打字稿：Map <>与strictNullChecks的使用

Question

Given the following simple class:

class Observer {
        private subscribers: Map<string, Array<((data: any) => void)>> = new Map();     

        public subscribe(event: string, callback: (data: any) => void) {
            if (!this.subscribers.has(event)) {
                this.subscribers.set(event, []);
            }

            this.subscribers.get(event).push(callback); //tsc says: Object is possibly 'undefined'
        }
    }

Furthermore, in tsconfig.json, the flags strictNullChecks and strict are enabled.

Although subscribers is checked for a key of the current event, the typescript compiler complains with the error message shown above ( this.subscribers.get(event) is possibly undefined).

If I'm not completely wrong, this.subscribers.get(event) can never be undefined in this case.

How can I get rid of that message?

Answer 1

Typing of Map explicitly states that get can result in undefined :

interface Map<K, V> {
    ...
    get(key: K): V | undefined;
    ...
}

That's why you're getting error with strictNullChecks enabled.

You can use non-null assertion operator to inform the compiler that you're sure that it actually has value:

this.subscribers.get(event)!.push(callback);

Another option (the better one in my opinion) is to refactor your code in following way:

public subscribe(event: string, callback: (data: any) => void) {
    let callbacks = this.subscribers.get(event);
    if (!callbacks) {
        callbacks = []
        this.subscribers.set(event, callbacks);
    }

    callbacks.push(callback);
}