从这段代码中错过`[]`是什么意思？

I have written this bit of code

我写了这段代码

let content = new Immutable.Map({
    metadata: new Immutable.Map({
      cities: 2
    }),
    data: new Immutable.List(
      new Immutable.Map({
        code: 'a',
        label: 'Sydney',
        percentage: 35 / 40,
      }),
      new Immutable.Map({
        code: 'b',
        label: 'Melbourne',
        percentage: 22 / 40,
      }),
    ),
   })
let