One aspect of control flow based type analysis is that the TypeScript compiler narrows the type of a variable within a type guard.

This lesson explores how you can define functions and type predicates to create your own type guards similar to the Array.isArray() method.

const numbers = [0, 1, 2, [3, 4], 5, [6], [7], 8, [9]];

 
function isFlat<T>(array: (T | T[])[]): array is T[] {
    console.log(!array.some(Array.isArray));
    return !array.some(Array.isArray);
}

if (isFlat(numbers)) {
    numbers;
}

isFlat function return value is a boolean value. We add ‘array is T[]‘ that adds additional information for types.

isFlat(numbers): numbers type is ‘(number|number())[]‘

but inside if statement: numbers is ‘number[]‘, because we tell typescript, array is T[] in the return value.

[TypeScript] Define Custom Type Guard Functions in TypeScript