I'm having trouble using a function within a Node.js module. I'm using a custom sort function to sort an array of objects by the value of a certain property.
exports.getResult = function(cards) {
cards.sort(sortByField('suit'));
...
...
...
return cards;
}
function sortByField(fieldName) {
return function(card1, card2) {
return card1[fieldName] < card2[fieldName]
};
}
When I use the getResult function NOT as an export, and call it from the same file, everything works as expected. The card object with the highest suit value is first and the card object with the lowest value is last.
However, when I call the function as an export from my index.js, the sort doesn't happen. There is no error, the rest of the function just executes with the array in the same order that it was before the sort function.
I've tried everything I can think of. I've done module.exports.sortByField(), I've tried requiring the .sortByField() in a completely separate module. I'm sure there's a simple answer, but I just can't figure it out.
I suspect that it's working the same in both cases, but the problem is that the sort callback is incorrect.
A sort callback must return a negative number, 0, or a positive number, not a boolean: A negative number if the first entry should be before the second, 0 if they're the same, a positive number if the first should come after the second.
If cards contains numbers, you want:
return card1[fieldName] - card2[fieldName];
...if you want to sort ascending.
If it's strings, you can use localeCompare:
return card1[fieldName].localeCompare(card1[fieldName]);
So I hope I can be forgiven for being new to all this, but it turns out that the code I posted originally wasn't the problem at all. Long story short, the array that I passed to the getResult() function was an array of arrays, instead of an array of objects.
I was adding cards to an array of played cards with this code:
playedCards.push(game.playCard(1, players[0].hand, 3));
and the playCard() function was exporting an array:
exports.playCard = function(amount, hand, index) {
const cards = hand.splice(index, amount);
return cards;
};
but I should have been exporting an individual object:
exports.playCard = function(amount, hand, index) {
const cards = hand.splice(index, amount);
return cards[0];
};
Related
Issue Description
I'm currently trying to make updates on passwords using sequelize.fn.
I've seen numerous sites, including the Sequelize's manual, yet I found too few articles that approaches my case, and none of them seems to solve it.
Moreover, the function to encrypt the passwords need nested function, so I need to gradually construct the nested function
by putting the successive arguments into variables.
I have not found anything on constructing the functions, thus using sequelize.fn outside the arguments of the functions find and updates.
Additional context
In my case, " crypt('password', gen_salt('bf', 4)) " (in Postgres) needs to become
sequelize.fn('crypt', 'password', sequelize.fn('gen_salt', 'bf', 4)) and to be stored into a variable
When I inject it into the update function, an Fn Object is injected, and the result in the database seems to be a memory address instead of the result of the crypt function.
I may post a code example so you can have a better understanding of what I'm trying to do and I think should be added into the documentation
I did not made this doc issue to resolve my issue, only to point out what I think is lacking in the documentation
I will ask StackOverflow for help regarding my issue.
//Take a format and a value, and applies the functions inside the format onto the value
function recursSequelizeFn (format, value) {
const sequelize = db.sequelize;
let nextId = 0;
let matches;
let finalArgs = {};
//The RegEx finds the function calls of which parameters aren't functions themselves
//As the format string is modified on each loop, I need to initialize the RegEx on each loop
while ((matches = (/(\w+)\(([^(,)]+)?(?:,([^(,)]+))*\)/g).exec(format)) !== null) {
//matches[0] is the whole expression
//matches[1] is the function's name
//matches[>=2] are the arguments
let func = matches[1];
let args = matches.slice(2);
for (let argNo in args) {
//Transforms DB's columns into Sequelize-interpreted columns
//I do not check the column that is found, because I do not have others columns than the one that is being treated
args[argNo] = args[argNo].replace(/seqCol_(\w+)/g, (correspondance, p1, decalage, chaine)=>{return value;});
//If the parameter was previously saved, we replace its reference by its value
args[argNo] = args[argNo].replace(/(#\d+)/g, (correspondance, p1, decalage, chaine)=>{return finalArgs[p1];});
}
//finally, we save the value of the function and replace the whole expression by a reference
finalArgs['#'+nextId]=sequelize.fn(func, ...args);
format = format.replace(matches[0], '#'+nextId);
nextId++;
}
//When the treatment is finished, we return the last value saved into the array
//Or we return the original value if we have not made any change (the array will be empty)
return finalArgs['#'+(nextId-1)]||value;
}
Object.keys(req.message.attributes).map((key)=>{
//req.message.attributes[key] contains the formatting needed to be put onto the value
//e.g. crypt(password, gen_salt('bf', 4))
//req.body[key] contains the value
//e.g. '1234'
let seqValue = util.recursSequelizeFn(req.message.attributes[key], req.body[key]);
req.body[key] = seqValue;
});
//mainModel is a sequelize model
mainModel.update(req.body, {where: {id:req.params.id}, logging:console.log})
.then() //...
