Q:

Get a value inside a Promise Typescript

One of function inside a typescript class returns a Promise<string>. How do I unwrap/yield the value inside that promise.
functionA(): Promise<string> {
   // api call returns Promise<string>
}

functionB(): string {
   return this.functionA() // how to unwrap the value inside this  promise
}

A:

How do I unwrap/yield the value inside that promise

You can do it with async/await.Don't be fooled into thinking that you just went from async to sync, async await it is just a wrapper around .then. 
functionA(): Promise<string> {
   // api call returns Promise<string>
}

async functionB(): Promise<string> {
   const value = await this.functionA() // how to unwrap the value inside this  promise
   return value;
}

Further

TypeScript Deep Dive docs

A:

Try this
functionB(): string {
   return this.functionA().then(value => ... );
}

