You can use a type guard[1] as an argument to Array.filter, but the function has to be explicitly typed as such. I don't know why the type isn't narrowed in A

You can use a type guard[1] as an argument to Array.filter, but the function has to be explicitly typed as such. I don't know why the type isn't narrowed in Array.filter like it is in if statements without this weird workaround. const array: (number | string)[] = []; const mixedArray = array.filter(value => typeof value === 'string'); // mixedArray: (number | string)[] const arrayOfString = array.filter((value): value is string => typeof value === 'string'); // arrayOfString: string[] This example in Typescript playground: https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhgJwXAngLhgCjAVwLYBGApgjAD4zQICWYA5gJQDaAujALwysDcAUL6Eiw81AB5EAJgEEkqDvFkoAdADNqAGyglMANzjqcRDgD4YUFAAciIFTD0Gj7JzADkVWnRcM+Aeh8wRcWlFDGx8YlIKd3pmFgFwaAVkFAB5FQBlKBp6eURk1Q0tBExdfUMGDHtDGGoISiyPEzNLa1sqx2c3BvovX3881DTM7LoMaLpWIA https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhgJwXA... [1]: https://www.typescriptlang.org/docs/handbook/advanced-types.html#type-guards-and-differentiating-types https://www.typescriptlang.org/docs/handbook/advanced-types....