symbol is a new data type introduced in ECMAScript 6. Symbols are the foundation for adding any metadata support and customizing built-in behaviors in JavaScript. TypeScript supports the symbol type, but it doesn't create equivalent code if the target environment doesn't support it. This is because no equivalent code can be easily produced. Moreover, it is not possible to create a polyfill that can be relied upon to always behave like the symbol type. Some polyfills try to simulate symbols with strings, but they might fail in very unlikely situations. Luckily, symbols are supported by all mainstream browsers, the sole exception being Internet Explorer (just Internet Explorer; Edge does support symbol).
Since symbol is not included in ES5, in order to use it in TypeScript, either we change the TypeScript target to ES6, or we manually add the symbol library to...