请问该怎么做？我正在尝试使用 ReactiveVar 设置占位符的值，但一直出现此错误。它显示 ReactivarVar 无法看到它的设置函数。

这是错误

Uncaught TypeError: Cannot read property 'set' of undefined
    at Object.change #identity (login.js:19)
    at blaze.js?hash=a1ff2d6d5ecd59ee11e2ba260b8650a9d1140f59:3775
    at Function.Template._withTemplateInstanceFunc (blaze.js?hash=a1ff2d6d5ecd59ee11e2ba260b8650a9d1140f59:3744)
    at Blaze.View.<anonymous> (blaze.js?hash=a1ff2d6d5ecd59ee11e2ba260b8650a9d1140f59:3774)
    at blaze.js?hash=a1ff2d6d5ecd59ee11e2ba260b8650a9d1140f59:2617
    at Object.Blaze._withCurrentView (blaze.js?hash=a1ff2d6d5ecd59ee11e2ba260b8650a9d1140f59:2271)
    at Blaze._DOMRange.<anonymous> (blaze.js?hash=a1ff2d6d5ecd59ee11e2ba260b8650a9d1140f59:2616)
    at HTMLSelectElement.<anonymous> (blaze.js?hash=a1ff2d6d5ecd59ee11e2ba260b8650a9d1140f59:863)
    at HTMLDivElement.dispatch (jquery.js?hash=e15504227515793c40a6a3a8ad340399cb657500:5282)
    at HTMLDivElement.elemData.handle (jquery.js?hash=e15504227515793c40a6a3a8ad340399cb657500:4934)

这是代码

Template.login.onCreated(() => {
    this.identify = new ReactiveVar( "" );
});

Template.login.helpers({
    identify() {
        return Template.instance().identify.get();
      }
})

Template.login.events({
    'change #identity'(event, template){
        event.preventDefault();

        if ($(event.target).val() == "one") {
            console.log("Enter Phone Number");
            template.identify.set( "Enter Phone Number" );
        }
        if ($(event.target).val() == "two") {
            console.log("Enter Phone Number");
            template.identify.set( "Enter Age" );
        }
        if ($(event.target).val() == "one") {
            console.log("Enter Phone Number");
            template.identify.set( "Enter ID Card Number" );
        }

    }
});

这是我的模板文件

<form class="startup" id="startup">
    <select name="identity" style="width: 310px; height: 40px;" id="identity" >
        <option selected value="select">Select an option</option>
        <option value="one">One</option>
        <option value="two">Two</option>
        <option value="three">Three</option>
    </select>
    <input type="text" name="identify" id="identify" required="true" placeholder= {{identify}} >
    <input type="password" name="Password" placeholder="Password" required="true">
    <input type="submit" value="Sign Up">
</form>