Update: Resolve npm installed formatters (#5900)

docs/user-guide/command-line-interface.md

@@ -333,6 +333,17 @@ Example:
 
     eslint -f ./customformat.js file.js
 
+An npm-installed formatter is resolved with or without `eslint-formatter-` prefix.
+
+Example:
+
+    npm install eslint-formatter-pretty
+
+    eslint -f pretty file.js
+
+    // equivalent:
+    eslint -f eslint-formatter-pretty file.js
+
 When specified, the given format is output to the console. If you'd like to save that output into a file, you can do so on the command line like so:
 
     eslint -f compact file.js > results.txt

lib/cli-engine.js

@@ -26,10 +26,14 @@ const fs = require("fs"),
     validator = require("./config/config-validator"),
     stringify = require("json-stable-stringify"),
     hash = require("./util/hash"),
+    ModuleResolver = require("./util/module-resolver"),
+    Plugins = require("./config/plugins"),
     pkg = require("../package.json");
 
 const debug = require("debug")("eslint:cli-engine");
 
+const resolver = new ModuleResolver();
+
 //------------------------------------------------------------------------------
 // Typedefs
 //------------------------------------------------------------------------------
@@ -670,15 +674,28 @@ class CLIEngine {
             // replace \ with / for Windows compatibility
             format = format.replace(/\\/g, "/");
 
+            let cwd = this.options ? this.options.cwd : process.cwd();
+
+            const namespace = Plugins.getNamespace(format);
+
+            if (namespace) {
+                cwd = `${cwd}/${namespace}`;
+                format = Plugins.removeNamespace(format);
+            }
+
             let formatterPath;
 
             // if there's a slash, then it's a file
             if (format.indexOf("/") > -1) {
-                const cwd = this.options ? this.options.cwd : process.cwd();
-
                 formatterPath = path.resolve(cwd, format);
             } else {
-                formatterPath = `./formatters/${format}`;
+                const npmFormat = /^eslint-formatter-/.test(format) ? format : `eslint-formatter-${format}`;
+
+                try {
+                    formatterPath = resolver.resolve(npmFormat, cwd);
+                } catch (_) {
+                    formatterPath = `./formatters/${format}`;
+                }
             }
 
             try {

tests/fixtures/cli-engine/@somenamespace/eslint-formatter-foo/index.js

@@ -0,0 +1 @@
+module.exports = function() {};

tests/fixtures/cli-engine/eslint-formatter-bar/index.js

@@ -0,0 +1 @@
+module.exports = function() {};

tests/lib/cli-engine.js

@@ -2685,6 +2685,42 @@ describe("CLIEngine", () => {
             assert.isFunction(formatter);
         });
 
+        it("should return a function when a formatter prefixed with eslint-formatter is requested", () => {
+            const engine = new CLIEngine({
+                    cwd: getFixturePath("cli-engine")
+                }),
+                formatter = engine.getFormatter("bar");
+
+            assert.isFunction(formatter);
+        });
+
+        it("should return a function when a formatter is requested, also when the eslint-formatter prefix is included in the format argument", () => {
+            const engine = new CLIEngine({
+                    cwd: getFixturePath("cli-engine")
+                }),
+                formatter = engine.getFormatter("eslint-formatter-bar");
+
+            assert.isFunction(formatter);
+        });
+
+        it("should return a function when a formatter is requested within a scoped npm package", () => {
+            const engine = new CLIEngine({
+                    cwd: getFixturePath("cli-engine")
+                }),
+                formatter = engine.getFormatter("@somenamespace/foo");
+
+            assert.isFunction(formatter);
+        });
+
+        it("should return a function when a formatter is requested within a scoped npm package, also when the eslint-formatter prefix is included in the format argument", () => {
+            const engine = new CLIEngine({
+                    cwd: getFixturePath("cli-engine")
+                }),
+                formatter = engine.getFormatter("@somenamespace/eslint-formatter-foo");
+
+            assert.isFunction(formatter);
+        });
+
         it("should return null when a customer formatter doesn't exist", () => {
             const engine = new CLIEngine(),
                 formatterPath = getFixturePath("formatters", "doesntexist.js");