how javascripts startsWith function works

const startsWith = (stringValue, searchValue, position) => {
    if (typeof stringValue === "string" && (["string", "boolean", "number"].includes(typeof searchValue) || searchValue instanceof Array)) {
        if (position) return stringValue.slice(position).slice(0, searchValue.toString().length) === searchValue.toString()
        else return stringValue.slice(0, searchValue.toString().length) === searchValue.toString()
    } else return false
}

// example
// code | expected output

startsWith("testing oof", "testing") // true
startsWith("testing oof", "oof", 8)  // true
                                     //
startsWith()                         // false
startsWith("testing oof")            // false
startsWith("testing oof", "oof")     // false
startsWith("testing oof", "oof", 10) // false