[Bug Report] how to use validateField in typescript

[mu-yu]

Element UI version

0.1.0

OS/Browsers version

typescript: 2.6.2

Vue version

2.5.9

Reproduction Link

https://jsfiddle.net/mmx38qxw/

Steps to reproduce

使用typescript配合element-ui，在表单验证某个字段的时候使用了如下方法：

(this.$refs[formRef] as ElForm).validateField('packageNames', function(errMsg: string): void {})

但是会报错：TS2349: Cannot invoke an expression whose type lacks a call signature. Type 'void' has no compatible call signatures.

What is Expected?

callback应该为可选项，且可以传入简单的箭头函数

What is actually happening?

callback为必选项，且传入简单的箭头函数会报错：
TS2349: Cannot invoke an expression whose type lacks a call signature. Type 'void' has no compatible call signatures.

[element-bot]

Translation of this issue:

Element UI version

0.1.0

OS/Browsers version

Typescript: 2.6.2

Vue version

2.5.9

Reproduction Link

https://jsfiddle.net/mmx38qxw/

Steps to reproduce

Using typescript with element-ui, the following method is used to verify a field in the form:

(this.$refs[formRef] as ElForm.ValidateField ('packageNames'), function (errMsg: string): void {}). But there will be a mistake:TS2349: Cannot invoke an expression whose type lacks a call signature. Type

What is Expected?

Callback should be optional and can be introduced into a simple arrow function

What is actually happening?

Callback is a necessary option, and the introduction of a simple arrowhead function will be wrong:

` TS2349: Cannot invoke an expression whose type lacks a call signature. Type

[baixiaoji]

I meet the same problem

[tudi2d]

I´m facing the same problem. Writing this.$refs["form"]["validateField"]("packageNames") avoids the type checking. This is not a solution but a work around.