var casesCount = 1000;

var keys = [];
for (let i = 0; i < casesCount; ++i) {
  keys.push('key_' + Math.random());
}

var map = {};
var switchCode = 'function withSwitch(key) { switch (key) {';

for (var i = 0; i < casesCount; ++i) {
  map[keys[i]] = 'case_' + keys[i];
  switchCode += ' case ' + JSON.stringify(keys[i]) + ': return ' + JSON.stringify(map[keys[i]]) + ';';
}

switchCode += ' }}';
eval(switchCode);

function withMap(key) {
  return map[key];
}

// ----------

var testKeys = [];
for (var i = 0; i < 5; ++i) {
  testKeys.push(keys[Math.floor(casesCount * (i + 1) / 5 - 1)]);
}