Javascript: "undefined" value for variable

Question

newbee to Javascript...

I've got a problem with updating a global variable witht he result of a function. When I output the variable it says 'undefined'.

What I am trying to do is loop though an array (problemArr) and create a string in to a variable (stringA).

Then add stringA to another variable stringMessage and output the value of stringMessage.

Eg: Your biggest problems are : Prob1, Prob2, Prob3,

I already have an array called problemArr which gets updated from another function which I haven't included in this snippet of code. (This part works I'm able to demonstrate that the array gets updated).

I've read a few posts on here about Function Scope and hoisting, which I think may have something to do with it. Not sure.

var stringA = ' ';//initialize variable which will form list of problems 
var stringMessage ='Your biggest problems are :' + stringA; // Output
var problemArr[]; //Empty array. Gets elements from another function - this part works, I've checked. 
//create list of problems
function messageString(){
 for(i in problemArr){
 stringA = stringA + problemArr[i] + ',';
 }
 return stringA;
}

• mike.k: problemArr does not exist anywhere, start by defining it or passing it to the function.
• Felix Kling: The code doesn't output anything. You are also not calling messageString. Why don't you provide a complete example?
• itzmurd4: where is problemArr?
• nurdyguy: Is problemArr also a global variable? Where is the array defined?
• Mindastic: Assuming your problemArr is already defined, why don't you do var stringMessage ='Your biggest problems are :' + messageString(); instead of var stringMessage ='Your biggest problems are :' + stringA;? You are setting the stringMessage with the current value of stringA when you define it and stringMessage won't change despite you are changing it on your loop.

Answer 1

You need to define your array it is missng, then call the function you have created as follows.

var stringA = ' ';//initialize variable which will form list of problems 
var problemArr = ['1','2','3']; // <<<<<<<<< define the array
//create list of problems
function messageString(){
 for(i in problemArr){
 stringA += problemArr[i] ; 
 
 // do not add a comma after the last array item
 if(i < problemArr.length - 1)
 {
 stringA += ',';
 }
 }
}
messageString(); // <<<<<<<<< call the function
var stringMessage ='Your biggest problems are :' + stringA; // Output
document.write(stringMessage);

Edit

To match your case, call the function to create the message string and fill stringA, then set it to the final output afterward var stringMessage ='Your biggest problems are :' + stringA; // Output

• Felix Kling: What's wrong with stringA = stringA + problemArr[i] + ',';? Not using += is perfectly fine. The OP certainly does not need to use it.
• Mindastic: Why are you writing to the document?
• KAD: writing just to view the result, @FelixKling I have edited the post

Answer 2

trying to replace a string with a string, strings always have to be a new variable, so rather than stringA = stringA you'd need newString = stringA + ","

i've refactored your code, and this will do what you want it to do:

(function () {
 var stringA;
 var stringMessage = "Your biggest problems are : ";
 var problemArr = [1, 2, 3, 4, 5, 6];
 for (var i = 0; i < problemArr.length; i++) {
 stringA = problemArr[i] + ",";
 stringMessage += stringA;
 }
 alert(stringMessage);
})()

the "+=" operator appends something to an existing object e.g 1 += 2 would equal 3 e.g "hello" += "world" would equal "helloworld"

Answer 3

Looks like you're just trying to add the content of an array to a string variable. Why don't you just do it like this:

var problemArr = ["99 problems","but a b****","ain't one"];
//or just use the array you created before
var problemsAsString = problemArr.join(", ");
//converts the array to a string and puts a ", " between each element
var stringMessage = 'Your biggest problems are : ' + problemsAsString; 
alert(stringMessage); //output, or do whatever you want with it

JSFiddle