This function should do the job :

function humanize(str) {
  var i, frags = str.split('_');
  for (i=0; i<frags.length; i++) {
    frags[i] = frags[i].charAt(0).toUpperCase() + frags[i].slice(1);
  }
  return frags.join(' ');
}


humanize('sample_orders');
// > Sample Orders

JSFiddle

.replace replace only the first occurrence of the target input. In order to replace all the occurrences, use .replaceAll.

 var string = 'sample_orders'

 string = string.replaceAll('_', ' ')

Further converting it to the proper case could be accomplished through regEx

string = string.replace(/(^\w|\s\w)/g, firstCharOfWord => firstCharOfWord.toUpperCase());

Where:

^\w : First char of the string

| : or

\s\w : First char after whitespace

g : global scope(Match all occurrences)

The second argument is a function that accepts the first character of each word computed using the regular expression and converts it to an upper case. Binding the above logic into a function:

function formatString(string){
 
 return string.replaceAll('_', ' ').replaceAll(/(^\w|\s\w)/g, firstCharOfWord => firstCharOfWord.toUpperCase());

}

let formattedString = formatString('sample_orders')
console.log(formattedString) //Sample Orders

Caveat: You might encounter an error saying .replaceAll is not a function if you are running on an older browser or runtime environment. The .replaceAll method was added in ES2021/ES12. The best workaround to run such functionalities on older versions of JS is to provide native support to older versions that do not support newly added methods or features ( .replaceAll) in this case.

function formatString(string){

 return string.replace(/_/g, ' ').replace(/(^\w|\s\w)/g, firstCharOfWord => firstCharOfWord.toUpperCase());

}
let formattedString = formatString('sample_orders')
console.log(formattedString) //Sample Orders

Note that we're using the replace method with regular expressions on a global scope. This could also be used at instances where the strings might not contain underscores.