Javascript 设置发送到API的商户定义数据字段的最大长度

这是发送到Cybersource的商户定义的数据字段3，它返回一个错误，因为字符长度太长，我想将其减少到255个字符

[merchant_defined_data3] => Jadau Polki Design,Jadau Polki Design,All,Jhumar,Jadau Polki Design,Jadau Polki Design,All,Earrings,Jadau Polki Design,Diamond Design,All,Teeka,Jadau Polki Design,Jadau Polki Design,All,Teeka

代码如下： //获取购物车项目

$cart_items = $woocommerce->cart->get_cart();   
$cb_items = array();
if( ! empty( $cart_items ) ){
    if( count($cart_items) == 1 ){

        $x = 0;
        foreach( $cart_items  as $values ){

            if( $values[ 'product_id' ] ){
                $product = wc_get_product( $values['product_id']);
                $products_cats = $product->get_category_ids();
            } else {
                $product = wc_get_product( $values['variation_id']);
                $products_cats = $product->get_category_ids();
            }


            $_product = $values['data']->post;
            $cb_items[ 'merchant_defined_data4' ] = $_product->post_title;
            if( is_array($products_cats) && ! empty($products_cats)){
                $c = array();
                foreach ($products_cats as $value) {
                    if( $term = get_term_by( 'id', $value, 'product_cat' ) ){
                    $c[] = $term->name;
                }   
                }
                $products_cats = implode( ',', $c );
            } else {
                $products_cats = '';
            }
            $cb_items[ 'merchant_defined_data3' ] = $products_cats;
            $x++;
        }
    } else {
        $x = 0;
        foreach( $cart_items  as $values ){

            if( $values[ 'product_id' ] ){
                $product = wc_get_product( $values['product_id']);
                $products_cats = $product->get_category_ids();
            } else {
                $product = wc_get_product( $values['variation_id']);
                $products_cats = $product->get_category_ids();
            }


            $_product               = $values['data']->post;

            $mdd_product[ 'name' ][]  = $_product->post_title; 
            if( ! empty($products_cats) && is_array($products_cats)){
                foreach ( $products_cats as $key => $value) {
                    $mdd_product[ 'cats' ][]  = $value; 
                }
            } else {
                $mdd_product[ 'cats' ]  = $products_cats; 
            }

            $x++;
        }
        if( ! empty( $mdd_product[ 'name' ] ) && is_array($mdd_product[ 'name' ])){
            $cb_items[ 'merchant_defined_data4' ] = implode( ',', $mdd_product[ 'name' ] );
        }
        if( ! empty($mdd_product[ 'cats' ]) && is_array($mdd_product[ 'cats' ] )){
            $c = array();
            foreach ($mdd_product[ 'cats' ] as  $value) {
                if( $term = get_term_by( 'id', $value, 'product_cat' ) ){
                    $c[] = $term->name;
                }
            }
            $cb_items[ 'merchant_defined_data3' ] = implode( ',', $c);
        }

    }
}

---

你只是想任意把它砍掉255个字符，或者你想把它砍掉255个字符之前的最后一个逗号，或者什么？我只是想任意把它砍掉255个字符，不管它们在哪里结束。这对接收数据的API有意义吗？一个简单的应该这么做对不起，这实际上不是我的代码，它来自CyberSource，所以我不知道在哪里放置substr（）以减少长度。