Add check method to Prettier Node API. (#1104)

* Add check method to Prettier. Make CLI use that method for list-different

* Catch in check and return false if it throws

* Remove catch/finally from Prettier list-different bin

* remove try catch in prettier bin for list-different

README.md

@@ -175,7 +175,7 @@ exit 1
 
 ### API
 
-The API is a single function exported as `format`. The options
+The API has two functions, exported as `format` and `check`. The options
 argument is optional, and all of the defaults are shown below:
 
 ```js
@@ -212,6 +212,9 @@ prettier.format(source, {
 });
 ```
 
+`check` checks to see if the file has been formatted with prettier given the those options and returns a Boolean.
+This is similar to the `--list-different` parameter in the CLI and is useful for running Prettier in CI scenarios.
+
 ### Excluding code from formatting
 
 A JavaScript comment of `// prettier-ignore` will exclude the next node in the abstract syntax tree from formatting.

bin/prettier.js

@@ -228,6 +228,14 @@ if (stdin) {
       return;
     }
 
+    if (argv["list-different"]) {
+      if (!prettier.check(input, options)) {
+        console.log(filename);
+        process.exitCode = 1;
+      }
+      return;
+    }
+
     const start = Date.now();
 
     let output;
@@ -267,11 +275,6 @@ if (stdin) {
       if (output) {
         console.log(output);
       }
-    } else if (argv["list-different"]) {
-      if (input !== output) {
-        console.log(filename);
-        process.exitCode = 1;
-      }
     } else {
       // Don't use `console.log` here since it adds an extra newline at the end.
       process.stdout.write(output);

index.js

@@ -95,6 +95,14 @@ module.exports = {
   format: function(text, opts) {
     return formatWithShebang(text, normalizeOptions(opts));
   },
+  check: function(text, opts) {
+    try {
+      const formatted = this.format(text, opts);
+      return formatted === text;
+    } catch (e) {
+      return false;
+    }
+  },
   version: version,
   __debug: {
     formatAST: function(ast, opts) {