javascript: Cannot set properties of undefined-CodePudding

I need to dynamically add a new property and try a couple of ways, but nothing works.

var streams = new Map();
streams.set(a, { id: 123 });
streams[a].context = some not-null object;

This is the error.

Cannot set properties of undefined (setting 'context')

So I try to pre-define context as an object.

var streams = new Map();
streams.set(a, { id: 123, context: {} });
streams[a].context = some not-null object;

But it still complains.

CodePudding user response：

var streams = new Map();
streams.set('a', { id: 123 });
streams.get('a').context = some not-null object;

You cannot get data like object/array from map. You have to use get to get the data

CodePudding user response：

Using the JavaScript Map class, you can get properties by using the get function. Here is an example:

var streams = new Map();
streams.set('a', { id: 123 });
streams.get('a').context = "";     // non-null object

You could also use an Object as so:

var streams = {
    a: {
        id: 123
    }
};

streams.a.context = "";
streams['a'].context = "";        // does the same thing