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How to define a custom TypeScript definition of an array where all elements are number except first

November 22, 2021

The arrays in question are SVG path segments, for instance ['L', 0, 0] I’m basically using this to define these arrays:

// doSomethingToSegment.js

/** 
 * typedef {(string|number)[]} segment
 */
  
/**
 * @param {segment} seg input segment
 * @param {object} options
 */
function doSomethingToSegment(seg, options) {}

Now the doSomethingToSegment() function will say that Math.round(someVar,3) is not ok because someVar is not ok with type string, or otherVar.toUpperCase() is not ok with type number.

So, how to define a custom type definition that satisfies this specific need?

>Solution :

type A = [string, ...number[]];

More info about rest elements in tuple types:
https://www.typescriptlang.org/docs/handbook/2/objects.html#tuple-types

Here are examples from the docs:

Tuples can also have rest elements, which have to be an array/tuple
type.

type StringNumberBooleans = [string, number, ...boolean[]];
type StringBooleansNumber = [string, ...boolean[], number];
type BooleansStringNumber = [...boolean[], string, number];