t.replace不是函数（...）修剪

The following is producing the error in the question when I check in console.log and it breaks it

var map = L.map('map').setView([0, 0], 2);

<?php 
    $classesForCountries = [];
    if (have_posts()) : while (have_posts()) : the_post();
        $classesForCountries[ get_field('country') ] += get_field('year') + ' ';
    endwhile; endif;
?>

// Now this should look something like {"Australia": "2006 2010 "}

var classNameMap = <?php echo JSON_encode($classesForCountries); ?>;
geojson = L.geoJson(statesData, {
    style: function(feature) {

        // Here is where we got the issue

        var classes = classNameMap[feature.properties.sovereignt];
        if (classes) {
            return {className: classes};
        }
    },
}).addTo(map);

UPDATE

by looking at console.log, it points me to a line in the library:

...t.replace(/^\s+|\s+$/g,"")},splitWords:function(t){return o.Util.trim(t).split(/\s+/)}...

"@Fred-ii- lol superb. Thanks a lot. Put that in an answer and I will accept it. – rob.m"

As per OP's request:

This += get_field('year') + ' '

You may have come from a JS/C background and think that the + signs can be used for concatenation in PHP. PHP is interpreting the plus signs in thinking you want to do math.

It is the dots that concatenate in PHP:

.= get_field('year') . ' '