typeof with template literal does not narrow union

[canac]

TypeScript Version: 2.7.0-dev.20171230

Code

interface ProcessEnv {
  [key: string]: string | undefined;
}

const env: ProcessEnv = { FOO: 'bar' };
const envVar: string | undefined = env.FOO;
if (typeof envVar === `string`) {
  // error TS2532: Object is possibly 'undefined'.
  console.log(envVar.slice(0));
}
if (typeof envVar === 'string') {
  console.log(envVar.slice(0));
}

Expected behavior:
Should compile fine.

Actual behavior:
error TS2532: Object is possibly 'undefined'. typeof envVar === `string` does not narrow the string | undefined union down to string.

[RyanCavanaugh]

Doesn't really matter, but I'm curious why you wrote it that way?

[canac]

@RyanCavanaugh After reading this article a while back, I became persuaded of template literals' superiority over normal single/double quoted strings and have been using them exclusively ever since.

[mhegazy]

thanks @Kingwl !