NodeJS unit testing

Possibly a naive question but I have a class similar to(mine has more data read from database and caching them):

var genericClass = class {

    constructor() {
        this.genericStructure = {};
    }

    async getState(key) {
        //some more core logic
        return this.genericStructure[key];
    }

    async putState(key, genericJson) {
        this.genericStructure[key] = genericJson;
        return true;
    }
}

its getState and putState are used multiple times in a consumer class, I want to mock all these occurences from a local map object in my test class. Can it be done?

I am using chai, mocha, sinon

"devDependencies": {
        "chai": "^4.1.2",
        "chai-as-promised": "^7.1.1",
        "eslint": "^4.19.1",
        "mocha": "^5.2.0",
        "nyc": "^12.0.2",
        "sinon": "^6.0.0",
        "sinon-chai": "^3.2.0"
    }

Solution

Instead of letting your class initialize the structure it wants, you should provide (inject) it the the structure on initialization. This is the principle of Dependency Injection ( Constructor injection ).

var genericClass = class {

    constructor(initialStructure) {
        this.genericStructure = initialStructure;
    }

    async getState(key) {
        //some more core logic
        return this.genericStructure[key];
    }

    async putState(key, genericJson) {
        this.genericStructure[key] = genericJson;
        return true;
    }
}

And now in your tests you can pass whatever you like for initialStructure.

Updated Answer

From your comment seems like the question was a little confusing. I think what you need is to mock the class methods as well to change their functionality, and at the same time support dependency injection. You can use something like below -

function testClassProxy(map) {

    let handleSetProps = function(args){
      map.set(args[0], args[1]);
      return true;
    }

    let handleGetProps = function(args){
      return map.get(args[0]);
    }

    let handler = {
        get(target, propKey) {
            return function (...args) {
                switch(propKey){
                  case 'setProps':{
                    return handleSetProps(args)
                  } 
                  case 'getProps': {
                    return handleGetProps(args)
                  }
                }
            };
        }
    };

    return new Proxy(map, handler);
}


let proxy = testClassProxy(new Map());


console.log(
  proxy.setProps('a', '1'),
  proxy.getProps('a')
);