wp_localize_script() │ WP 2.2.0

wp_localize_script() is recommended to be used only for passing localization strings, not for passing data - an object that your script will use. For passing data, it's better to use wp_add_inline_script().

Let's consider an example:

Previously, you could (and still can) do the following:

wp_enqueue_script( 'my-script', 'https://url-to/my-script.js' );

wp_localize_script( 'my-script', 'myScriptData', [
	'ajax_url'  => admin_url( 'admin-ajax.php' ),
	'param_one' => 'some value',
	'param_two' => 25,
] );

But it's better to do it differently to preserve the types of the transmitted data - numbers will remain numbers, booleans true/false, and will not turn into strings:

wp_enqueue_script( 'my-script', 'https://url-to/my-script.js' );

$js = 'window.myScriptData = ' . wp_json_encode( [
	'ajax_url'  => admin_url( 'admin-ajax.php' ),
	'param_one' => 'some value',
	'param_two' => 25,
] );

wp_add_inline_script( 'my-script', $js, 'before' );

Note that you need to add 'before' as the third parameter.

Now in the JS script, you can access this data like this:

console.log( myScriptData.ajax_url );  // "https://example.com/wp-admin/admin-ajax.php"
console.log( myScriptData.param_one ); // "some value"
console.log( myScriptData.param_two ); // 25

An example of inserting global javascript data before the JS script (in the example, the script is jquery). Use this code in functions.php:

add_action( 'wp_enqueue_scripts', 'action_function_name_7714', 99 );

function action_function_name_7714(){

	wp_localize_script( 'jquery', 'object_name', array(
		'some_string' => __( 'Some string to translate' ),
		'some_num' => '10'
	) );
}

We'll get it in the head part of the site:

<script type='text/javascript'>
/* <![CDATA[ */
var object_name = {"some_string":"Some string to translate","some_num":"10"};
/* ]]> */
</script>
<script type='text/javascript' src='http://example.com/wp-includes/js/jquery/jquery.js?ver=1.8.3'></script>

Let's add the script and then the global data to it. Here we add a javascript file with an AJAX request. The file uses the variable MyAjax.ajaxurl as the URL to direct the AJAX request. We will define this URL through wp_localize_script():

add_action( 'wp_enqueue_scripts', 'action_function_name_7714' );

function action_function_name_7714(){

	wp_enqueue_script( 'my-ajax', get_template_directory_uri() . '/js/custom_script.js' );

	wp_localize_script( 'my-ajax', 'MyAjax', [ 'ajaxurl' => admin_url( 'admin-ajax.php' ) ] );

}

we get:

<script type='text/javascript'>
/* <![CDATA[ */
var MyAjax = {"ajaxurl":"http://example.com/wp-admin/admin-ajax.php"};
/* ]]> */
</script>
<script type='text/javascript' src='http://example.com/wp-content/themes/twentyeleven/js/custom_script.js?ver=3.5'></script>

Access to variables in Javascript is as follows (for Example 2):

<script>
// in the file custom_script.js we can use a variable: MyAjax.ajaxurl
alert( MyAjax.ajaxurl ); // print http://example.com/wp-admin/admin-ajax.php
</script>

Note, calling this function multiple times in the a single session with the same object name will overwrite previous values:

wp_localize_script( 'some_handle', 'object_name', $value1 );
... code executed again...
wp_localize_script( 'some_handle', 'object_name', $value2 ); 
// object_name is now set to $value2.

If you need to have multiple data sets associated with the same script (handle), then you need to rename your object,

wp_localize_script( 'some_handle', 'object_name1', $value1 );
... code executed again...
wp_localize_script( 'some_handle', 'object_name2', $value2 );

Variant

Or remove the original code and replace with the code you need:

global $wp_scripts;
$data = $wp_scripts->get_data( 'enqueued-script', 'data' );
if( empty( $data ) ){
	wp_localize_script( 'enqueued-script', 'obj', $localized_data );
}
else{
	if( ! is_array( $data ) ){
		$data = json_decode( str_replace( 'var obj = ', '', substr( $data, 0, -1 ) ), true );
	}
	foreach( $data as $key => $value ){
		$localized_data[ $key ] = $value;
	}
	$wp_scripts->add_data( 'enqueued-script', 'data', '' );
	wp_localize_script( 'enqueued-script', 'obj', $localized_data );
}