I recently began learning javascript, and this is the answer to a practice problem (Write a function that swaps the cases of each letter in a string):

var swapCase = function(letters){
    var newLetters = "";
    for(var i = 0; i<letters.length; i++){
        if(letters[i] === letters[i].toLowerCase()){
            newLetters += letters[i].toUpperCase();
        }else {
            newLetters += letters[i].toLowerCase();
        }
    }
    console.log(newLetters);
    return newLetters;
}

var text = "Life is 10% what happens to you, and 90% of how you REACT to it";
var swappedText = swapCase(text);

OUTPUT:

"lIFE IS 10% WHAT HAPPENS TO YOU, AND 90% OF HOW YOU react TO IT"

The code is perfectly functional and does exactly what it needs to do, but I am confused on the use of letters. This is not my code.

The parameter letters is not linked to anything anywhere, and this is what confuses me. I believe it represents each individual letter, but where is it defined to do so? I am familiar with Python, so I was expecting something like for i in list.

Thanks in advanced.

I recently began learning javascript, and this is the answer to a practice problem (Write a function that swaps the cases of each letter in a string):

var swapCase = function(letters){
    var newLetters = "";
    for(var i = 0; i<letters.length; i++){
        if(letters[i] === letters[i].toLowerCase()){
            newLetters += letters[i].toUpperCase();
        }else {
            newLetters += letters[i].toLowerCase();
        }
    }
    console.log(newLetters);
    return newLetters;
}

var text = "Life is 10% what happens to you, and 90% of how you REACT to it";
var swappedText = swapCase(text);

OUTPUT:

"lIFE IS 10% WHAT HAPPENS TO YOU, AND 90% OF HOW YOU react TO IT"

The code is perfectly functional and does exactly what it needs to do, but I am confused on the use of letters. This is not my code.

The parameter letters is not linked to anything anywhere, and this is what confuses me. I believe it represents each individual letter, but where is it defined to do so? I am familiar with Python, so I was expecting something like for i in list.

Thanks in advanced.

• javascript
• function
• for-loop

• 1 WHats your question? WHenever you call the function swapCase letters is gonna be whatever you pass to it in parenthesis. swapCase(text) is just saying letters will be equal to text – code Commented Jul 24, 2015 at 15:51
• 1 when you put text in the parameter, text bees letters inside the function – FutoRicky Commented Jul 24, 2015 at 15:52
• 1 letters is placeholder/ argument that has been provided when you call swapCase(text);, so text bees letters inside the function – vinayakj Commented Jul 24, 2015 at 16:04

4 Answers 4

When you put text in the parameter, text bees letters inside the function.

var swapCase = function(letters){  //anything you put as a parameter in this function will bee 'letters'
var newLetters = "";
for(var i = 0; i<letters.length; i++){
    if(letters[i] === letters[i].toLowerCase()){  //letters[i] represents the character in the 'i' position (which is assigned in the for loop) in the string you added as a parameter.
        newLetters += letters[i].toUpperCase();
    }else {
        newLetters += letters[i].toLowerCase();
    }
}
console.log(newLetters);
return newLetters;
}

var text = "Life is 10% what happens to you, and 90% of how you REACT to it";
var swappedText = swapCase(text); // You are adding the text string as a parameter in the function, thus it being the letter variable inside the function

letters is a function parameter, so basically when you call swapCase(text), the function takes text and assign it to letters. If you call the function without parameter like this swapCase() then you basically pass undefined to this function and that is assign to letter. You can do a quick check at the beginning of the function to check for that.

if(letters === undefined) return false;

The parameter letters is not linked to anything anywhere

It is defined here — function(letters){ — as an argument name on the function.

It is passed a value when the function is called here — swapCase(text); — where text is a variable defined as a string on the line above.

I believe it represents each individual letter, but where is it defined to do so?

It's a string. You can access characters in a string using square bracket notation.

When you write the line of code swapCase(text) you are passing the variable text into the function swapCase. The letters variable inside the function gets assigned the value of whatever text is.