Explicit typing - Array of objects

Question

In the below syntax,

interface IPerson{
    firstName: string;
    lastName?: string;
}

const personList = [
    "p1": {firstName: "F1", lastName: "L1"},
    "p2": {firstName: "F2"},
    "p3": {firstName: "F3"}
];

// or

const personList = [
    {"p1": {firstName: "F1", lastName: "L1"}},
    {"p2": {firstName: "F2"}},
    {"p3": {firstName: "F3"}}
];

personList is an array of key:value pairs, where key is of type string and value is of type IPerson

Edit:

Below syntax allows more than one key:value pair at an index of an array

const personList:{[key:string]:IPerson}[] = [
    {
     "p1": {firstName: "F1", lastName: "L1"},
     "p2": {firstName: "F1", lastName: "L1"}   
    },
    {"p2": {firstName: "F2"}},
    {"p3": {firstName: "F3"}}
];

How to explicitly type personList?

First example of const personList is not a valid JS array. — Niladri Basu
– Niladri Basu, Commented Apr 27, 2018 at 15:27
Both posted answers are as good as you're going to get, there's no way in the type system to constrain an object to a single property. — Aaron Beall
– Aaron Beall, Commented Apr 27, 2018 at 20:46

bugs · Accepted Answer · 2018-04-27 15:19:53Z

3

const personList: { [key: string]: IPerson }[]  = [
    {"p1": {firstName: "F1", lastName: "L1"}},
    {"p2": {firstName: "F2"}},
    {"p3": {firstName: "F3"}}
];

Define it as an array of objects whose key is a string and whose value is of type IPerson

answered Apr 27, 2018 at 15:19

bugs

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1 Comment

overexchange Over a year ago

There is a problem with your syntax. Edited

Aaron Beall · Accepted Answer · 2018-04-27 20:59:14Z

1

An index type cannot be constrained to a single property, so both the posted answers are probably as good as you'll get.

As an alternative you could use a tuple type:

const personList: [string, IPerson][] = [
    ["p1", { firstName: "F1", lastName: "L1" }],
    ["p2", { firstName: "F2"}],
    ["p3", { firstName: "F3" }]
];

This will constrain each element of the array to an array of [string, IPerson]:

personList.forEach(item => {
    const [key, person] = item;
    // `key` is type string (p1, p2, p3...)
    // `person` is type IPerson
});

answered Apr 27, 2018 at 20:59

Aaron Beall

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4 Comments

overexchange Over a year ago

Can't I have first element as key?let personList: [[id: string], IPerson][] = [] Because having keys in personList is most important

Aaron Beall Over a year ago

No. Array uses index, object uses keys.

overexchange Over a year ago

you mean tuple is a n element array?

Aaron Beall Over a year ago

Basically, yes, it describes an array with known types at explicit indices.

JackFrost · Accepted Answer · 2018-04-27 15:20:07Z

0

You can create a map interface:

interface Map<T> {
    [K: string]: T;
}

const personList: Map<IPerson> = [];

answered Apr 27, 2018 at 15:20

JackFrost

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1 Comment

overexchange Over a year ago

Your syntax also has same problem. Query already edited. map<IPerson>[] does not work

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