html中使用js计算字符串所占宽度

String.prototype.getWidth = function (fontSize) {
    var span = document.getElementById("_getwidthID");
    if (span == null) {
        span = document.createElement("span");
        span.id = "_getwidthID";
        span.style.visibility = "hidden";
        span.style.whiteSpace = "nowrap"
    } else {
        span.style.display = "inline-block"
    }
    span.innerText = this;
    span.style.fontSize = fontSize + "px";
    var fontWidth = span.offsetWidth;
    span.style.display = "none";
    return fontWidth;
}

var str=“测试”;

str.getWidth(12);