The description of Javascript function parameters on W3Schools wasn't very clear, so I just want to clarify.
From my understanding, there isn't a type restriction; the "real value" of the parameters are passed into the method. Is there a way to pass objects or elements? Or is that what is meant by "real value"?
For example:
The function displayText meant to take input text and set a display to show a new word in the given input text, going to the next word every time it's called.
function displayText() {
var text = document.getElementById("words").value;
// Since text is initialized
// every time the method is called,
// it will always start at the beginning of the text area.
// Not sure how to fix this since making `text`
// a global variable doesn't work
var list = text.split(/[ \t\n]+/);
displayNext(list, "display");
}
There is a "helper" method, displayNext, which is supposed to shift to the next word in the list and sets the display to that word.
function displayNext(list, textboxID) {
document.getElementById(textboxID).innerHTML = list.shift();
}
This isn't working as it is intended. I'm fairly sure it's because I've mucked up something with the parameters, since displayNext sets innerHTML to null. list must have not passed properly. I'm not sure how to fix this.
I'm sure there's a more efficient way to do this, but this is a good opportunity to learn how Javascript parameters actually work, so I thought I'd ask.
JSFiddle
Based on the comments in your code, it sounds like you want displayText() to display the "next" word each time. To do that, you have to create some place to store some state about which word is the next one to display. As you have it now, you create a new array every time and always display the first word.
The simplest way is to create a variable outside your function in some lasting scope where you store the word number:
var wordNum = 0;
function displayText() {
var text = document.getElementById("words").value;
var list = text.split(/\s+/);
if (list.length !== 0) {
// make sure we aren't off the end of the list
wordNum = wordNum % list.length;
displayNext(list[wordNum++], "display");
}
}
function displayNext(text, textboxID) {
document.getElementById(textboxID).innerHTML = text;
}
For a lot more info about arguments to Javascript functions and even how you can detect what arguments were passed or overload arguments, see this answer: How to overload functions in javascript? and for more info about how arguments are passed: Javascript by reference vs. by value
Related
My Requirement is
I want to display this output when I gave a(0) the output should come 5 and when I gave a(5) the output should come 0.
a(0) = 5
a(5) = 0
like this
Hint:
Using this function
function A(num){}
like this
Please Help me how to do this I'm new in JS
Please give me different kind of solutions its more useful to my career.
function swap (input) {
if( input == 0)
return 5;
return 0;
}
i think there is no description needed
I think I see what you are getting at.
You want to input one variable into a function, and return another variable that is not defined yet. Then you want to define that variable by inputting it into the same function, and get the first input as the output. And so you should end up with 2 outputs.
Now this is technically impossible, because there are undefined variables at play. However, programming is about imagination and I think I have a solution (it's technically a hack but it will work):
var i = 1;
var output1;
var output2;
function swap(input) {
function func1(input) {
output2 = input;
i++;
}
function func2(input) {
output1 = input;
i = 1;
alert(String(output1) + "\n" + String(output2));
}
if (i === 1) {
func1(input);
}
else if (i === 2) {
func2(input);
}
}
while(true) {
swap(prompt("Enter your first input (this will be your second output):"));
swap(prompt("Enter your second input (this will be your first output):"));
}
The swap function goes back and forth between the values 1 and 2 in the variable i. That is how it keeps track of first or second inputs and their exact opposite outputs. The input, or parameter of the swap function is whatever the user types into the prompt boxes. Feel free to make it user-friendly, this is just the dirty code behind it. The reason they are outputted together is because the second input is undefined, and so the machine cannot guess what you were going to input. So first my little program collects all the data and just reverses the order when it is time to output. But to the user who knows nothing about JavaScript and what is going on underneath the hood, this would work perfectly in my opinion.
This should work for any data types inputted, I tested it myself with objects, strings, numbers, and arrays. Hope this helps!!
Shorter alternative to #mtizziani's answer:
let swap = x => !x * 5 // yes this is all
console.log(swap(0));
console.log(swap(5));
We toggle the input, so x is now 1 or 0
We multiple by 5.
Job done.
I am looking at the source of pace.js and it has a pretty long source code of about a thousand lines. You can view the source here.
I need to debug this code. Is there any tool or method in JavaScript using which one can identify how many unique functions are there in a given plugin? I found one way which is:
Paste the code in a text editor
Identify each function individually
Paste a console.log("i am so and so function").
Run the script and copy paste the result from the console in a text editor
Count the number of functions
Is there a easier way to do this?
