The compiler's functionality is determined by the specific lib you designate for it to use.
You can control which lib the compiler utilizes in two ways: through the target and lib compiler options.
As described in the link above:
If --lib is not specified, a default library will be included. The default
libraries injected are:
► For --target ES5: DOM,ES5,ScriptHost
► For
--target ES6: DOM,ES6,DOM.Iterable,ScriptHost
All the different libs can be found as part of the project.
If you're aiming at es3 or es5, then features like Number.isInteger() cannot be used since it's considered an exclusive feature of es6.
If you have a polyfill available, you can still target es5 by including the es6 lib:
--target es5 --lib DOM,ES6,ScriptHost
Alternatively, you can manually insert the definition from lib.es6.d.ts:
interface NumberConstructor {
isInteger(number: number): boolean;
}
The reason behind being able to utilize keywords like let, const, for/of regardless of the target is that the compiler has mechanisms in place to generate equivalent code even when the feature isn't supported for the selected target.
For instance:
const arr = [1, 2, 3];
for (let num of arr) {}
Will be compiled to:
var arr = [1, 2, 3];
for (var _i = 0, arr_1 = arr; _i < arr_1.length; _i++) {
var num = arr_1[_i];
}
If no target is specified.
As seen, const and let get converted into var, and for/in turns into a standard for loop.
Number.isInteger() presents a different scenario, being a feature absent in certain targets such as Promise and 'Symbol`.
The burden of adding the polyfill rests on you, alongside informing the compiler about its presence.