I have defined this object in the TypeScript file and have use the following reduce() method and there is no issue with it.

  sampleList = {
    totalRecordsTest: {},
    sampleList: [
      {
        parent: 'Sample Text 2',
        children: [
          {
            id: 1,
            text: "Sample Text 1"
          },
        ],
      },
      {
        parent: 'Sample Text 2',
        children: [
          {
            id: 2,
            text: "Sample Text 2"
          },
        ],
      },
      {
        parent: 'Sample Text 3',
        children: [
          {
            id: 3,
            text: "Sample Text 3"
          },
        ],
      },
    ]
  }

    let grouped = this.sampleList.sampleList.reduce((groups, current) => {
      groups[current.parent] = groups[current.parent] || [];
      groups[current.parent].push(...current.children);
      return groups;
    }, Object.create(null));

    this.groupedSampleList = Object.keys(grouped).map((key) => ({
      parent: key,
      children: grouped[key],
    }));

However, when I tried to create a model interface file for the "sampleList" in another file I am getting the error Type 'undefined' cannot be used as an index type in the following method for the current variable.

TypeScript File

let grouped2 = this.sampleList.reduce((groups, current) => {
  groups[current.parent] = groups[current.parent] || [];
  groups[current.parent].push(...current.children);
  return groups;
}, Object.create(null));

Import model in TypeScript

sampleList: SampleList[] = []

Model File

export interface SampleListModel {
    parent?: string | '-',
    children?: ChildrenSampleModel[];
}

export class SampleList implements SampleListModel {
    constructor(
        public parent?: string | '-',
        public children?: ChildrenSampleModel[]
    ) { }
}

export interface ChildrenSampleModel {
    id?: number,
    text?: string
}

export class ChildrenSample implements ChildrenSampleModel {
    constructor(
        public id?: number,
        public text?: string
    ) { }
}

Solution

The parent is declared as optional which is why it can be undefined and when I use it as an index it will throw the error "Type 'undefined' cannot be used as an index type in the following method for the current variable."

export interface SampleListModel {
    parent: string,
    children: ChildrenSampleModel[];
}

export class SampleList implements SampleListModel {
    constructor(
        public parent: string,
        public children: ChildrenSampleModel[]
    ) { }
}