JSFiddle - React, Tailwind, and code Playground

JavaScript

var Arr = [ 'h78em', 'w145px', 'w13px' ]

function stringToNum(str){
  num = 0;
  for (i = 0; i < str.length; i++) 
    if (str[i] >= '0' && str[i] <= '9') 
      num = num * 10 + parseInt(str[i]);
  return num;
}

alert(stringToNum(Arr[0]));