and filters the keys by instances where the key has a true value. The code filter(key => changesObj[key]) returns the key if the key is not null, undefined, or false.
this.selectedNames = this.checkboxGroup.valueChanges.pipe(map(changesObj => Object.keys(changesObj).filter(key => changesObj[key])));
This is essentially just shorthand for this:
function propNotFalse (changes, prop) {
return changes[prop] == true;
}
this.selectedNames = this.alternateFilter = Object.keys(this.checkboxGroup.valueChanges).filter(this.propNotFalse.bind(null, this.checkboxGroup.valueChanges));
I’m making a collection of React Elements and displaying them; what follows is a trivial example to frame the problem of how-would-one-modify-an-preexisting-instantiated-element only.
var c = [
<div>A</div>,
<div>B</div>,
// ...
<div>Z</div>
];
var ListComponents = React.createClass({
render: function() {
return <div>{c}</div>;
}
});
ReactDOM.render(<ListComponents/>, document.getElementById('root'));
While the code above “works,” it renders a console message I’d rather not ignore:
Warning: Each child in an array or iterator should have a unique "key" prop.
Check the render method of `ListComponents`.
See https://fb.me/react-warning-keys for more information.
Superficially, I could just add a unique key="…" string to each element in c and be done with it.
However, that seems a quite verbose, especially since I have the data in an indexed array and a functional language that in theory can assign each key its matching index value without manually having to enter it as a source literal.
I’d love to be able to just do this...
c.forEach( (e,i) => e.key = i ); // ...or call some setter
What’s the *right* React-way to do this -and- keep the code clean?
ADDENDUM:
...for the curious or those that want to just say add a key field...
The collection I'm using is actually an array of tuples containing meta-data and a corresponding React Element, a custom Component, or some huge JSX block. The example above overly trivializes what the actual data looks like as well as its irregularities.
As the source data itself is quite long, updated often, and not maintained by a developer; it is highly error prone to missed key fields or duplicates values from manual entry. Hence the desire to do it entirely programmatically. I can not count on the data owners to do it properly. They can't read code, so ideally I'd rather not mess up the data structures with a lot of "programming goop."
The collection is manipulated a few times, putting various runs of certain elements into other dynamically created wrappers, so that the final collection is actually generated by a few transformations, filters, and maps before it is ultimately displayed.
A major shout out to Wes Bos, who came up with a clever solution that works!
The code is a simple one liner and does exactly what I was looking for:
c = c.map( (el,key) => React.cloneElement(el, {key} ));
We're building a new collection using the .cloneElement() method, which I was unaware of. That was what I needed, it turns out.
In the .map() operation, the lambda function is passed both the element and the index. It's return value is a cloned element, but with the key property set.
By cleverly naming the index element key, it allows the short notation for the expression { "key" : key }. This object augments the cloned object.
In the end, I end up with a new collection of identical objects, each with a key property set to the index.
I am a little confused by the functionality of ImmutableJS when working with an array of objects. The following example shows that even though the List x is immutable, I can still modify properties of objects inside the list both with and without using Immutable List's update() function.
My question is, why would I use Immutable if I can still modify the contents of my objects? I expected this module to protect me from that. I realize that I will not be able to add or remove entire objects to/from the list, but that doesn't fully protect me from modifying the list, which when working with a list in React state, I do not want to be able to do.
The other interesting thing I noticed is that when I directly modify the name after first performing the update, x.get(0).name and y.get(0).name are both changed. I thought that the resulting list from update() would not contain references to the same objects in the list.
How and why is ImmutableJS really helping me in this case?
var x = Immutable.List.of({name: 'foo'});
console.log(x.get(0).name);
var y = x.update(0, (element) => {
element.name = 'bar';
return element;
});
console.log(x.get(0).name);
console.log(y.get(0).name);
x.get(0).name = 'baz';
console.log(x.get(0).name);
console.log(y.get(0).name);
Output:
foo
bar
bar
baz
baz
https://jsfiddle.net/shotolab/rwh116uw/1/
Example of #SpiderPig's suggestion of using Map:
var x = Immutable.List.of(new Immutable.Map({name: 'foo'}));
console.log(x.get(0).get('name'));
var y = x.update(0, (element) => {
return element.set('name', 'bar');
});
console.log(x.get(0).get('name'));
console.log(y.get(0).get('name'));
Output:
foo
foo
bar
While the last example shows what I was trying to accomplish, ultimately I don't know if I will end up using Map or List or even ImmutableJS at all. What I don't like is the alternate APIs (especially for a mapped object). I am afraid that when I hand my project off to another developer, or as others join the team, using these immutable objects and lists correctly will completely fall apart without the proper governance.
