为什么当我在记录项目时知道事实时 Map.get() 返回 undefined？

Question

I'm curious if I'm missing something here -- When I call a method that should get something from the collection because I have proof it exists, it doesn't get it still My Object of Collections

export const databaseCache = {
  /* Other collections ommitted */
  roles: new Collection<string, Role>(),
};

My method

export function getRoleByName(name: string) {
  return databaseCache.roles.get(name);
}

and my call

const feedbackMod = getRoleByName("pack feedback helper");

feedbackMod is undefined even though I have proof it exists because I logged the collection, (collection is considered a Map object as Collection is a class that extends Map)

https://imgur.com/hD2RP2S image of proof of collection item existing

Reproduceable Code:

interface Role {
    name: string;
    roleId: number;
}

const myCollections = {
    roles: new Map<string, Role>()
};

myCollections.roles.set("pack feedback helper", { name: "pack feedback helper", roleId: 543576460363956244 });
const item = myCollections.roles.get("pack feedback helper");

console.log(item);

Although when tested online, this works, locally it does not for me.

Answer 1

Almost certainly there is something more going on that your example shows. The likely problem is that you have a String vs. a string kicking around on one side or the other (eg an objectified string vs. a primitive string):

let key = "pack feedback helper";
let memo = new Map();
memo.set(key, "some value here");
memo.get(key); // Works
memo.get("pack feedback helper"); // Works

let objectifiedKey = new String(key);
memo.get(objectifiedKey); // DOES NOT WORK

The simplest test to validate that this is the case is to wrap both the set and the get calls' key argument in String to de-objectify the key:

// Continuing from the example above
let deObjectifiedKey = String(objectifiedKey);  // Note the lack of `new` here
memo.get(deObjectifiedKey); // WORKS AGAIN!

If it is the case that de-objectifying works, then you can trace which side is causing the problem by checking typeof key . It will be object for "objectified" values and string for primitive ones.

Alternatively, test in another browser. If it works in one and not another, then you may have a browser bug.

Answer 2

I found my answer; it is too much code to paste but in short and simple terms it was because some of my methods were lexically scoped, thus being read by the compiler first, where the collection still had no items in it. When I converted the lexical scoped methods to block (es6 arrow functions), everything was able to load in correctly and be used accordingly.