What's the difference between String[] and [String] in typescript ?

Question

What's the difference between String[] and [String] in typescript ? What is the best option to choose between the two ?

Answer 1

They are not the same!

Update: Since TypeScript 2.7 this is not entirely correct anymore (see below for additions):

• string[] says the variable is an array with values of type string (it can be of any size, even empty).
• [string] says the variable is an array of size >= 1 and the first entry is a string
  • since 2.7: size === 1
• the [type] syntax can be extended, like [type1,type2,type3,typeN] and then requires the array to have a size of at least N and the first N types are as specified, while the following types are the union of those types.

Some different examples that illustrate this issue:

const testA: string[] = []; // good
const testB: [string] = []; // Error, array must have a string at position 0
const testC: [string, number] = ["a", 0]; // good
const testC1 = testC[0]; // testC1 is detected to be string
const testC2 = testC[1]; // testC2 is detected to be number

const testD: [string, number] = ["a", 0, "1"]; // good before 2.7, bad afterwards
const testD1 = testD[2]; // testD1 is detected to be string | number
const testE: [string, number] = ["a", 0, 1]; // good before 2.7, bad afterwards
const testE1 = testE[2]; // testE1 is detected to be string | number
const testF: [string, number] = ["a", 0, null]; // Error, null is not of type string|number

Since TypeScript 2.7

The size is by default fixed. So if you want [string] to allow more than one entry, you need to specify in a very ugly way:

interface MinimumStrTuple extends Array<number | string> {
    0: string;
}