Maybe this is more of a commentary on React, but if React intends for the state to be immutable, but it's not enforced, it just seems to me like this will end up a mess in a project that is moving quickly with multiple developers. I was trying my best not to mutate the state, but forgetting that modifying an object in a list/array is very easy mistake to make.
The immutable.js does not provide true immutability in the sense that you could not modify the Objects directly - it just provides API which helps you to maintain the immutable state.
The update -function should return completely new version of the indexed object:
var y = x.update(0, (element) => {
return { name : "bar"};
});
But doing something like this is a big no-no: x.get(0).name = 'baz';
Here is a much better explanation of the whole thing than I could ever write:
https://github.com/facebook/immutable-js/issues/481
The point of immutable.js is to allow re-use of objects which are not modified, which consumes less memory and gives a good practical performance.
There is also library "Seamless immutable", which freezes the objects, so that they can not be modified, but this comes with some performance penalty under JavaScript: https://github.com/rtfeldman/seamless-immutable
Does it make sense to use an array (which may be frequently push'ed and splice'd) as the value in an ES6 Map, or should I stick to plain associative arrays?
eg something like the following Node server code which would track which users have already downloaded a specific file
// initialisation
var map = new Map();
map.set ("foo.png",[]);
// push a new user id to array associated with hashString
function userHasFile(filename,userId) {
// retrieve array of all userIds that have downloaded the file
var users = map.get(filename);
// delete the map reference since we are going to update it
map.delete(filename);
// push the new userId to the array of users that have downloaded file
users.push(userId);
// update the Map key to the new array
map.set(filename,users);
}
// example usage
userHasFile("foo.png","user1");
userHasFile("foo.png","user2");
I'm new to ES6 and the simplicity of the Map API appeals to me, so I'm wondering if there are any benefits\downsides (such as performance, memory, GC etc) to this approach over using standard associate arrays.
Absolutely.
Having a key-list store is a useful enough pattern that it's one of the data types in Redis.
Keeping an array in a Map is a totally legitimate and useful thing to do. It's a great way to store a list of objects that match some criteria and is especially efficient in JavaScript (and other languages that use references). The map doesn't actually hold a copy of the array, just a reference to it, so you can modify the array from outside of the map and the map will update without knowing or resizing.
However, you should be careful to use more functional patterns if they make sense. If you have incoming items, you may want to stream them into a map/reduce (using reduce to group them by key) rather than keeping a map and modifying the underlying arrays. You are relying on references and side effects here.
If you want to prevent the arrays from being updated at some point, you can replace them with immutable arrays, using a library like immutable.js.
If you're working in a server, say, with incoming requests that you need to record but you don't know when they will arrive (the kind of cache-and-flush that statsd does), a map makes sense:
let responses = new Map();
server.on('request', function (url, responseTime) {
if (!responses.has(url)) {
responses.set(url, []);
}
const times = responses.get(url);
times.push(responseTime);
});
const flushTimer = setInterval(function () {
stats.write(responses);
responses = new Map();
}, 10*1000);
I have a javascript application, that calls an api, and the api returns json. With the json, I select a specific object, and loop through that.
My code flow is something like this:
Service call -> GetResults
Loop through Results and build Page
The problem though, is sometimes that api returns only one result, so that means it returns an object instead of an array, so I cant loop through results. What would be the best way to go around this?
Should i convert my object, or single result to an arrary? Put/Push it inside an array? or should I do a typeof and check if the element is an array, then do the looping?
Thanks for the help.
//this is what is return when there are more than one results
var results = {
pages: [
{"pageNumber":204},
{"pageNumber":1024},
{"pageNumber":3012}
]
}
//this is what is returned when there is only one result
var results = {
pages: {"pageNumber": 105}
}
My code loops through results, just using a for loop, but it will create errors, since sometimes results is not an array. So again, do I check if its an array? Push results into a new array? What would be better. Thanks
If you have no control over the server side, you could do a simple check to make sure it's an array:
if (!(results.pages instanceof Array)) {
results.pages = [results.pages];
}
// Do your loop here.
Otherwise, this should ideally happen on the server; it should be part of the contract that the results can always be accessed in a similar fashion.
Arrange whatever you do to your objects inside the loop into a separate procedure and if you discover that the object is not an array, apply the procedure to it directly, otherwise, apply it multiple times to each element of that object:
function processPage(page) { /* do something to your page */ }
if (pages instanceof Array) pages.forEach(processPage);
else processPage(pages);
Obvious benefits of this approach as compared to the one, where you create a redundant array is that, well, you don't create a redundant array and you don't modify the data that you received. While at this stage it may not be important that the data is intact, in general it might cause you more troubles, when running integration and regression tests.