Typescript does assign to an object properties that do not exist in the object type definition-CodePudding

I don't understand why Typescript doesn't highlight this as an error ...

When I conditionally add to an object a new property that doesn't exist in that object type definition, Typescript does assign it

  type Filters = {
      keywords: Array<string>
    }
    
    const condition = 1;
    
    let filters: Filters = {keywords: ['keyword']}
    
    filters = {
      ...filters,
        ...(condition && {...{ tags: ['tag']}}),
      }

Results:

filters: {
  keywords:["keyword"]
  tags:["tag"]
}

While I expected to get this error:

Object literal may only specify known properties, and 'tags' does not exist in type 'Filters'.

PS: I get the expected error when I try to add the same property this way:

filters = {...filters, tags: ['tag']}

CodePudding user response：

This is currently a missing feature of TypeScript; excess property checks only happen in certain contexts, and so far nobody has implemented the sort of contextual checking of the contents of inline objects that are spread into outer objects.

There's an open feature request for this at microsoft/TypeScript#39998. It's currently marked as "Awaiting More Feedback" so you might want to go to that issue, give it a