• Go

t := strings.Map(func(r rune) rune {
	if unicode.IsSpace(r) {
		return -1
	}
	return r
}, s)

• C++

auto t = s;
t.erase(std::ranges::remove_if(t, [](const char c) { return std::isspace(c); }).begin(),t.end()) ;

• C#

var t = new string(s.Where(c => !Char.IsWhiteSpace(c)).ToArray());

• Erlang

[Ch || Ch <- "This is a\tstring with\nwhite\rspaces", Ch /= 8, Ch /= 9, Ch /= 10, Ch /= 13, Ch /= 32 ].

• Haskell

t = filter (not . isSpace) s

• JS

let t = s.replace(/\s/g,'');

• Java

String t = s.replaceAll("\\s+", "");

• Java

String t = "";
for (char c : s.toCharArray())
    if (!isWhitespace(c)) t = t + c;

• Java

String t = "";
for (char c : s.toCharArray())
    switch (c) {
        case ' ', '\t', '\n', '\r' -> {}
        default -> t = t + c;
    }

• Java

String t = s.chars()
    .filter(c -> !isWhitespace(c))
    .mapToObj(c -> valueOf((char) c))
    .collect(joining());

• Lisp

(defun remove-whitespace (s &aux |t|)
  "Return a copy of S (a sequence) excluding all the characters space, tab, linefeed, return, and formfeed."
  (setf |t|
    (remove-if
        (lambda (item) (find item '(#\Space #\Tab #\Linefeed #\Return #\Page)))
        s))
  |t|)

• Pascal

  SetLength(t, Length(s));
  i := 0;
  for Ch in S do
    if (Ch > #32) then
    begin
      Inc(i);
      t[i] := Ch;
    end;
  SetLength(t, i);

• Pascal

t := ReplaceRegExpr('\s', s, '');

• Perl

my $t = $s;
$t =~ s/\s*//g;

• Python

t = re.sub('\\s', '', s)

• Python

f = lambda x: x not in whitespace
t = ''.join(filter(f, s))

• Python

t = ''.join(s.split())

• Ruby

t = s.gsub(/[[:space:]]/, "")

• Ruby

t = s.gsub(/\s/, "")

• Rust

let t: String = s.chars().filter(|c| !c.is_whitespace()).collect();