Find the string that all array elements begin with in Javascript

Question

I have an array of strings ["samantha", "mike", "john", "sammy", "carla"] and an input value of s.

const input = "s";
let names = ["samantha", "mike", "john", "sammy", "carla"];
let filteredNames = names.filter(name => name.startsWith(input));

This gives me "samantha" and "sammy" as a result.

How can I extract the string that all elements of filteredNames start with? So basically: How can I find out that sam is the matching string for this array?

Are you trying to determine the most left-side matches for every string in filteredNames? e.g. ["hat","hats","ham"] -> ha? — Marty
– Marty, Commented Nov 19, 2017 at 23:09
@Marty Yes, exactly. I had difficulties describing what I'm looking for but your question brings it to the point. — Robert Strauch
– Robert Strauch, Commented Nov 19, 2017 at 23:11

Nenad Vracar · Accepted Answer · 2017-11-19 23:39:54Z

4

You can use reduce() on first element of your result array and then use every() to check if current letter is same in every element with same index.

let names = ["samantha", "sammy"];

var found = true;
var r = names[0].split('').reduce(function(r, e, i) {
  var check = names.every(elem => elem[i] == e);
  check && found ? r += e : found = false;
  return r;
}, '')

console.log(r)

edited Nov 19, 2017 at 23:39

answered Nov 19, 2017 at 23:13

Nenad Vracar

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1 Comment

Robert Strauch Over a year ago

It works. However I need to dig into your statements to get a better understanding :-)

jl005 · Accepted Answer · 2017-11-19 23:19:15Z

0

const input = "s"
let names = ["samantha", "mike", "john", "sammy", "carla"]
let fN = names.filter(name => name.startsWith(input))
let result = ""
if(fN.length){
    result = input
    for(let i=1; fN.every(x=>x[i]==fN[0][i]); i++)
        result += fN[0][i]
}
console.log(result)

answered Nov 19, 2017 at 23:19

jl005

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Comments

Dave Cousineau · Accepted Answer · 2017-11-19 23:26:37Z

Not necessarily efficient, but something like:

function findCommonPrefix(array) {
   let prefix = "";
   const smallest = findSmallestString(array);

   while (prefix.length < smallest) {
      let test = prefix + array[0][prefix.length];
      if (!(array.every(item => item.indexOf(test) == 0)))
         return prefix;
      prefix = test;
   }
   return prefix;
}

function findSmallestString(array) {
   if (array.length <= 0)
      return -1;

   let smallest = Number.MAX_SAFE_INTEGER;
   for (let x = 0; x < array.length; x++) {
      const len = array[x].length;
      if (smallest > len)
         smallest = len;
   }
   return smallest;
}

let stuff = ["sammy", "samuel"];
console.info(findCommonPrefix(stuff));

stuff = ["sammy", "samuel", "susan"];
console.info(findCommonPrefix(stuff));

stuff = ["sammy", "samuel", "susan", "joe"];
console.info(findCommonPrefix(stuff));

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