Question 184390 in Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript – Specify rest arguments first in Typescript

I have a function which receives a variable number of arguments (each of type string), and a final argument (of a complex type, called Expression).

In JavaScript, this looks like:

function layerProp(...args) {
  const fields = args.slice(0, -1)
  const fallbackExpression = args.slice(-1)[0]

How can I write a type definition for this function?

Tags:

4

Answers


  1. 0

    For the function argument you can write as ...args: string[], From the function implementation fallbackExpression is a simple string. So it would be better to use a interface instead of a type

    function layerProp(...args: string[]) {
  const fields = args.slice(0, -1)
  const fallbackExpression = args.slice(-1)[0]
  // rest of the code
}
    Login or Signup to reply.
  2. 0

    You can declare a union type like:

    (string | Expression)[]

    Then your code becomes:

    function layerProp(...args: (string | Expression)[]) {
  const fields = args.slice(0, -1)
  const fallbackExpression = args.slice(-1)[0]
  // rest of the code
}
    Login or Signup to reply.
  3. 0

    If your method gets one complex Expression object, and a bunch of strings, you might want to consider flipping the positions of the arguments, and then you can do this:

    function layerProp(expression: Expression, ...fields: strings[]) {
  // ... your code
}
    Login or Signup to reply.
  4. 0

    You can use the following type:

    function layerProp(...args: [...string[], Expression]) {}

    However, it won’t type the slice and the easiest solution would be just to assert the last element to Expression as follows:

    function layerProp(...args: [...string[], Expression]) {
  const fields = args.slice(0, -1);
  const fallbackExpression = args.slice(-1)[0] as Expression;
}

    Usage:

    type Expression = {
  a: string;
}

layerProp('a', 'b', 'c', { a: 'sd' });
layerProp('a', 'b', 'c', { a: 'sd' }, 'a'); // expected error
layerProp('a', 'b', 'c', 'a'); // expected error

    playground

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search