00_1_useful-methods-reversing.md

Reversing: strings, numbers or arrays

Examples:

• String > Hello should return olleH
• Number > -10 should return -1
• Array > [1,2,3] should return [3,2,1]

Solutions summary

1. Reverse string using reverse()
2. Reverse string with .map() and .unshift()
3. Reverse string with for loop prepending each element to a new string
4. Reverse string with for...of prepending each element to a new string
5. Reverse number and preserve the sign (if negative)
6. Reverse array with for...of and spread operator

Solution 1

Since we are going to reverse using the .reverse() array method, we need first to convert our string to array. Then, use the method, convert back the array to a string and return it.

Example:

const str1 = "I'm a string";

const reverseString = (str) => {
  return str.split('').reverse().join('');
}

console.log(reverseString(str1))

Result:

gnirts a m'I

Solution 2

The previous example is probably what you would do in a real non-interview context. However, your interviewer could want a "more engaged" solution. The following 3 examples are going to be based on iterations.

Example:

const str1 = "I'm a string";

const reverseString = (str) => {
  let results = [];
  const strToArry = str.split('');
  strToArry.map((element) => {
    results.unshift(element);
  })

  return results.join('');
}

console.log(reverseString(str1))

Result:

gnirts a m'I

Here, we are also converting the string to array. Then, we .map() or loop through each element adding each to the start of the array with unshift(). Then, we join() the string and return it.

Solution 3

Before, we were converting our string input to array. We can avoid that extra steep and loop through each element of the string adding it to the start of a temporal new variable and return it.

This is the conventional for loop solution approach.

Example:

const str1 = "I'm a string";

const reverseString = (str) => {
  let results = '';
  for(let i = 0; i < str.length; i++) {
    results = str[i] + results;
  }
  return results;
}

console.log(reverseString(str1))

Result:

gnirts a m'I

Solution 4

This is similar to the previous example, but using the elegant syntax of for...of. If you don´t need to move "your pointer" several indexes (for example i + 2) on each iteration, this should be the preferred solution to prevent typos.

Example:

const str1 = "I'm a string";

const reverseString = (str) => {
  let results = '';
  for(char of str) {
    results = char + results;
  }
  return results;
}

console.log(reverseString(str1))

Result:

gnirts a m'I

Solution 5

For reversing a number, we can convert that number to a string and re-use some of the previous logic. Then, convert back to number and multiply the result by its sign with Math.sign(originalNumber) to preserve the - (if negative).

Example:

const reverseNumber = (num) => {
  let converted = num.toString();
  let results = '';
  for(char of converted) {
    results = char + results;
  }
  return parseInt(results) * Math.sign(num);
}

console.log(reverseNumber(-10))
console.log(reverseNumber(-125))
console.log(reverseNumber(310))

Result:

-1
-521
13

Solution 6

As before, we loop the array. The only difference is we are using the "spread operator" creating a new array where first we add the element, and then spread the value of tempArr.

Graphic explanation. The first time we iterate, tempArr is empty and el is 1. So we add 1 at the start of the array and spread nothing. Result: [1].

Now, tempArr is [1] and el is 2. We replicate the same process with the following result: tempArr holds [2, 1]

Example

const arr1 = [1,2,3,4,5,6]

const reverseArray = (arr) => {
  let tempArr = []
  
  for (let el of arr) {
    tempArr = [el, ...tempArr]
  }
  
  return tempArr
}

console.log(reverseArray(arr1))

Result:

[6, 5, 4, 3, 2, 1]