Need to concatenate strings from two arrays [duplicate]

I'm struggling with this area of jQuery

That's simply because the jQuery library has no tools for this kind of work.

Use the native JavaScript functionality instead, specifically the String split method, the Array join method, the string concatenation operator + and for-loops:

var myFirstVariable = "green,blue"
var mySecondVariable = "circle,triangle";

var firstArr = myFirstVariable.split(","),
    secondArr = mySecondVariable.split(","),
    thirdArr = [];
for (var i=0; i<firstArr.length; i++)
    for (var j=0; j<secondArr.length; j++)
        thirdArr.push(firstArr[i]+secondArr[j]);

var myThirdVariable = thirdArr.join(",");

You can use the plain old string split method to get 2 arrays. http://www.w3schools.com/jsref/jsref_split.asp

You could then use nested for loops to push the new strings into your 3rd array and then use the join method to create the final string. http://www.w3schools.com/jsref/jsref_join.asp

Try

var myFirstVariable = 'green,blue'
var mySecondVariable = 'circle,triangle'

var myThirdVariable = fn(myFirstVariable, mySecondVariable);
console.log(myThirdVariable)

function fn(fv, sv){
    var fa = fv.split(','), sa = sv.split(','), ta = [];
    for(var i = 0; i < fa.length; i++){
        for(var j = 0; j < sa.length; j++){
            ta.push(fa[i] + sa[j])
        }
    }
    return ta.join(',')
}

Demo: Fiddle

OK so you don't need jquery to achieve this, just JavaScript.

check out this answer here to help you:

How to merge two arrays in Javascript and de-duplicate items