为什么字符串数组中的第一个字母不转换为大写？

Question

I take on input string "ZpglnRxqenU"

and must return string with that view "Z-Pp-Ggg-Llll-Nnnnn-Rrrrrr-Xxxxxxx-Qqqqqqqq-Eeeeeeeee-Nnnnnnnnnn-Uuuuuuuuuuu"

i almost done, but i don't know why first letter in array elements don't change to upper case.

 function accum(s) { let str = s.toLowerCase(); let arr = []; for(let i=0; i<str.length;i++){ arr.push(str[i].repeat(i+1)); arr[i][0].toUpperCase(); } return arr.join('-'); } console.log(accum("ZpglnRxqenU")); // must be "Z-Pp-Ggg-Llll-Nnnnn-Rrrrrr-Xxxxxxx-Qqqqqqqq-Eeeeeeeee-Nnnnnnnnnn-Uuuuuuuuuuu" 

Answer 1

Strings are immutable and just using .toUpperCase() will not update existing string, to do that you can try below code

function accum(s) {
  let str = s.toLowerCase();
  let arr = [];

  for(let i=0; i<str.length;i++){
      arr.push(str[i].repeat(i+1));
      var [firstLetter, ...rest] = arr[i]
      arr[i] = firstLetter.toUpperCase() +rest.join('')
  }

  return arr.join('-');
}

Answer 2

Strings in Javascript are immutable , so it's got to be arr[i] = arr[i][0].toUpperCase() + arr[i].slice(1) if you want to convert it.

toUpperCase() only returns the character(s), it won't manipulate the String it was called on.

The same is true for any other method on String.prototype like eg String.replace() .

To make it even more clear: The only way in Javascript to ever change a variable containing a String is assigning a new value to the variable.

 function accum(s) { let str = s.toLowerCase(); let arr = []; for(let i=0; i<str.length; i++){ arr.push(str[i].repeat(i+1)); arr[i] = arr[i][0].toUpperCase() + arr[i].slice(1); } return arr.join('-'); } console.log(accum("ZpglnRxqenU")); 

Answer 3

You can to do it like this way and remove unnecessary this line arr[i][0].toUpperCase();

 function accum(s) { let str = s.toLowerCase(); let arr = []; for(let i=0; i<str.length;i++){ let gStr = str[i].repeat(i+1); arr.push(gStr[0].toUpperCase() + gStr.slice(1)); } return arr.join('-'); } console.log(accum("ZpglnRxqenU")); 

Answer 4

String are inmutable. Using toUpperCase() will not change the original string. Instead of this, it will return you a new string. You have to save this new string in the position that you want:

arr[i][0] = arr[i][0].toUpperCase();

Answer 5

You could take the power of Array.from and get the string in characters and map the first upper case character and the repeated characters for a new string.

At the end join all items to a single string.

 function accum(s) { return Array .from(s.toLowerCase(), (c, i) => c.toUpperCase() + c.repeat(i)) .join('-'); } console.log(accum("ZpglnRxqenU"));