reverse a string in Javascript using unshift function-CodePudding

Can anyone explain what's the mistake in the below code?

function reverseString(str) {
  let reversedStr = "";
  for (let i = 0; i < str.length; i  ) {
    reversedStr.unshift(str[i]);
    return str;
  }
}
console.log(
 reverseString("hello")
 )

CodePudding user response：

You need an array to do unshift and the return outside the loop

const reverseString = str => {
  const rev = [];
  for (let i = 0; i < str.length; i  ) rev.unshift(str[i])
  return rev.join("")
};
console.log(
  reverseString("hello")
)

but here is a oneliner

const reverseString = str => str ? [...str].reverse().join("") : str;

console.log(
  reverseString("hello")
)

CodePudding user response：

If you meant to do the string reverse without using a util function, you can do it like below. If the string length is n then the time complexity of the below code is O(n/2).

function reverseString(str) {
  // get single characters into array
  const arrOfChars = str.split("");
  // get the half size of the array
  const halfLengthOfArr = Math.floor(arrOfChars.length / 2);
  // start index
  let i = 0;
  // swap chars until reach the middle of the array
  // h e l l o
  // ^       ^
  // o e l l h
  //   ^   ^
  // o l l e h
  while (i < halfLengthOfArr) {
    // swap coupterparts from start and end with same distance
    const start = arrOfChars[i];
    arrOfChars[i] = arrOfChars[arrOfChars.length - (i   1)];
    arrOfChars[arrOfChars.length - (i   1)] = start;
    i  ;
  }
  // return the output array as a string
  return arrOfChars.join("");
}
console.log(reverseString("hello"));