var casesCount = 1000;

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

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

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

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