TypeScript: Parse & Stringify partially known / dynamic JSON. Using classes (type safe).

json.ts

// npm install --save-dev class-transformer

import 'reflect-metadata';

import { classToPlain, deserialize, Exclude, Type } from 'class-transformer';

const inpJsonStr: string = `
{
    "topKnownKey": 1,
    "topUnknownKey": 2,
    "nestedKnownArray": [
        {
            "nestedKnownKey": 3,
            "nestedUnknownKey": 4
        },
        {
            "nestedKnownKey": 5,
            "nestedUnknownKey": 6
        }
    ],
    "nestedUnknownArray": [
        {
            "nestedKnownKey": 7,
            "nestedUnknownKey": 8
        }
    ]
}
`;

export class Top {
    topKnownKey!: number;
    @Type(() => NestedKnownArray) nestedKnownArray!: NestedKnownArray[];
    @Exclude() excludedKnownKey!: number; // Field sample for internal needs, excluded from JSON processing.
}

export class NestedKnownArray {
    nestedKnownKey!: number;
    @Exclude() excludedKnownKey!: number; // Field sample for internal needs, excluded from JSON processing.
}

// Decode.
const top: Top = deserialize(Top, inpJsonStr);

// Modify.
top.topKnownKey = 100;
top.excludedKnownKey = 200;
top.nestedKnownArray[0].nestedKnownKey = 300;
top.nestedKnownArray[0].excludedKnownKey = 400;

// Encode.
const outJsonObj: Object = classToPlain(top);
const outJsonStr: string = JSON.stringify(outJsonObj, null, 2);

// Print output.
console.log(outJsonStr);