[Typescript] 3 un-typed exports from @material-ui/core

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

@material-ui/core exports 3 references that are not available in the typescript definitions:

isString, isNumber, and styleFunction

Expected Behavior 🤔

@material-ui/core should either not export these variables or provide Typescript types for them.

Steps to Reproduce 🕹

See the testExports.ts file and the console output in this demo: https://codesandbox.io/s/optimistic-wilbur-98lq2

Context 🔦

We are trying to dynamically generate an index.ts file that re-exports all members of @material-ui/core, except those which we have defined in our wrapping library. e.g., we want to customize the function of the TextField elements, so want to override that element with our own export but just re-export most other Material components.

While generating this file, we realized that not all of the actually-exported members of @material-ui/core had types for them.

Your Environment 🌎

Tech	Version
Material-UI	v4.8.3

[oliviertassinari]

@JonKrone Do you want to take care of the modules in styles/transitions.js 😃? (We can handle styleFunction later on)

[oliviertassinari]

I don’t think that any of the modules of styles/transitions.js, aside duration and easing, should be available in the barrel index. I would suggest:

diff --git a/packages/material-ui/src/styles/transitions.js b/packages/material-ui/src/styles/transitions.js
index 1cc6e1ad1..b4839a8a8 100644
--- a/packages/material-ui/src/styles/transitions.js
+++ b/packages/material-ui/src/styles/transitions.js
@@ -28,9 +28,9 @@ export const duration = {
   leavingScreen: 195,
 };

-export const formatMs = milliseconds => `${Math.round(milliseconds)}ms`;
-export const isString = value => typeof value === 'string';
-export const isNumber = value => !isNaN(parseFloat(value));
+function formatMs(milliseconds) {
+  return `${Math.round(milliseconds)}ms`;
+}

 /**
  * @param {string|Array} props
@@ -52,6 +52,9 @@ export default {
     } = options;

     if (process.env.NODE_ENV !== 'production') {
+      const isString = value => typeof value === 'string';
+      const isNumber = value => !isNaN(parseFloat(value));
+
       if (!isString(props) && !Array.isArray(props)) {
         console.error('Material-UI: argument "props" must be a string or Array.');
       }

I believe these methods were only exported recently with #18306.