title: Golang's syscall/js js.NewCallback is undefined
body: <p>I am new to both go and wasm and it looks like these projects are pretty fast moving. <a href="https://www.sitepen.com/blog/compiling-go-to-webassembly/" rel="noreferrer">I was reading an introduction to using wasm in go</a> and it seems although it was written in January, its already out of date.</p>
  
  <p>I am attempting to call a single go function when a JavaScript function is invoked. The example says to do something like this:</p>
  
  <pre><code> js.Global().Set("jsFunctionName", js.NewCallback(goCallback))
  </code></pre>
  
  <p>Although when I am trying to compile this, I am getting this error:</p>
  
  <pre><code>./webassembly.go:54:35: undefined: js.NewCallback
  </code></pre>
  
  <p><a href="https://golang.org/pkg/syscall/js/" rel="noreferrer">I checked the documentation</a> and see no reference to <code>NewCallback</code>. What is the current "correct" way that this should be done?</p>