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How to format license key using JavaScript ?

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Given a string and the task is to format the string into the License key.


  • The input string contains alphanumeric characters and dashes
  • The input string is separated by N+1 groups using N dashes.
  • The output string must be grouped by K characters using a dash.
  • The first group can be less than K characters but not Zero.
  • The output string must not contain any lowercase character.

You have given the input string str and number K


Input:  str = "dsf354g4dsg1"
          k = 4
Output: "DSF3-54G4-DSG1"

Input:  str = "d-sf354g4ds-g1dsfgdf-sfd5ds65-46"
          k = 6
Output:  "D-SF354G-40DSGS-FGFSFD-DS6546"

Input:  str = "   d-sf354';.;.';.'...,k/]gcs-hsfgdf-sfs6-46"
Output:   k = 5

Output: "DS-F354K-GCSHS-FGDFS-FS646"

To solve this problem we use the following steps:

  • Create a function that takes two arguments str and k.
  • Remove leading and ending white spaces using String.trim() method.
  • Replace all the special characters using the String.replace() method and by searching with regex /[^a-zA-Z0-9]/g.
  • Transform the string characters into uppercase by using String.toUpperCase() method.
  • Convert the string into Array using the String.split() method. Now you have str is an array in which all the elements are uppercase characters and numbers (in the string form).
  • Use a for loop and initialize the loop with the length of str and run it until the i is greater than 0 and after every iteration, the value of i decreases by k. Basically, we want a loop that runs from the backside of the str.
  • Now concatenate the string at every iteration with a dash “-“
  • Now on the str using Array.join() method convert the str into String and return the string from the function.


    function format(str, k) {
        str = str
            // Remove the white spaces
            // Replace all the special
            // characters with ""
            .replace(/[^a-zA-Z0-9]/g, "")
            // Transform the string into
            // uppercase characters
            // Convert the string into array
        // Store the length of the
        // array into len
        let len = str.length;
        for (let i = len; i > 0; i = i - k) {
            if (i != len) {
                // Concatenate the string with "-"
                str[i - 1] = str[i - 1] + "-";
        //  Join the array to make it a string
        return str.join("");
    console.log(format("dsf354g4dsg1", 4))
        "d-sf354g40ds-gsfgf-sfdds65-46", 6))
"   d-sf354';.;.';.'...,k/]gcs-hsfgdf-sfs6-46", 5))





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Last Updated : 25 Jun, 2021
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