Can you explain the distinction between `any[]` and `{ [s: string]: any }`?

I was attempting to make this code snippet function properly:

test(a: any[]) {
    let b: string[][] = [];
    b.push(Object.keys(a[0]));
    b.push(...a.map(e => Object.values(e)));
  }

However, the compiler is throwing an error for the 

b.push(...a.map(e => Object.values)));
line: "TS2345: Argument of type 'unknown[]' is not assignable to parameter of type 'string[]'. Type 'unknown' is not assignable to type 'string'"

Surprisingly, changing the parameter type to test(a: { [s: string]: any }) { fixes the issue.

My queries:

  • Why does it work with the unconventional type?
  • What exactly does the peculiar type signify? It seems like a confusing object with keys (possibly of array type) that map to any.
  • When using a.map(e =>, without the odd type it infers the type of e as e?: any, but with the unique signature, it deduces as e: any. What is the meaning of e?: any?
typescript

Answer №1

Can you explain that unusual type to me?

You are asking about the type { [s: string]: any }, which is known as an index signature:

When you don't know all property names of a type in advance but have an idea of their values, an index signature can be used.

This allows you to describe the types of potential values

(highlighted by me)

As mentioned by kelly in the comments on the question, this represents an object with limited information about its properties, where the values for these properties are of type any.

How does it function with this unique type?

In reality, it "functions" when you define the argument a as an array of this type:

function test2(a: { [s: string]: any }[]) { // Defined `a` as an array of objects with the distinct type
  let b: string[][] = [];
  b.push(Object.keys(a[0]));
  b.push(...a.map(e => Object.values(e)));
  //              ^? (parameter) e: { [s: string]: any; }
}

Since a is an array, the compiler understands that a.map goes through the elements of that array. These elements are objects with values defined as any, compatible with string.

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