How to discriminate type and array of type in typescript?

Question

In typescript, how can I discriminate between a user defined type and an array of such type?

type SomeType = { foo: string; };

function doSomething(x: SomeType | SomeType[]) {
    // Is x an array?
}

When the argument is a primitive type or an array, it is straightforward with typeof, but this does not work with user defined types.

Jérémie B · Accepted Answer · 2021-02-06 16:30:21Z

2

Use Array.isArray :

interface A { }

function someFunction(prop: A|A[]) {
    if (Array.isArray(prop)) {
        prop.length
    }
}

answered Feb 6, 2021 at 16:30

Jérémie B

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