Add key-value to nested json error - Element implicitly has an 'any'

I have a nested json like so, which I cannot modify as it is part of some package

const myjson = {
properties: {
'firstname':'david'
},
class: 'primary'
}

I want to add another key value to properties, so that it looks like

const myjson = {
properties: {
'firstname':'david',
'lastname' :'watson'
},
class: 'primary'
}

So I used the following

myjson.properties['lastname'] = 'watson';

This gives the error

Element implicitly has an 'any' type because expression of type '"lastname"' can't be used to index type '{ firstname: string; }'.
Property 'lastname' does not exist on type '{ firstname: string; }'.(7053)

I think I have to use as keyof typeof something like below which is wrong syntax and doesn't fix the error

myjson.properties['lastname' as typeof keyof myjson.properties] = 'watson';

Runnable code with the error can be seen here https://www.typescriptlang.org/play?#code/MYewdgzgLgBAtgTwFYXDAvDA3gKBvmABwCcRCBTYqAS3IgC5s8CWByAM2uOjAEM5yreqwAmvAG7URrZvgC+AGlkxgAG14QGMViWpxexBDLk5EKcADoSZSjToBtVup79BMDTCgIKIdjADW5Ai+8MioYFakFFS0EAC6GNoA7rxQ4awA3DigkCCq5BaqIADmABRm4QCUGUA

Edit#1: I made a small error in the order of typeof keyof in the above statement. Now the error is gone. The correct statement is

myjson.properties['lastname' as keyof typeof myjson.properties] = 'watson';

Corrected code is here https://www.typescriptlang.org/play?ssl=7&ssc=76&pln=7&pc=1#code/MYewdgzgLgBAtgTwFYXDAvDA3gKBvmABwCcRCBTYqAS3IgC5s8CWByAM2uOjAEM5yreqwAmvAG7URrZvgC+AGlkxgAG14QGMViWpxexBDLk5EKcADoSZSjToBtVup79BMDTADW5BCHYwoBAo-eGRUMCtSCipaCABdDG0Ad14ocNYAbhxQSBBVcgtVEABzAAozcIBKDKA

Edit#2: Found one more way to do this without the error

Object.assign(myjson.properties, {'lastname': 'watson'});

The inferred type of myjson is

{
properties: {
firstname: string;
};
class: string;
}

You cannot add new properties to this type - you need to let TS compiler know that properties can accept additional keys.
You need to add an explicit type to myjson

Option 1: Index signature

Use Index signature if you have:

• arbitrary property names
• all sharing same type

interface MyData {
properties: {
[key: string]: string
},
class: string
}
const myjson: MyData = {
properties: {
'firstname':'david'
},
class: 'primary'
}
myjson.properties.lastname = 'watson';

Option 2: optional properties

Use optional properties if you have:

• a few well-defined property names
• possibly with different types

interface MyData {
properties: {
firstname?: string,
lastname?: string,
age?: number
},
class: string
}
const myjson: MyData = {
properties: {
'firstname':'david'
},
class: 'primary'
}
myjson.properties.lastname = 'watson';

Option 3: create new object

const myjson: MyData = {
properties: {
'firstname':'david'
},
class: 'primary'
}
const newJson = {
...myjson,
properties: {
...myjson.properties,
lastname: 'watson'
}
}