Javascript - Transfer all letters in string into uppercase or lowercase

Question

I am a beginner in Javascript.
I tried to make a function about covert string into uppercase or lowercase. But I am confused why it can get expected output. Following is my function purpose and codes.Thank you!

1. Function purpose :

When letter in string is uppercase, it will change into lowercase. When letter in string is lowercase, it will change into uppercase. For example: "Peter" will transfer into "pETER"

2. Question:

I can't understand why my code ends up with "Peter" rather then "pETER"

function swap(str) {
 var name = ''
 for (i = 0; i <= str.length - 1; i++) {
 if (str[i] >= 'a' && str[i] <= 'z') {
 str[i].toUpperCase()
 } else {
 str[i].toLowerCase()
 }
 name += str[i]
 }
 return name
}
console.log(swap('Peter'))

I am not sure whether the problem is in this line.

if(str[i] >= 'a' && str[i] <= 'z'){
 str[i].toUpperCase()
}

Can anyone help me , thanks!!

• Nick Parsons: Strings are immutable in JS, so doing str[i].toUpperCase() won't make the i-th letter in your string uppercase, instead, it will return the uppercase version, which you can use to build a new string.
• Harmandeep Singh Kalsi: Does this answer your question? convert uppercase and lowercase in javascript
• wibbly: @NickParsons Thank you so much ! I know what my problem is.
• wibbly: @HarmandeepSinghKalsi Yes!! it helps me, sorry I haven't found this useful answer before.

Answer 1

I think your problem is to think that str[i].toUpperCase() or str[i].toLowerCase() will change the value of the str[i], but it doesn't. These functions will change the char value to uppercase or lowercase and they'll return the result of the function call, but the original variable (str[i]) will remain its value.

Try with this version:

function swap(str) {
 var name = ''
 var string;
 for (i = 0; i <= str.length - 1; i++) {
 string = str[i]; 
 if (str[i] == string.toUpperCase()) {
 name += string.toLowerCase();
 } else {
 name += string.toUpperCase();
 }
 }
 return name;
}
console.log(swap('PeTeR'));

• wibbly: Thank you! You are right ! I think it will change the original str[i] value, but i ignore that strings are immutable in JS.

Answer 2

Use reduce to accumulate your new string and lowercase/upercase JS functions to check your letters hence do the conversion.

const swap = (str) => str.split('').reduce((acc, char) =>
 acc += char === char.toLowerCase() ? char.toUpperCase() :
 char.toLowerCase(), '')
console.log(swap('Peter'))

Answer 3

This works:

function swap(str) {
 var name = ''
 for (i = 0; i <= str.length - 1; i++) {
 if (str[i] >= 'a' && str[i] <= 'z') {
 name += str[i].toUpperCase()
 } else {
 name += str[i].toLowerCase()
 }
 }
 return name
}
console.log(swap('Peter'))

Answer 4

I have changed your code but it should work now

function swap(str) {
 var name = ''
 var lowers = "abcdefghijklmnopqrstuvwxyz";
 for (i = 0; i <= str.length - 1; i++) {
 if (lowers.includes(str[i])) {
 name +=str[i].toUpperCase()
 } else {
 name +=str[i].toLowerCase()
 }
 }
 return console.log(name);
 }
swap("Peter");

• wibbly: Thank you ! I know where the problem is.

Answer 5

function name(str){
 let newArr = str.split('')
 let output = [];
 for(let x of newArr){
 output.push(x.toUpperCase());
 }
 return output.join('');
}
name('hello')