JS - access the object value by key but occur the message [TypeError: Cannot read property 'XXX' of undefined]

Question

I have following data

indexMap = {"A": 0, "B": 1};
self.ctx.chart.data.datasets = [
    {
        "label" : "A",
        "data" : [0,0,0,0,0]
    },
    {
        "label" : "B",
        "data" : [0,0,0,0,0]
    },
]

When I want to access data array of the first object of the datasets array,
I write the code below:

var idx = indexMap["A"];
var data = self.ctx.chart.data.datasets[idx].data; // <--- error occur [TypeError: Cannot read property 'data' of undefined]

But the error occurs, that says:

TypeError: Cannot read property 'data' of undefined

But the self.ctx.chart.data.datasets[idx] can print the value like below:

{
    "label" : "A",
    "data" : [0,0,0,0,0]
}

the variable self.ctx.chart.data.datasets[idx] really has the key 'data'

How do I fix it?
BTW, when I using the code below is no problem, why?

var idx = 0;
var data = self.ctx.chart.data.datasets[idx].data; // <--- this is ok!!! why!!!!

Answer 1

It is working fine if the data provided are correct

 let indexMap = {"A": 0, "B": 1};
    
    this.ctx = [
        {
            "label" : "A",
            "data" : [0,0,0,0,0]
        },
        {
            "label" : "B",
            "data" : [0,0,0,0,0]
        },
    ];
    
    let id = indexMap["A"];
    console.log((id))
    
     this.result = this.ctx[id].data; 

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