I don't have much JavaScript experience. My question is this:
When I'm writing a JavaScript library, and many of the functions I'm writing functions are meant to call each other and users can call the functions I'm defining on each other in ways I might not have predicted but are valid, how do I keep the iterators in functions that have iterating loops straight?
Do I have to come up with new names for each iterator in a for loop every time I do a for-loop just to be safe that I haven't accidentally used the same variable in two functions where one function might nest inside the other in a situation I haven't predicted or thought of?
These are just a couple examples of functions that have iteration in them. Everything I'm writing is for working with interacting with Qualtrics surveys (shown in gif examples below).
function watchSet(set, mathFunction) {
var setSize = set.length;
for (var i=0; i < setSize; i++) {
set[i].down().observe("keyup", mathFunction );
}
}
function mathSum(set, output) {
var setTotal = 0;
for (var j=0; j < (set.length); j++) {
var setInputValue = parseInt(set[j].down().value, 10);
if (isNaN(setInputValue)) { setInputValue = 0; }
setTotal = setTotal + setInputValue;
}
output.value = setTotal;
}
function validateError(array, color) {
if (color === undefined) {
color = "pink";
}
color = color.concat(";");
for (var k=0; k < array.length; k++) {
array[k].down().setAttribute("style", "background-color: ".concat(color));
}
$('NextButton') && $('NextButton').hide();
}
function cellRange(startCell, endCell) {
var r1 = /^[A-Z]/;
var r2 = /[0-9]{1,3}$/;
var startCellColumn = r1.exec(startCell)[0].charCodeAt(0) - 61;
var endCellColumn = r1.exec(endCell)[0].charCodeAt(0) - 61;
var startCellRow = parseInt(r2.exec(startCell)[0], 10);
var endCellRow = parseInt(r2.exec(endCell)[0], 10);
var tempRange = [];
for (var q=startCellColumn; q<=endCellColumn; q++) {
for (var r=startCellRow; r<=endCellRow; r++) {
tempRange.push(q);
tempRange.push(r);
}
}
var outputRange = [];
for (var s=0; s < tempRange.length; s+=2) {
outputRange.push(cell(String.fromCharCode(tempRange[s]+61).concat(tempRange[s+1])));
}
return outputRange;
}
Gif Examples:
setting equivalency-validation
summing a couple cells
No, you don't need unique variable names in different functions.
Variables declared with var are local to the function scope in which they are declared in. They will not and do not conflict with anything outside that scope. So, your three functions watchSet(), mathSum() and validateError() can all use var i just fine and will not conflict with each other or with any third party code outside of those functions. Local variables like this are created uniquely each time the function is run and can be referred to only from within that function.
If you did not use var to explicitly declare your loop variables, then Javascript would "implicitly" create global variables by that name and then, yes, your different functions could collide if one function doing this called another so thus they were both trying to use the same global at the same time. But, as long as your variables are declared with var and your code is in a function (thus not running at the global scope), this will not happen.
You can also run your code in strict mode (highly recommended) because then an accidential implicit global is an immediate error and the interpreter will immediately show you where the problem is.
Or use .forEach()
You can also use .forEach() on arrays and not have to create your own iteration index at all.
function watchSet(set, mathFunction) {
set.forEach(function(item) {
item.down().observe("keyup", mathFunction );
});
}
Or, use let in an ES6 environment
In an ES6 environment, you can use let instead of var and the variable will be scoped to only the for loop too.
function watchSet(set, mathFunction) {
var setSize = set.length;
// when declaring with let in a for loop, the variable is scoped to
// only inside the for loop
for (let i=0; i < setSize; i++) {
set[i].down().observe("keyup", mathFunction );
}
// with let in a for declaration, even another use in the same function
// does not conflict
// this is a completely different variable than the one above
for (let i=0; i < setSize; i++) {
set[i].up().observe("keyup", mathFunction );
}
}
Related
I am triying to create a constructor function in order to aferwards be able to instance it to create game boards objects. The program is simple, I just passed the constructor the number of rows and it has to create them.
The for loop is not working and I dont understand the reason, ySize variable is defined. If I remove the for the program works sucessfully and create the section with no problem. Which is the problem with the for loop?
Thank you very much for your time
function AlgorithmType(ySize) {
this.ySize = ySize;
this.createBoard = function() {
let selector; let row
for(let i = 0; i ++; i <= this.ySize) {
debugger; // Debugger is not executed
selector = document.querySelector(".mainContainer")
row = document.createElement("section")
selector.appendChild(row)
}
}
}
let testBoard = new AlgorithmType(5)
testBoard.createBoard() //expected to create 5 rows
Your function rebinds this, which is why this.ySize is not defined. Use an arrow function to access this of the lexical scope (parent scope):
function AlgorithmType(ySize) {
this.ySize = ySize;
this.createBoard = () => {
let selector; let row
for(let i = 0; i <= this.ySize; i++) {
debugger; // Debugger is not executed
selector = document.querySelector(".mainContainer")
row = document.createElement("section")
selector.appendChild(row)
}
}
}
Side note: As #atomNULL pointed out, your for-loop syntax is incorrect as well. See his answer
Your for loop syntax is wrong, the correct syntax would be:
for (let i = 0; i <= this.ySize; i++)
Also you should use an arrow function to access this instead of rebinding it current way.