this.breakintoletters=()=>
this.lengthi!==0?(this.title2=this.title,this.title2.split(),this.title2.
map((x)=>this.arol.push(new letter(x))))
:!!false
So basically this is the code. It's supposed to break the string into letters and then push pertinent objects into an array..
Checks for the length of the string, if not 0 proceeds, returns an errors where MAP function is at...) TypeError telling me it's not a function. Editor is not showing errors. Would appreciate help
I suggest to use a different approach by checking this.lengthi in advance and return either false, or later the mapped length of the pushed values.
this.breakintoletters = () => {
if (!this.lengthi) return false;
this.title2 = this.title;
// this.title2.split(); the result is not taken, not even splitted
return this.title2.map(x => this.arol.push(new letter(x)));
};
You're not assigning splited value back to this.title2 and than using map on this.split2 which is a string
this.breakintoletters=()=>
this.lengthi ? (this.title2=this.title,
this.title2=this.title2.split(),
this.title2.map((x)=>this.arol.push(new letter(x))))
:false
IMO you should try to make your code consice only upto a point where it stays readabale you can simply it in following manner
this.breakintoletters = ( ) => {
if(this.lengthi === 0 ) return false;
this.title2=this.title;
this.title2=this.title2.split();
return this.title2.map((x) => this.arol.push(new letter(x))))
}
(this.title2=this.title, this.title2.split(), this.title2.
map((x)=>this.arol.push(new letter(x))))
Is this.arol the name of an array?
Try restructuring it to be:
(this.title.split().map((x) => this.arol.push(new letter(x)))
Methods like split() join() map() etc can be chained together.
I would rethink using the map function here though and the ternary which other commenters covered above. I mean it works technically, but if the goal is to iterate through the string in order to push certain values, it'd be better to use a for loop. Map functions are more when you want to iterate in order to apply the same specified method to each individual character
Also this is just a formatting thing but it makes it a lot easier to read and understand your code when you have some spaces between variables and operators, and choosing variable names that make sense for what you are doing (this.bookLength, this.reverseAr), or at least using the generic this.array or this.arr It makes it easier to ask questions like this because you'll get less clarifying questions about typos, and also if you are ever planning to work on a larger code base it's important to write clean code that is understandable to someone who doesn't know you
I wrote a simple function to sort objects not expecting it to work but it does:
function sortObjs(objArr,field) {
objArr.sort(
function(a,b) {
if( isNaN(a[field]) ) {
return a[field].localeCompare(b[field]);
} else {
return parseFloat(a[field]) - parseFloat(b[field])
}
}
);
return objArr;
}
When I call this function I get my sorted objects without issue. But I was not expecting it to work because I thought the first two return statements would exit the function before it got to the last statement: return objArr.
You have a nested function. The first two returns will exit the inner function, while the last one will exit the outer one.
EDIT:
You can think of function returns as "replacing" the function with the returned value. For example:
var i = getSum(1, 3);
function getSum(a, b) {
return a + b;
}
The function getSum returns the sum of a and b. The line var i = getSum(1, 3) will execute the lines of code contained in the function with a = 1 and b = 3. The value that is returned "replaces" the function call. So now the line of code looks like var i = 4;. Although this is not exactly how it works, it's a good way to conceptualize it. This is a special case because you aren't actually running the inner method here, you're passing it as a variable.
