KMP prefix table

Question

I am reading about KMP for string matching.
It needs a preprocessing of the pattern by building a prefix table.
For example for the string ababaca

Answer 1

I have tried my hands using the Javascript, Open for suggestions.

const prefixArray = function (p) {
let aux = Array(p.length).fill(0);

// For index 0 the matched index will always be 0, so we will we start from 1
let i = 1;
let m = 0; // mismatched index will be from 0th

// run the loop on pattern length
while ( i < p.length) {

    // 3 Cases here
    // First when we have a match of prefix and suffix of pattern
    if(p.charAt(i) === p.charAt(m)) {
        // increment m
        m++;
        // update aux index
        aux[i] = m;
        // update the index.
        i++;
    } 
    // Now if there is no match and m !=0 means some match happened previously
    // then we need to move back M to that index
    else if(p.charAt(i) !== p.charAt(m) && m !== 0) {
        m = aux[m-1];
        // we dont want to increment I as we want to start comparing this suffix with previous matched
    } else {
        // if none of the above conditions then
        // just update the current index in aux array to 0
        aux[i] = 0; // no match
        i++; // shift to the next char
    }
}

return aux; 
}