As you asked, here's how to do it:

1. Mapping the array to an object
2. Converting the object to JSON

let array = [{
    Field: 'Key',
    Value: '7'
  },
  {
    Field: 'City',
    Value: 'Some City'
  },
  {
    Field: 'Description',
    Value: 'Some Description'
  }
];

// #1 Mapping the array to an object...
let obj = {};
array.forEach(item => obj[item.Field] = item.Value);

// #2 Converting the object to JSON...
let json = JSON.stringify(obj);

console.log(json);

Bonus (ES6 + reduce):

const obj = array.reduce((acc, { Field, Value }) => ({ ...acc, [Field]: Value }), {});

@Jaromanda X is correct- you are looking for JSON.parse. Something like this would suffice:

responseArray =  "[{"id":"blah", "type":"blah", ...}, {"id":"blah2",.... },...]"
<Item[]> JSON.parse(responseArray)

Obviously, this doesn't do any validation of the response (which is bad practice). You should ideally perform some validation of the result:

responseArray =  "[{"id":"blah", "type":"blah", ...}, {"id":"blah2",.... },...]"

var result;
try {
   itemListResponse = <Item[]>JSON.parse(responseArray);

   if(!itemListResponse.has("id") ||
      !itemListResponse.has("type") ||
      !itemListResponse.has("state")){

      throw "Invalid Item";
   }
} catch (e){

}

Or, alternatively, use a JSON Schema validator like ajv.

The interface has an iD field and the json has a date field. They need to be the same types. You also might need to use JSON.parse(stringValue) depending on how you are getting the JSON data.

To convert any JSON to array, use the below code:

const usersJson: any[] = Array.of(res.json());

TypeScript type assertions

TypeScript allows you to override its inferred and analyzed view of types any way you want to. This is done by a mechanism called "type assertion".

Type assertions have two forms. One is the "angle-bracket" syntax:

let someValue: any = "this is a string";
let strLength: number = (<string>someValue).length;

And the other is the as-syntax:

let someValue: any = "this is a string";
let strLength: number = (someValue as string).length;

Answer to this question

In your case the problem is not the type assertion though. You are trying to assigning an array of plain JSON objects to users variable and then trying to call a method that doesn't exist on an object (users[0].isDriver is undefined).

You can't simply cast a plain object to a JavaScript/TypeScript class instance. You have to instantiate a User first. There are a number of techniques for doing it, and generally involve copying data.

For an example solution of "casting" a JSON to an instance of TS class please see one of the following answers:

1. https://stackoverflow.com/a/22886730/6103920
2. https://stackoverflow.com/a/29759472/6103920

This should do the job:

names: string[] = jsonObject.map(person => person.name); 

// => ["Bill Gates", "Max Payne", "Trump", "Obama"]

If the response is what you included then it's simple:

JSON.parse(data.text()).forEach(item => {
    console.log(item.name);
});

As your body is an array of objects of this interface:

interface ObjectInResponseArray {
    name: string;
}

You gave incorrect type to your parsed data. Should be something like this:

interface MyObj {
  name: string
  description: string
}

let obj: { string: MyObj[] } = JSON.parse(data.toString());

So it's not MyObj, it's object with property string containing array of MyObj. Than you can access this data like this:

console.log(obj.string[0].name, obj.string[0].description);

Instead of using anonymous type, you can also define interface for it:

interface MyRootObj {
  string: MyObj[];
}

let obj: MyRootObj = JSON.parse(data.toString());