Find difference between two strings in JavaScript

Question

I need to find difference between two strings.

const string1 = \'lebronjames\';
const string2 = \'lebronnjames\';

The expected output is t

Answer 1

    function getDifference(a, b)
    {
        var i = 0;
        var j = 0;
        var result = "";

        while (j < b.length)
        {
         if (a[i] != b[j] || i == a.length)
             result += b[j];
         else
             i++;
         j++;
        }
        return result;
    }
    console.log(getDifference("lebronjames", "lebronnjames"));

Answer 2

Another option, for more sophisticated difference checking, is to make use of the PatienceDiff algorithm. I ported this algorithm to Javascript at...

https://github.com/jonTrent/PatienceDiff

...which although the algorithm is typically used for line-by-line comparison of text (such as computer programs), it can still be used for comparison character-by-character. Eg, to compare two strings, you can do the following...

let a = "thelebronnjamist";
let b = "the lebron james";

let difference = patienceDiff( a.split(""), b.split("") );

...with difference.lines being set to an array with the results of the comparison...

difference.lines: Array(19)

0: {line: "t", aIndex: 0, bIndex: 0}
1: {line: "h", aIndex: 1, bIndex: 1}
2: {line: "e", aIndex: 2, bIndex: 2}
3: {line: " ", aIndex: -1, bIndex: 3}
4: {line: "l", aIndex: 3, bIndex: 4}
5: {line: "e", aIndex: 4, bIndex: 5}
6: {line: "b", aIndex: 5, bIndex: 6}
7: {line: "r", aIndex: 6, bIndex: 7}
8: {line: "o", aIndex: 7, bIndex: 8}
9: {line: "n", aIndex: 8, bIndex: 9}
10: {line: "n", aIndex: 9, bIndex: -1}
11: {line: " ", aIndex: -1, bIndex: 10}
12: {line: "j", aIndex: 10, bIndex: 11}
13: {line: "a", aIndex: 11, bIndex: 12}
14: {line: "m", aIndex: 12, bIndex: 13}
15: {line: "i", aIndex: 13, bIndex: -1}
16: {line: "e", aIndex: -1, bIndex: 14}
17: {line: "s", aIndex: 14, bIndex: 15}
18: {line: "t", aIndex: 15, bIndex: -1}

Wherever aIndex === -1 or bIndex === -1 is an indication of a difference between the two strings. Specifically...

• Element 3 indicates that character " " was found in b in position 3.
• Element 10 indicates that character "n" was found in a in position 9.
• Element 11 indicates that character " " was found in b in position 10.
• Element 15 indicates that character "i" was found in a in position 13.
• Element 16 indicates that character "e" was found in b in position 14.
• Element 18 indicates that character "t" was found in a in position 15.

Note that the PatienceDiff algorithm is useful for comparing two similar blocks of text or strings. It will not tell you if basic edits have occurred. Eg, the following...

let a = "james lebron";
let b = "lebron james";

let difference = patienceDiff( a.split(""), b.split("") );

...returns difference.lines containing...

difference.lines: Array(18)

0: {line: "j", aIndex: 0, bIndex: -1}
1: {line: "a", aIndex: 1, bIndex: -1}
2: {line: "m", aIndex: 2, bIndex: -1}
3: {line: "e", aIndex: 3, bIndex: -1}
4: {line: "s", aIndex: 4, bIndex: -1}
5: {line: " ", aIndex: 5, bIndex: -1}
6: {line: "l", aIndex: 6, bIndex: 0}
7: {line: "e", aIndex: 7, bIndex: 1}
8: {line: "b", aIndex: 8, bIndex: 2}
9: {line: "r", aIndex: 9, bIndex: 3}
10: {line: "o", aIndex: 10, bIndex: 4}
11: {line: "n", aIndex: 11, bIndex: 5}
12: {line: " ", aIndex: -1, bIndex: 6}
13: {line: "j", aIndex: -1, bIndex: 7}
14: {line: "a", aIndex: -1, bIndex: 8}
15: {line: "m", aIndex: -1, bIndex: 9}
16: {line: "e", aIndex: -1, bIndex: 10}
17: {line: "s", aIndex: -1, bIndex: 11}

Notice that the PatienceDiff does not report the swap of the first and last name, but rather, provides a result showing what characters were removed from a and what characters were added to b to end up with the result of b.

EDIT: Added new algorithm dubbed patienceDiffPlus.

After mulling over the last example provided above that showed a limitation of the PatienceDiff in identifying lines that likely moved, it dawned on me that there was an elegant way of using the PatienceDiff algorithm to determine if any lines had indeed likely moved rather than just showing deletions and additions.

In short, I added the patienceDiffPlus algorithm (to the GitHub repo identified above) to the bottom of the PatienceDiff.js file. The patienceDiffPlus algorithm takes the deleted aLines[] and added bLines[] from the initial patienceDiff algorithm, and runs them through the patienceDiff algorithm again. Ie, patienceDiffPlus is seeking the Longest Common Subsequence of lines that likely moved, whereupon it records this in the original patienceDiff results. The patienceDiffPlus algorithm continues this until no more moved lines are found.

Now, using patienceDiffPlus, the following comparison...

let a = "james lebron";
let b = "lebron james";

let difference = patienceDiffPlus( a.split(""), b.split("") );

...returns difference.lines containing...

difference.lines: Array(18)

0: {line: "j", aIndex: 0, bIndex: -1, moved: true}
1: {line: "a", aIndex: 1, bIndex: -1, moved: true}
2: {line: "m", aIndex: 2, bIndex: -1, moved: true}
3: {line: "e", aIndex: 3, bIndex: -1, moved: true}
4: {line: "s", aIndex: 4, bIndex: -1, moved: true}
5: {line: " ", aIndex: 5, bIndex: -1, moved: true}
6: {line: "l", aIndex: 6, bIndex: 0}
7: {line: "e", aIndex: 7, bIndex: 1}
8: {line: "b", aIndex: 8, bIndex: 2}
9: {line: "r", aIndex: 9, bIndex: 3}
10: {line: "o", aIndex: 10, bIndex: 4}
11: {line: "n", aIndex: 11, bIndex: 5}
12: {line: " ", aIndex: 5, bIndex: 6, moved: true}
13: {line: "j", aIndex: 0, bIndex: 7, moved: true}
14: {line: "a", aIndex: 1, bIndex: 8, moved: true}
15: {line: "m", aIndex: 2, bIndex: 9, moved: true}
16: {line: "e", aIndex: 3, bIndex: 10, moved: true}
17: {line: "s", aIndex: 4, bIndex: 11, moved: true}

Notice the addition of the moved attribute, which identifies whether a line (or character in this case) was likely moved. Again, patienceDiffPlus simply matches the deleted aLines[] and added bLines[], so there is no guarantee that the lines were actually moved, but there is a strong likelihood that they were indeed moved.

Answer 3

this will return the first difference between two string

Like for lebronjames and lebronnjames is n

const string1 = 'lebronjames';
const string2 = 'lebronnjabes';


const findFirstDiff = (str1, str2) =>
  str2[[...str1].findIndex((el, index) => el !== str2[index])];


// equivalent of 

const findFirstDiff2 = function(str1, str2) {
  return str2[[...str1].findIndex(function(el, index) {
    return el !== str2[index]
  })];
}



console.log(findFirstDiff2(string1, string2));
console.log(findFirstDiff(string1, string2));