This approach first finds all the functions in the window object. Then passes those function references to 'getInnerFunction()' which matches the function against a regular expression to detect any inner functions. Finally the count of functions is returned.
However it will not be able to detect inner functions of native function present in the browser, since they return
function FUNCTION NAME {
[native code]
}
this as the to string output.
For other cases this should work. Just call fnCount() and you will receive the number of functions present (subtract 2 from the result to exclude these 2 functions).
** Please correct me if there is any problem with the function matching regular expression.
function fnCount(){
var keys = Object.keys(window);
var property;
var count = 0;
for(var i=0;i<keys.length; i++){
property = window[keys[i]];
if(typeof(property) === 'function'){
count += getInnerFunction(property);
}
}
return count;
}
function getInnerFunction(property){
var fn = property.toString();
var fnCount = fn.match(/function.*\(.*\).*{.*/g).length;
return fnCount;
}
Open notepad++, press CTRL+F, type function, click Find all...
I have been stuck on this homework:
Create a generic function that outputs one line of the countdown on
the web page, followed by an alert, and receives the data to output as
an input parameter.
Use that function to output each line of the countdown, and an alert.
Please note that you are outputting the countdown to the browser
window this time, not to an alert!
The alert is only being used to signal when to output the next line
I need help in how to come up with a generic function that passes only one argument and then can be called 13 times. To write a for loop that output the numeric part of a countdown.
I think the key here is that they're asking for "Generic".
That means one function that doesn't have to know anything but what it's doing.
It also usually means that it shouldn't remember anything about what it did last time, or what it's going to do next time it's called (unless you're writing a generic structure specifically for remembering).
Now, the wording of the specification is poor, but a generic function which:
takes (input) data to write
writes the input to the page
calls an alert
is much simpler than you might think.
var writeInputAndAlert = function (input) {
// never, ever, ***ever*** use .write / .writeLn in the real world
document.writeLn(input);
window.alert("next");
};
If I was your teacher, I would then rewrite window.alert to handle the non-generic portion.
It's non-generic, because it knows the rules of the program, and it remembers where you are, and where you're going.
var countFrom = 100,
currentCount = countFrom,
countTo = 0;
var nextCount = function () {
currentCount -= 1;
if (currentCount >= countTo) { writeInputAndAlert(currentCount); }
};
window.alert = nextCount;
edit
var countdownArray = ["ten", "nine", "eight", "Ignition Start", "Lift Off", "We have Lift Off"],
i = 0, end = countdownArray.length, text = "",
printAndAlert = function (item) {
alert();
document.write(item);
};
for (; i < end; i += 1) {
text = countdownArray[i];
printAndAlert(text);
}
This really doesn't need to be any harder than that.
printAndAlert is a generic function that takes one input, writes that input and triggers an alert.
You call it inside of a for loop, with each value in your array.
That's all there is to it.
If I understand correctly, you want to create a function that will allow you to pass the data once, and then you can call that function to output the data line by line.
To do this exactly that way isn't possible, but this method is almost the same:
function createOutputFunction(dataArray)
{
return function() {
document.write(dataArray.shift()); // This writes the first element of the dataArray to the browser
};
}
//It can then be used like this
outputFunction = createOutputFunction(["Banana", "Mango", "Apple"]);
outputFunction();
outputFunction();
outputFunction();
The "createOutputFunction" function returns a function that can read the "dataArray" variable and print its first element every time it is called.
I'm a javascript newbie so I'm writing ugly code so far sometimes due to my lack of experience and how different it is to the languages I'm used to, so the code I'll post below works, but I'm wondering if I'm doing it the right way or perhaps it works but it's a horrible practice or there is a better way.
Basically, I have a little dude that moves within a grid, he receives from the server an action, he can move in 8 directions (int): 0:up, 1: up-right, 2: right... 7: up-left.
the server will send him this 0 <= action <= 7 value, and he has to take the correct action... now, instead of using a switch-case structure. I created a function goUp(), goLeft(), etc, and loaded them in an array, so I have a method like this:
var getActionFunction = actions[action];
actionFunction();
However, what to set all this up is this:
1) create a constructor function:
function LittleDude(container) {
this.element = container; //I will move a div around, i just save it in field here.
