```ts

// solution.ts

export function fillBetween(entries: string[], filler: string): string[] {
    if (!Array.isArray(entries) || entries.length < 2) {
        return [];
    }

    const result = [];
    for (let i = 0; i < entries.length - 1; ++i) {
        result.push(entries[i], filler);
    }
    result.push(entries[entries.length - 1]);

    return result;
}
```