How to make Typescript recognize recursive type without falling on string

Question

I need to write a Typescript program that models numbers tree as nested array. Meaning, it should give an error when some leaf is not a number, but for example boolean or string. I wrote the code:

interface Tree {
    [index: number]: number | Tree;
}
let a:Tree = [1, 3, [4, 8, [], 5, 9, true, [[[0]]]]];
let b:Tree = [true];
let c:Tree = true;
let d:Tree = 3;

and it works as expected, with four compilation errors. First two because true is not a number, last two because the value is not an array. Unfortunately, the code

interface Tree {
    [index: number]: number | Tree;
}
let e:Tree = [1, 3, [4, 8, [], 5, 9, "abc", [[[0]]]]];
let f:Tree = ["abc"];
let g:Tree = "abc";

compiles, despite "abc" leaf is neither a number nor an array of Trees. Somehow, string is recognized as recursive type. Is there a way to make Typescript to see the problem? Maybe different Tree definition, or something else? I know I can solve it easily in JS, using typeof, but I need Typescript (meaning, static) solution. Thank you.

Gorka Hernandez · Accepted Answer · 2017-03-24 15:16:56Z

3

It happens because when accessing an object, numerical keys are treated as if they where strings. You can do the following to constrain the value of the properties though:

interface Tree {
    [key: number]: Array<Tree | number> | number;
}

answered Mar 24, 2017 at 15:16

Gorka Hernandez

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1 Comment

khitron23 Over a year ago

Thank you very much for your help, it works perfect.

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