How can I get a post field value using javascript?

Can I use the API and a JS ajax function to get the two values?

Yes, absolutely.

Step 1: (PHP) Registers the custom fields (i.e. add them to the WordPress REST API response for Posts)

register_meta( 'post', 'address1', [
    'type'         => 'string', // string/boolean/integer/number
    'single'       => true,
    'show_in_rest' => true,
] );

register_meta( 'post', 'address2', [
    'type'         => 'string',
    'single'       => true,
    'show_in_rest' => true,
] );

See Modifying Responses | REST API Handbook and register_meta().

Step 2 (PHP): Loads the official client/JS library

add_action( 'wp_enqueue_scripts', function(){
    wp_enqueue_script( 'wp-api' );
} );

See Backbone JavaScript Client | REST API Handbook.

Step 3: Add the JS code

.. which is as simple as: (refer to your question for the postId variable)

var post = new wp.api.models.Post( { id: postId } );

post.fetch().done( function(){
    // Use `post.getMeta( 'KEY' )` to retrieve the meta value.
    alert( post.getMeta( 'address1' ) );
    alert( post.getMeta( 'address2' ) );
} );

See https://developer.wordpress.org/rest-api/using-the-rest-api/backbone-javascript-client/#model-examples ..

---

UPDATE: An alternative to the wp-api script.

If you don’t want to use the official client/JS library, or if you want just a simple jQuery script, then…

Step 2:

Don’t queue/load the wp-api script.

Add this somewhere on the page: (the variable name can be changed, but make sure to also change the one in the JS code in step #3)

<script>
var restApiSettings = {
    nonce: '<?= wp_create_nonce( 'wp_rest' ) ?>',
    root: '<?= esc_url_raw( rest_url( 'wp/v2' ) ) ?>'
};
</script>

Or with PHP/WordPress, use wp_localize_script() like the official client/JS library does it.

wp_localize_script( 'your-script', 'restApiSettings', [
    'root'  => esc_url_raw( rest_url( 'wp/v2' ) ),
    'nonce' => wp_create_nonce( 'wp_rest' )
] );

Step 3: The JS/jQuery code:

jQuery.get( restApiSettings.root + '/posts/' + postId, {
    _wpnonce: restApiSettings.nonce
}, function( res ){
    if ( res.meta ) {
        alert( res.meta.address1 );
        alert( res.meta.address2 );
    }
} );