Add jest-platform section to the docs (#5114)

Add section to the docs that showcases the various npm packages that jest publishes.

Author: GAumala

docs/JestPlatform.md

@@ -0,0 +1,174 @@
+---
+id: jest-platform
+title: Jest Platform
+---
+
+You can cherry pick specific features of Jest and use them as standalone packages. Here's a list of the available packages:
+
+## jest-changed-files
+
+Tool for identifying modified files in a git/hg repository. Exports two functions:
+
+-  `getChangedFilesForRoots` returns a promise that resolves to an object with the changed files and repos. 
+- `findRepos` returns a promise that resolves to a set of repositories contained in the specified path.
+
+### Example
+
+``` javascript
+const {getChangedFilesForRoots} = require( 'jest-changed-files');
+
+// print the set of modified files since last commit in the current repo
+getChangedFilesForRoots(['./'], {
+  lastCommit: true,
+}).then(result => console.log(result.changedFiles));
+
+```
+
+You can read more about `jest-changed-files` in the [readme file](https://github.com/facebook/jest/blob/master/packages/jest-changed-files/README.md).
+
+## jest-diff
+
+Tool for visualizing changes in data. Exports a function that compares two values of any type and returns a "pretty-printed" string illustrating the difference between the two arguments. 
+
+### Example
+
+``` javascript 
+const diff = require('jest-diff');
+
+const a = {a: {b: {c: 5}}};
+const b = {a: {b: {c: 6}}};
+
+const result = diff(a, b);
+
+// print diff
+console.log(result);
+```
+
+## jest-docblock
+
+Tool for extracting and parsing the comments at the top of a JavaScript file. Exports various function to manipulate the data inside the comment block.
+
+### Example
+
+``` javascript
+const { parseWithComments } = require('jest-docblock');
+
+const code = `
+/**
+ * This is a sample
+ *
+ * @flow
+ */
+ 
+ console.log('Hello World!');
+`;
+
+const parsed = parseWithComments(code);
+
+// prints an object with two attributes: comments and pragmas.
+console.log(parsed)
+```
+You can read more about `jest-docblock` in the [readme file](https://github.com/facebook/jest/blob/master/packages/jest-docblock/README.md).
+
+## jest-get-type
+
+Module that identifies the primitive type of any JavaScript value. Exports a function that returns a string with the type of the value passed as argument. 
+
+### Example
+
+``` javascript
+const getType = require('jest-get-type');
+
+const array = [1, 2, 3];
+const nullValue = null;
+const undefinedValue = undefined;
+
+// prints 'array'
+console.log(getType(array));
+// prints 'null'
+console.log(getType(nullValue));
+// prints 'undefined'
+console.log(getType(undefinedValue));
+```
+
+## jest-validate
+
+Tool for validating configurations submitted by users. Exports a function that takes two arguments: the user's configuration and an object containing an example configuration and other options. The return value is an object with two attributes: 
+
+- `hasDeprecationWarnings`, a boolean indicating whether the submitted configuration has deprecation warnings, 
+- `isValid`, a boolean indicating whether the configuration is correct or not. 
+
+### Example 
+
+``` javascript
+const { validate }  = require('jest-validate');
+
+const configByUser = {
+  transform: '<rootDir>/node_modules/my-custom-transform'
+}
+
+const result = validate(configByUser, {
+  comment: '  Documentation: http://custom-docs.com',
+  exampleConfig: { transform: '<rootDir>/node_modules/babel-jest' },
+});
+
+console.log(result);
+```
+
+You can read more about `jest-validate` in the [readme file](https://github.com/facebook/jest/blob/master/packages/jest-validate/README.md).
+
+## jest-worker
+
+Module used for parallelization of tasks. Exports a class `Worker` that takes the path of Node.js module and lets you call the module's exported methods as if they where class methods, returning a promise that resolves when the specified method finishes its execution in a forked process.  
+
+### Example 
+
+``` javascript
+// heavy-task.js
+
+module.exports = {
+  myHeavyTask: (args) => {
+    // long running CPU intensive task.
+  }
+}
+```
+
+``` javascript
+// main.js
+
+async function main() {
+  const worker = new Worker(require.resolve('./heavy-task.js'));
+
+  // run 2 tasks in parellel with different arguments
+  const results = await Promise.all([
+    worker.myHeavyTask({ foo: 'bar' }), 
+    worker.myHeavyTask({bar: 'foo' }),
+  ]); 
+
+  console.log(results);
+}
+
+main();
+```
+
+You can read more about `jest-worker` in the [readme file](https://github.com/facebook/jest/blob/master/packages/jest-worker/README.md).
+
+## pretty-format
+
+Exports a function that converts any JavaScript value into a human-readable string. Supports all built-in JavaScript types out of the box and allows extension for application-specific types via user-defined plugins.
+
+### Example
+
+``` javascript
+const prettyFormat = require('pretty-format'); 
+
+const val = {object: {}};
+val.circularReference = val;
+val[Symbol('foo')] = 'foo';
+val.map = new Map([['prop', 'value']]);
+val.array = [-0, Infinity, NaN];
+
+console.log(prettyFormat(val));
+```
+
+You can read more about `pretty-format` in the [readme file](https://github.com/facebook/jest/blob/master/packages/pretty-format/README.md).

website/i18n/en.json

@@ -9,6 +9,7 @@
     "getting-started": "Getting Started",
     "api": "Globals",
     "jest-object": "The Jest Object",
+    "jest-platform": "Jest Platform",
     "manual-mocks": "Manual Mocks",
     "migration-guide": "Migrating to Jest",
     "mock-function-api": "Mock Functions",

website/sidebars.json

@@ -6,6 +6,7 @@
       "asynchronous",
       "setup-teardown",
       "mock-functions",
+      "jest-platform",
       "more-resources"
     ],
     "API Reference": [