Prefix Function Implemented Using Javascript (used in Knuth Morris Pratt, ...)

prefix_function.js

/**
 * We start by setting element [0] on 0
 * Check every time of the index of arr found by getting prefix[i] is the same, 
 * if so do +1 else start over from 0
 **/
function calculate_prefix(patternArr) {
  var prefix = [];

  // First index is 0
  prefix[0] = 0;
  
  // Calculate the prefix 
  for (var i = 1; i < arr.length; i++) {
      if (arr[prefix[i - 1]] == arr[i]) {
          prefix[i] = prefix[i - 1] + 1;
      } else {
          prefix[i] = 0;
      }
  }

  return prefix;
}

var arr = ['a', 'b', 'a', 'b', 'a', 'b', 'a', 'b', 'c', 'a'];
console.log(calculate_prefix(arr));