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 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!!

• javascript
• string
• uppercase
• lowercase

• 2 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. – Nick Parsons Commented Aug 15, 2020 at 8:48
• Does this answer your question? convert uppercase and lowercase in javascript – Harmandeep Singh Kalsi Commented Aug 15, 2020 at 8:48
• 1 @NickParsons Thank you so much ! I know what my problem is. – wibbly Commented Aug 15, 2020 at 12:05
• 1 @HarmandeepSinghKalsi Yes!! it helps me, sorry I haven't found this useful answer before. – wibbly Commented Aug 15, 2020 at 12:16

5 Answers 5

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'));

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'))

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'))

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");

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

name('hello')