Implementation
JS

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Other implementations
import java.util.Map;
m.containsKey(k)
_, ok := m[k]
k in m
array_key_exists($k, $m)
exists $m->{$k}
k in m
use std::collections::HashMap;
m.contains_key(&k)
if (k in m) { }
m.containsKey(k)
uses fgl, classes;
m.IndexOf(k) >= 0
m.include?(k)
Map.has_key?(m, k)
(/=Nothing) (lookup k m)
maps:is_key(K, M).
m[k] ~= nil
#include <map>
bool key_exists = m.find(k) != m.end();
m.key?(k)
m.has_key?(k)
(assoc k m)
import qualified Data.Map.Strict as Map
Map.member k m
bool keyExists = m.ContainsKey(key)
def contains[K](map: Map[K, _], key: K): Boolean =
  map.contains(key)
import kotlin.collections.map
map.containsKey(key)
(contains? m k)
_m.has(_k)
_, ok := m[key]
#include <map>
bool key_exists = m.count(k) != 0;
#include <map>
bool key_exists = m.contains(k);
If m.ContainsKey(k) Then...