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!

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"

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!!

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
– Nick Parsons, Commented Aug 15, 2020 at 8:48
Does this answer your question? convert uppercase and lowercase in javascript — Harmandeep Singh Kalsi
– Harmandeep Singh Kalsi, Commented Aug 15, 2020 at 8:48
@HarmandeepSinghKalsi Yes!! it helps me, sorry I haven't found this useful answer before. — wibbly
– wibbly, Commented Aug 15, 2020 at 12:16

cooper · Accepted Answer · 2020-08-15 09:19:01Z

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'));

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

EugenSunic · Accepted Answer · 2020-08-15 09:13:20Z

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'))

Harmandeep Singh Kalsi · Accepted Answer · 2020-08-15 08:59:23Z

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'))

Harmandeep Singh Kalsi · Accepted Answer · 2020-08-15 08:59:43Z

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");

M Hassan khan · Accepted Answer · 2021-02-27 08:51:45Z

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

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