In my project I have to do some two-deep loop procedure several (meaning a lot of) times. I'll have to do the same:
for (var i = 0; i < length; i++) {
something_here_maybe;
for (var j = 0; j < second_length; j++) {
something_else_here;
}
perhaps_other_thing_here;
}
Now I don't want to keep doing that, so I tried some:
function traverse(before, inside, after) {
for (var i = 0; i < length; i++) {
(before) ? before(i) : null;
for (var j = 0; j < second_length; j++) {
(inside) ? inside(i, j) : null;
}
(after) ? after(i) : null;
}
}
Of course, that seemed much more desirable for me, given that I thought I could do something like:
traverse(function(x) { blabla; }, function(x, y) { blabla; }, function(x) { blabla; });
Mbut ... I simply got to the point where those three functions need to interact with one another. And the variables in them are local - so they can't interact. I'd need to define those variables in traverse(), but I don't know beforehand what variables I'll need. I'd try to define another "initialize" parameter in traverse (as the first argument) which would be a function that initializes those values. But it would still be a function, and those variables would still be local to it, not taken by traverse();
Could you help me with any ideas about this approach ? Or it simply can't be done ? Any idea or advice would be much appreciated. Thank you in advance.
You could use inner functions to accomplish what you are describing, so, if the function traverse takes a parameter based on which the functions A, B and C get defined, define them by writing a function inside the traverse function which returns the three functions as it's return value, (you can return an array with the three functions in them) and then invoke the three functions in your loop.
example:
function traverse(param) {
function defineProcedures(args) {
/* local vars which have function scope(visible to the entire defineProcedures
body
*/
var funcA = function(params) { //blah };
var funcB = function(params) { //blah };
var funcC = function(params) { //blah };
return [funcA, funcB, funcC];
}
var procs = defineProcedures(param);
var firstFunc = procs[0],
secondFunc = procs[1],
thirdFunc = procs[2];
//for loops go here and invoke the functions appropriately.
}
When I run this sample code in Google Chrome, the intended behavior--loading an image within a placeholder image tag on the current page--does not occur. I checked the value of currPic when showPic() is called, and it is "undefined." I know if I change the parameter to showPic from 'anchors[i]' to 'this', then it will work, but was trying to understand why this is so.
function showPic(currPic) {
var srcLoc = currPic.getAttribute("href");
var placeHolder = document.getElementById("placeholder");
placeHolder.setAttribute("src", srcLoc);
var imgLabel = document.getElementById("imglabel");
var currLinkTitle = currPic.getAttribute("title");
imgLabel.firstChild.nodeValue = currLinkTitle;
}
function prepareGallery() {
if(!(document.getElementsByTagName && document.getElementById)) return false;
var imgGallery = document.getElementById("imagegallery");
if(imgGallery) {
var anchors = imgGallery.getElementsByTagName("a");
var i;
for(i = 0; i < anchors.length; i++) {
anchors[i].onclick = function() {
showPic(anchors[i]);
return false;
}
}
}
}
Inside the anonymous function, anchors[i] provides a runtime reference. At the time the click occurs, anchors[i] no longer exists. While it existed at the time the assignment was made, it falls out of scope at the time of the click (since it's just an array reference). However, using this provides a solid reference to the immediate object that is always available at the time of the click.
More succinctly, anchors[i] is a reference to a position in an array (which leaves scope once the for loop exits). this is a reference to the dom element itself.
Because this would also work: showPic(document.getElementById(anchors[i].id)); - do you "get" it now (pun very much intended)?
Didn't see the obvious statement regarding how closures work, so here's my take on it.
var i;
for(i = 0; i < anchors.length; i++) {
anchors[i].onclick = function() {
showPic(anchors[i]);
return false;
}
}
Notice how you reference the i variable inside the loop? By the end of your loop, the value of i equals anchors.length.
So, when any of your onclick function is executed, that reference to i now points one position past the last index of anchors; this is why you see currPic is undefined.
One solution to this problem has been given in other answers: use this to reference the current anchor and don't pass anchors[i] to the onclick function.
As you may encounter similar situations, I'll show you another solution by closing over the value of i like so:
var i;
for(i = 0; i < anchors.length; i++) {
anchors[i].onclick = (function(i) {
// inside this function, i is closed over and won't change anymore
return function() {
showPic(anchors[i]);
return false;
}
}(i));
}
I am learning Javascript for a project using online resources however i don't know how to get this function working.
var results =[[a1,a2,a3,a4,a5]];
var winner = 0;
function checkWinner (results)
{
for (var i = 0; i < results.length; i++)
if (results[0][i] > 50)
{
winner = results[0][i];
}
}
Just after the function, i use:
checkWinner(results);
In a HTML file i use alert to display the variable winner. But it obviously doesn't work. I realise it is a problem with my understanding of scope and global variables.
should be
var Results =[[a1,a2,a3,a4,a5]];
var winner = 0;
function checkWinner (results)
{
for (var i = 0; i < results[0].length; i++)
if (results[0][i] > 50)
{
winner = results[0][i];
}
}
checkWinner(Results);
To avoid name collisions name global variables from capital case.
