Update index.js

[zuck]

Resolve TypeError if title is not a string.

[yyx990803]

When does this happen? When submitting PRs, make sure to provide enough context so that we can determine whether this actually fixes the problem.

[ulivz]

Please give enough context, thanks!

[zuck]

Yeah, sorry guys. I was trying to port an existing (working) project from Hexo to Vuepress.

Following the "getting started" guide I simply ran vuepress dev from root folder , receiving the following error:

TypeError: str.replace is not a function
    at parseEmojis (/usr/lib/node_modules/vuepress/lib/util/index.js:3:14)
    at exports.inferTitle (/usr/lib/node_modules/vuepress/lib/util/index.js:38:12)
    at Promise.all.options.pageFiles.map (/usr/lib/node_modules/vuepress/lib/prepare.js:193:19)
    at <anonymous>

Of course I can guess this is related to the fact the root folder of my existing Hexo project contains many different kind of files, but docs are a little bit vague about project structure for beginners.

[meteorlxy]

I find what happened. I have ever met similar problem in another project.

If you set title in the frontmatter of a page (not home: true) to a pure Number value, it will try to call .replace() on a Number object, which leads to this error.

E.g. set the frontmatter of docs/config/README.md:

---
sidebar: auto
title: 233
---

Should be merged.

[ulivz]

@znck Thanks for this PR, but with some conflict and not a complete fix.
@meteorlxy thanks for your investigation.

completely fixed at 5023d19

[znck]

Not me @ulivz.

@zuck Thanks for the PR.