Let me know if you have any more questions!
To understand why the inner function's return statements would not have an effect on the return statement in the outer scope, you need to understand how the Array.prototype.sort() function works.
The function arr.sort([compareFunction]), takes a function as a parameter.
compareFunction
Optional. Specifies a function that defines the sort order.
If omitted, the array is sorted according to each character's Unicode code point value,
according to the string conversion of each element.
The logic you write inside the compareFunction gets executed when
the java script engine tries to compare two elements during its
comparison operation.
Hence for each comparison that it makes, the function would return a
value, based on which the elements would be ordered.
This implies that the compareFunction that we pass on as a parameter
would be used to just obtain a value based on which two elements can
be compared and not exit the sort operation, leave alone exiting the
outer function.
Having said this, the code return objArr;, takes no effect, since the array would be sorted in place.
I am trying to make a small program that prompts a user to add items to a grocery list.
I read about using recursion to loop. I understand a while loop would probably be better suited for this task, but I ran into the same problems with the while loop and I wanted to try recursion. It just sounds like I know what I'm doing... "Yeh, I used recursion to enumerate the array while prompting validation from the user... hur hur hur"... but, I digress.
Here is the code:
function addToArray() {
var array = [];
array.push(prompt("Add items to array or 'q' to stop"));
if (array.pop() == 'q') {
document.write(array)
}
else {
addToArray();
}
}
addToArray();
If you'll notice, it loops like its supposed to but it is not adding items to an array. I have tried the array[i] = i technique as well but to no avail, the array remains empty. Also, why is it that by using a function with no args am I not running into too much recursion? Is it because of the conditional statement?
If you know what I'm doing wrong, try and hint towards the right answer rather than just blurting it out. I'd like to have that 'Aha' moment. I think this all helps us learn a bit better.
Thanks guys. (and gals)
You're creating a new array instead of passing it to the recursive call.
Do this instead.
DEMO: http://jsfiddle.net/kDtZn/
function addToArray(array) {
array.push(prompt("Add items to array or 'q' to stop"));
if (array[array.length-1] == 'q') {
array.pop();
document.write(array)
}
else {
addToArray(array);
}
}
addToArray([]);
Now you start with an empty array, and for each recursive call, it passes the same array forward.
Also, I changed it so that it doesn't use .pop() in the if() condition, otherwise you'll always end up with an empty array when it comes time to write it. (The .pop() method actually removes the last item.)
Finally, make sure you're not using document.write after the DOM is loaded. If so, you need to change it to use DOM manipulation methods instead.
You could take a different approach so that you don't need .pop() at all.
DEMO: http://jsfiddle.net/kDtZn/1/
function addToArray(array) {
var item = prompt("Add items to array or 'q' to stop");
if (item == 'q') {
document.body.textContent = array;
} else {
array.push(item);
addToArray(array);
}
}
addToArray([]);
The reason your while loop didn't work is very likely because of the original .pop() issue.
Your function recreates var array = [] on every loop/recursion. I am not sure if recursion is the right tool for the job in your case - it does not seems like it - but if you're starting out with JavaScript/development and just trying it out then you're fine.
While an 'infinite loop' is probably what you really want (as it would probably make the code simpler), you can do this with recursion by defaulting the array and passing it as an argument to the function. Like so...
function addToArray( array ) {
var array = array || [];
array.push(prompt( "Add items to array or 'q' to stop" ));
if ( array[array.length - 1] === 'q' ) {
document.write(array.slice( 0, -1 ))
} else {
addToArray( array );
}
}
addToArray();
There's two issues with the code as you presented. One, as pointed out earlier, you're redefining your array variable every time you call your function. Second, array.pop() alters your array, so when you get to the document.write call, you'd be printing an empty array anyways.
I wrote a function yesterday to count the number of "a" characters in a string. My teacher told me to refactor the code into a recursive function and I don't really know how to do so.
