Make an array element nullable in Typescript

user_5405361 0
Viewed 1251

I am astonished not to have an error in this code:

let myList: string[] = require('something')

let foo: string | undefined = myList[7]

foo.length // no error!

How can I make foo have type string | undefined?

I want the typescript compiler to raise Object is possibly 'undefined'.ts(2532) on the third line.

typescript-typings typescript
2 Answers

It looks like TypeScript sees that myList is string[], so myList[7] is detected to be of type string.

Although it's assignable to something of type string | undefined, TypeScript determines that it can't be undefined, so it automatically narrows the type of foo to string only.

If you wanted this to produce an error, you would have to type myList as Array<string | undefined>:

let myList: Array<string | undefined> = require('something')

let foo: string | undefined = myList[7]
console.log(foo);
foo.includes('') // Error
foo.length // Error

If you want TypeScript to understand that accessing past the end of the array will result in undefined, use a tuple type instead:

let myList: [string, string] = require('something')
let foo = myList[7]

Then foo is typed as undefined, because it's past the length of the array.

If you don't know in advance how many elements the array has, the easiest way to get TypeScript to throw the error you want is really to type the array as Array<string | undefined>, since accessing any numeric index of the array may produce a string or undefined, despite the fact that the array's own-property values are all strings.

user_95152070

You can "cast" the result of the array access to (string|undefined)

let myList : string[] = require('something');
let v = myList[7] as (string | undefined); //v will have type string|undefined
let uv = v.length;  //will give an error that v is possibly undefined
user_37769270
Related