Magento 2.3 - How to send associative as a response using custom rest API?

Question

I need to send associative array in response. Keys can be dynamic like

{
 'a':'1',
 'b':'2'
}

I already tried mixed[], mixed, string, string[], array and array[]. But I always get following result.

{
 '1',
 '2'
}

Answer 1

First create this interface in your app/code/Vendor/Module/Api/Data

/**
 * Interface which represents associative array item.
 */
interface AssociativeArrayItemInterface
{
    /**
     * Get key
     * 
     * @return string
     */
    public function getKey();

    /**
     * Get value
     * 
     * @return string
     */
    public function getValue();
}

Apply @return \Vendor\Module\Api\Data\AssociativeArrayItemInterface[] datatype for on comment

/**
    * @return \Vendor\Module\Api\Data\AssociativeArrayItemInterface[]
    **/
    public function getSettings();

    /**
    * @return \Vendor\Module\Api\Data\AssociativeArrayItemInterface[]
    **/
    public function getExtra();

SECOND OPTION: send data in below formate

[{"a":"1","b":"2"}]