Property 'includes' does not exist on type 'string[]'

Add "ES2017" to your "lib" array in tsconfig.json:

{
  "compilerOptions": {
    ...
    "lib": ["es6", "dom", "es2017"],
    ...
    "target": "es5",
    ...
  }
}

This should work since TypeScript 2.1.

A related issue.

Explanation

The includes method on Array is supported since ES7 (ES2016). The above will add a missing library file to compilation.

The TypeScript compiler options are documented here.

Lib es2016 or es7 may be sufficient instead of es2017 (not tested).

If you don't want to change to es2016, just use arr.indexOf(valueToCheck) !== -1.

Changing the compiler target to "es2016" in tsconfig.js should solve this issue.