Dynamically assigning properties to an array of typed objects while mapping in Typescript

Question

this has an error:

interface TypeA {
  prop1: string
}
interface TypeB extends TypeA {
  prop2: string
}
const ArrA: TypeA[] = [{ prop1: "foo" }]
const ArrB: TypeB[] = ArrA.map(a => {
  a.prop2 = "bar"
  return a
})

this gets an error: [ts] Property 'prop2' does not exist on type 'TypeA'.

and i want to do this without using any in my interface definitions (like in this Stackoverflow question from 5 years ago). I can't find any other solution on Stackoverflow so I am posing this question here.

score 1 · Accepted Answer · answered Feb 05 '18 at 17:52

1

How about this:

const ArrB: TypeB[] = ArrA.map(a => {
  return {
    ...a,
    prop2: "bar"
  }
})

answered Feb 05 '18 at 17:52

Aaron Beall

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score 0 · Answer 2 · answered Feb 05 '18 at 17:27

I need to use the as keyword for casting and also add prop2 as an optional param on TypeA:

interface TypeA {
  prop1: string;
  prop2?: string;
}
interface TypeB extends TypeA {
  prop2: string;
}
const ArrA: TypeA[] = [{ prop1: "foo" }];
const ArrB: TypeB[] = ArrA.map(a => {
  a.prop2 = "bar";
  return a as TypeB;
});

this is the best solution i could find, would appreciate more idiomatic solutions that 1) dont involve modifying TypeA, and 2) dont involve the use of any. Thank you.

Dynamically assigning properties to an array of typed objects while mapping in Typescript

2 Answers2