我有一个带参数的函数：setTimeout("progressBar("+val+")",300);。为什么必须在val旁边加上+，使其成为+val+？
函数调用的上下文：

function progressBar(val){
    var pro_bar = document.getElementById("progress_bar");
    var stat = document.getElementById("status");
    stat.innerHTML = val + "%";
    pro_bar.value = val;
    val++;
    var load = setTimeout("progressBar("+val+")",300);
        if(val == 100){
            stat.innerHTML = "100%";
            pro_bar.value = 100;
            clearTimeout(load);
            document.getElementById("text").innerHTML = "Process Complete";
        }
    }       
    var initial_value = 0;
    progressBar(initial_value);