I would like some feedback on the subject, and by the way I'm an absolute beginner in JavaScript.
function numberOfA(n){
var numberA =0;
for (i=0; i<=n.length; i++){
if(n.charAt(i)== "a" ){
numberA++;}
}
return numberA;
}
to call the function following piece of code :
var n = prompt("type a word");
var output = numberOfA(n);
alert (output);
Thanks in advance !
The goal of recursion is to make a function which calls itself.
You might have mutual-recursion -- function A calls function B, calls function A... but that's certainly not needed here, and is better suited for when you know that you need to do two distinct things (one per function) and know that you need to do them in a leapfrog pattern.
Where recursion comes into play is when you're thinking about loops.
Normally, when you're doing things with loops, you might end up having two or three loops inside of one another.
Instead of worrying about managing loops, recursion is a way of thinking about what happens in a single-iteration of a loop, and writing ONLY the code needed to do that.
A really simple example of singular recursion might be to log all elements of an array to the console.
This is not a practical example -- it's a trivial example which has most of the pieces you need to make practical examples.
var array = [ "one", "two", "three", "four" ];
function listNextItem (array, index) {
var item = array[index];
if (!item) { return; }
console.log(item);
listNextItem(array, index + 1);
}
listNextItem(array, 0);
I've created a very simple function which looks like the inside of your innermost loop.
It sets an item variable, based on array[index].
If it doesn't exist, we're done, and we can return out of the function, so we don't try to go on forever (this is very important in recursion).
If it does exist, we log the item's value.
Then we call the exact same function, and pass it the exact-same array, but we pass it the value of index + 1.
Did this change anybody's life, or make loops obsolete?
Not really.
But it's the first step to getting recursion.
The next step is getting a return from recursion.
function recursiveAddOne (current, max) {
if (current === max) { return current; }
return 1 + recursiveAddOne(current + 1, max);
}
var total = recursiveAddOne(0, 3); // === 3 + 1 + 1 + 1
total; // 6
Normally in my return statement, I'd be sending the answer back to the variable in the outside world.
I'm still doing that, but here I'm adding a call to the same function, as part of my return.
What does that do?
Well, the outside function can't return a value until the inside function returns.
The inside function can't return a value until ITS inside function returns...
...and it goes all the way down until my termination-condition is met.
That condition returns a value to its outer function. That outer function returns that added value to ITS outer function... ...all the way up to where the outermost function gets handed the value of all of the other functions put together, and then returns THAT to the outside world.
It's like giving each Russian Matryoshka ("babushka") doll a piece of work.
You start with the biggest one, and go all the way inside to the tiniest one.
The tiniest one does its work first, and hands it back to the next one, which does its work and hands that back... ...all the way back until you're outside again.
Well, the basic concept of recursion is solving a problem with a smaller version of itself.
You have a function, numberOfA which gives you the length of a string(or maybe substring).
So let's say you have the string "javascript' the first string is at index 2.
It's logical to say that the number of as in your string is equal to 1 plus the number of as in the entire substring after the first a.
So what you do, is you add 1 to the number of as in the substring vascript
So here's some psudocode
function numA(str)
{
var substring = substr(index_of_first_a, str.length - index_of_first_a
return 1 + numA(substring);
}
function numberOfA(n, count){
if(!n.length) {
return count;
}
if(n.charAt(i)== "a") {
++count;
}
return numberOfA(n.substr(1), count);
}
var numberA = numberOfA('asdfafeaa', 0);
Try this:
function numberOfA(n) {
return n == "" ? 0 : (n.charAt(0) == "a" ? 1 : 0) + numberOfA(n.substring(1))
}
Here's how it works:
If n is the empty string, return 0 and finish the recursion. This is the base case of the recursion.
Else if the character at the first position in the string is an "a" add one, if not add zero and either way advance the recursion by removing the first character from the string. This is the recursive step of the recursion.
As you can see, every recursive solution must have at least a base case and a recursive step.
<!DOCTYPE html><html lang="en"><body><script>
var foo = function foo() {
console.log(arguments.callee); // logs foo()
// callee could be used to invoke recursively the foo function (e.g. arguments.callee())
}();
</script></body></html>
arguments.callee function will call the currently being executed method.