}
LittleDude.prototype.goUp() {
//do go up
this.element.animate(etc...);
}
LittleDude.prototype.actions = [LittleDude.prototype.goUp, LittleDude.prototype.goUpLeft, ...];
//In this array I can't use "this.goUp", because this points to the window object, as expected
LittleDude.prototype.doAction = function(action) {
var actionFunction = this.actions[action];
actionFunction(); //LOOK AT THIS LINE
}
Now if you pay attention, the last line won't work.. because: when i use the index to access the array, it returns a LittleDude.prototype.goUp for instance... so the "this" keyword is undefined..
goUp has a statement "this.element"... but "this" is not defined, so I have to write it like this:
actionFunction.call(this);
so my doAction will look like this:
LittleDude.prototype.doAction = function(action) {
var actionFunction = this.actions[action];
actionFunction.call(this); //NOW IT WORKS
}
I need to know if this is hackish or if I'm violating some sort of "DO NOT DO THIS" rule. or perhaps it can be written in a better way. Since it seems to me kind of weird to add it to the prototype but then treating it like a function that stands on its own.
What you are trying to do is one of the possible ways, but it is possible to make it more simple. Since object property names are not necessary strings, you can use action index directly on prototype. You even don't need doAction function.
LittleDude = function LittleDude(container) {
this.container = container;
}
LittleDude.prototype[0] = LittleDude.prototype.goUp = function goUp() {
console.log('goUp', this.container);
}
LittleDude.prototype[1] = LittleDude.prototype.goUpRight = function goUpRight() {
console.log('goUpRight', this.container);
}
var littleDude = new LittleDude(123),
action = 1;
littleDude[action](); // --> goUpRight 123
littleDude.goUp(); // --> goUp 123
actionFunction.call(this); //NOW IT WORKS
I need to know if this is hackish or if I'm violating some sort of "DO NOT DO THIS" rule. or perhaps it can be written in a better way.
No, using .call() is perfectly fine for binding the this keyword - that's what it's made for.
Since it seems to me kind of weird to add it to the prototype but then treating it like a function that stands on its own.
You don't have to define them on the prototype if you don't use them directly :-) Yet, if you do you might not store the functions themselves in the array, but the method names and then call them with bracket notation:
// or make that a local variable somewhere?
LittleDude.prototype.actions = ["goUp", "goUpLeft", …];
LittleDude.prototype.doAction = function(action) {
var methodName = this.actions[action];
this[methodName](); // calls the function in expected context as well
}
i'm implementing a charcounter in the UI, so a user can see how many characters are left for input.
To count, i use this simple function:
function typerCount(source, layerID)
{
outPanel = GetElementByID(layerID);
outPanel.innerHTML = source.value.length.toString();
}
source contains the field which values we want to meassure
layerID contains the element ID of the object we want to put the result in (a span or div)
outPanel is just a temporary var
If i activate this function, while typing the machine really slows down and i can see that FF is using one core at 100%. you can't write fluently because it hangs after each block of few letters.
The problem, it seems, may be the value.length() function call in the second line?
Regards
I can't tell you why it's that slow, there's just not enough code in your example to determine that. If you want to count characters in a textarea and limit input to n characters, check this jsfiddle. It's fast enough to type without obstruction.
It could be having problems with outPanel. Every time you call that function, it will look up that DOM node. If you are targeting the same DOM node, that's very expensive for the browser if it's doing that every single time you type a character.
Also, this is too verbose:
source.value.length.toString();
This is sufficient:
source.value.length;
JavaScript is dynamic. It doesn't need the conversion to a string.
I doubt your problem is with the use of innerHTML or getElementById().
I would try to isolate the problem by removing parts of the function and seeing how the cpu is used. For instance, try it all these ways:
var len;
function typerCount(source, layerID)
{
len = source.value.length;
}
function typerCount(source, layerID)
{
len = source.value.length.toString();
}
function typerCount(source, layerID)
{
outPanel = GetElementByID(layerID);
outPanel.innerHTML = "test";
}
As artyom.stv mentioned in the comments, cache the result of your GetElementByID call. Also, as a side note, what is GetElementByID doing? Is it doing anything else other than calling document.getElementById?
How would you cache this you say?
var outPanelsById = {};
function getOutPanelById(id) {
var panel = outPanelsById[id];
if (!panel) {
panel = document.getElementById(id);
outPanelsById[id] = panel;
}
return panel;
};
function typerCount(source, layerId) {
var panel = getOutPanelById(layerId);
panel.innerHTML = source.value.length.toString();
};
I'm thinking there has to be something else going on though, as even getElementById calls are extremely fast in FF.
Also, what is "source"? Is it a DOMElement? Or is it something else?