Also in your code you call the length of the "parent" array. You need to specify the length of the "Child" array
You've got to understand the concept of scope. The variables results and winner are not the same inside and outside the function.
Also, you've got to call the function and return something from it if you want to change the value of the variables outside the function (unless you use globals). This seems to be hard for novice programmers to understand, but merely defining a function doesn't do anything.
var results =[[a1,a2,a3,a4,a5]];
function checkWinner (results)
{
for (var result in results[0])
{
if (result > 50)
{
return result;
}
}
}
var winner = checkWinner(results);
Note that:
I used a for each loop, which has a cleaner syntax.
I am also iterating over results[0] instead of results, since you've got a nested array for whatever reason.
Because your function has an argument called results, it requires you to pass the global results in spite of it being a global. Another way to do this:
var results = [[a1,a2,a3,a4,a5]];
function checkWinner()
{
for (var result in results[0])
{
if (result > 50)
{
winner = result;
return;
}
}
}
checkWinner();
However, I would recommend against using global variables this way. Here's an explanation on why global variables are bad. It's for C++, but it applies to JavaScript as well.
You're iterating over the result[0] array (the array in result[0]), but using the length of the result array.
This question already has answers here:
JavaScript closure inside loops – simple practical example
(44 answers)
Closed 8 years ago.
I am very puzzled about this code:
var closures = [];
function create() {
for (var i = 0; i < 5; i++) {
closures[i] = function() {
alert("i = " + i);
};
}
}
function run() {
for (var i = 0; i < 5; i++) {
closures[i]();
}
}
create();
run();
From my understanding it should print 0,1,2,3,4 (isn't this the concept of closures?).
Instead it prints 5,5,5,5,5.
I tried Rhino and Firefox.
Could someone explain this behavior to me?
Fixed Jon's answer by adding an additional anonymous function:
function create() {
for (var i = 0; i < 5; i++) {
closures[i] = (function(tmp) {
return function() {
alert("i = " + tmp);
};
})(i);
}
}
The explanation is that JavaScript's scopes are function-level, not block-level, and creating a closure just means that the enclosing scope gets added to the lexical environment of the enclosed function.
After the loop terminates, the function-level variable i has the value 5, and that's what the inner function 'sees'.
As a side note: you should beware of unnecessary function object creation, espacially in loops; it's inefficient, and if DOM objects are involved, it's easy to create circular references and therefore introduce memory leaks in Internet Explorer.
I think this might be what you want:
var closures = [];
function createClosure(i) {
closures[i] = function() {
alert("i = " + i);
};
}
function create() {
for (var i = 0; i < 5; i++) {
createClosure(i);
}
}
The solution is to have a self-executing lambda wrapping your array push. You also pass i as an argument to that lambda. The value of i inside the self-executing lambda will shadow the value of the original i and everything will work as intended:
function create() {
for (var i = 0; i < 5; i++) (function(i) {
closures[i] = function() {
alert("i = " + i);
};
})(i);
}
Another solution would be to create yet another closure which captures the correct value of i and assigns it to another variable which would "get caught" in the final lambda:
function create() {
for (var i = 0; i < 5; i++) (function() {
var x = i;
closures.push(function() {
alert("i = " + x);
});
})();
}
Yes closures are working here. Each time you loop the function you are creating grabs the i. Each function you create shares the same i. The problem you are seeing is that since they all share the same i they also share the final value of i since it is the same captured variable.
Edit: This article by Mr. Skeet explains closures in some depth and addresses this issue in particular in a way that is much more informative then I have here. However be careful as the way that Javascript and C# handle closures have some subtle differences. Skip to the section called "Comparing capture strategies: complexity vs power" for his explanation on this issue.
John Resig's Learning Advanced JavaScript explains this and more. It's an interactive presentation that explains a lot about JavaScript, and the examples are fun to read and execute.
It has a chapter about closures, and this example looks a lot like yours.
Here's the broken example:
var count = 0;
for ( var i = 0; i < 4; i++ ) {
setTimeout(function(){
assert( i == count++, "Check the value of i." );
}, i * 200);
}
And the fix:
var count = 0;
for ( var i = 0; i < 4; i++ ) (function(i){
setTimeout(function(){
assert( i == count++, "Check the value of i." );
}, i * 200);
})(i);
Just defining an inner function, or assigning it to some variable:
closures[i] = function() {...
does not create a private copy of the whole execution context. The context isn't copied until the nearest outer function is exiting (at which point those external variables could be garbage collected, so we'd better grab a copy).
This is why wrapping another function around your inner function works - the middle guy actually executes and exits, cuing the innermost function to save his own copy of the stack.
Here is what you should do to achieve your result:
<script>
var closures = [];
function create() {
for (var i = 0; i < 5; i++) {
closures[i] = function(number) {
alert("i = " + number);
};
}
}
function run() {
for (var i = 0; i < 5; i++) {
closures[i](i);
}
}
create();
run();
</script>