From eeeb59d1db33c71c8d984bf76229d75dc6734646 Mon Sep 17 00:00:00 2001
From: Tobias Bieniek <tobias.bieniek@gmail.com>
Date: Sat, 23 Nov 2019 18:45:54 +0100
Subject: [PATCH] Collapse whitespace in `class` attributes

---
 .../hbs-minifier-plugin.test.js.snap          | 909 +++++++++++++++++-
 hbs-minifier-plugin.js                        |  26 +
 hbs-minifier-plugin.test.js                   |  30 +
 3 files changed, 962 insertions(+), 3 deletions(-)

diff --git a/__snapshots__/hbs-minifier-plugin.test.js.snap b/__snapshots__/hbs-minifier-plugin.test.js.snap
index 3db7e698..c0f12dfc 100644
--- a/__snapshots__/hbs-minifier-plugin.test.js.snap
+++ b/__snapshots__/hbs-minifier-plugin.test.js.snap
@@ -1,5 +1,306 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) collapses whitespace in concatenated \`class\` attributes 1`] = `
+Program {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              TextNode {
+                "chars": "btn ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [
+                  PathExpression {
+                    "data": false,
+                    "original": "this.isPrimary",
+                    "parts": Array [
+                      "isPrimary",
+                    ],
+                    "this": true,
+                  },
+                  StringLiteral {
+                    "original": "btn--primary",
+                    "value": "btn--primary",
+                  },
+                ],
+                "path": PathExpression {
+                  "data": false,
+                  "original": "if",
+                  "parts": Array [
+                    "if",
+                  ],
+                  "this": false,
+                },
+              },
+              TextNode {
+                "chars": " ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [
+                  PathExpression {
+                    "data": true,
+                    "original": "@stretch",
+                    "parts": Array [
+                      "stretch",
+                    ],
+                    "this": false,
+                  },
+                  StringLiteral {
+                    "original": "btn--stretch",
+                    "value": "btn--stretch",
+                  },
+                ],
+                "path": PathExpression {
+                  "data": false,
+                  "original": "if",
+                  "parts": Array [
+                    "if",
+                  ],
+                  "this": false,
+                },
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) collapses whitespace in concatenated \`class\` attributes 2`] = `
+"
+        <button
+          class=\\"
+            btn
+            {{if this.isPrimary \\"btn--primary\\"}}
+            {{if @stretch \\"btn--stretch\\"}}
+          \\"
+        />
+      
+---
+<button class=\\"btn {{if this.isPrimary \\"btn--primary\\"}} {{if @stretch \\"btn--stretch\\"}}\\"></button>"
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) collapses whitespace in concatenated \`class\` attributes 3`] = `
+Program {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              TextNode {
+                "chars": "foo ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@bar",
+                  "parts": Array [
+                    "bar",
+                  ],
+                  "this": false,
+                },
+              },
+              TextNode {
+                "chars": " baz",
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) collapses whitespace in concatenated \`class\` attributes 4`] = `
+"<button class=\\"   foo   {{@bar}}  baz  \\"/>
+---
+<button class=\\"foo {{@bar}} baz\\"></button>"
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) collapses whitespace in concatenated \`class\` attributes 5`] = `
+Program {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@foo",
+                  "parts": Array [
+                    "foo",
+                  ],
+                  "this": false,
+                },
+              },
+              TextNode {
+                "chars": " bar ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@baz",
+                  "parts": Array [
+                    "baz",
+                  ],
+                  "this": false,
+                },
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) collapses whitespace in concatenated \`class\` attributes 6`] = `
+"<button class=\\"{{@foo}}   bar   {{@baz}}\\"/>
+---
+<button class=\\"{{@foo}} bar {{@baz}}\\"></button>"
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) collapses whitespace in concatenated \`class\` attributes 7`] = `
+Program {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@foo",
+                  "parts": Array [
+                    "foo",
+                  ],
+                  "this": false,
+                },
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) collapses whitespace in concatenated \`class\` attributes 8`] = `
+"<button class=\\"   {{@foo}}   \\"/>
+---
+<button class=\\"{{@foo}}\\"></button>"
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) collapses whitespace in regular \`class\` attributes 1`] = `
+Program {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": TextNode {
+            "chars": "btn btn--primary btn--blue",
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) collapses whitespace in regular \`class\` attributes 2`] = `
+"
+        <button
+          class=\\"
+            btn
+            btn--primary
+            btn--blue
+          \\"
+        />
+      
+---
+<button class=\\"btn btn--primary btn--blue\\"></button>"
+`;
+
 exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) collapses whitespace into single space character 1`] = `
 Program {
   "blockParams": Array [],
@@ -981,10 +1282,311 @@ Program {
 }
 `;
 
-exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) strips leading and trailing whitespace from Program nodes 2`] = `
-"        {{foo}}        
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.35.11) strips leading and trailing whitespace from Program nodes 2`] = `
+"        {{foo}}        
+---
+{{foo}}"
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) collapses whitespace in concatenated \`class\` attributes 1`] = `
+Program {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              TextNode {
+                "chars": "btn ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [
+                  PathExpression {
+                    "data": false,
+                    "original": "this.isPrimary",
+                    "parts": Array [
+                      "isPrimary",
+                    ],
+                    "this": true,
+                  },
+                  StringLiteral {
+                    "original": "btn--primary",
+                    "value": "btn--primary",
+                  },
+                ],
+                "path": PathExpression {
+                  "data": false,
+                  "original": "if",
+                  "parts": Array [
+                    "if",
+                  ],
+                  "this": false,
+                },
+              },
+              TextNode {
+                "chars": " ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [
+                  PathExpression {
+                    "data": true,
+                    "original": "@stretch",
+                    "parts": Array [
+                      "stretch",
+                    ],
+                    "this": false,
+                  },
+                  StringLiteral {
+                    "original": "btn--stretch",
+                    "value": "btn--stretch",
+                  },
+                ],
+                "path": PathExpression {
+                  "data": false,
+                  "original": "if",
+                  "parts": Array [
+                    "if",
+                  ],
+                  "this": false,
+                },
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) collapses whitespace in concatenated \`class\` attributes 2`] = `
+"
+        <button
+          class=\\"
+            btn
+            {{if this.isPrimary \\"btn--primary\\"}}
+            {{if @stretch \\"btn--stretch\\"}}
+          \\"
+        />
+      
+---
+<button class=\\"btn {{if this.isPrimary \\"btn--primary\\"}} {{if @stretch \\"btn--stretch\\"}}\\"></button>"
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) collapses whitespace in concatenated \`class\` attributes 3`] = `
+Program {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              TextNode {
+                "chars": "foo ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@bar",
+                  "parts": Array [
+                    "bar",
+                  ],
+                  "this": false,
+                },
+              },
+              TextNode {
+                "chars": " baz",
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) collapses whitespace in concatenated \`class\` attributes 4`] = `
+"<button class=\\"   foo   {{@bar}}  baz  \\"/>
+---
+<button class=\\"foo {{@bar}} baz\\"></button>"
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) collapses whitespace in concatenated \`class\` attributes 5`] = `
+Program {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@foo",
+                  "parts": Array [
+                    "foo",
+                  ],
+                  "this": false,
+                },
+              },
+              TextNode {
+                "chars": " bar ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@baz",
+                  "parts": Array [
+                    "baz",
+                  ],
+                  "this": false,
+                },
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) collapses whitespace in concatenated \`class\` attributes 6`] = `
+"<button class=\\"{{@foo}}   bar   {{@baz}}\\"/>
+---
+<button class=\\"{{@foo}} bar {{@baz}}\\"></button>"
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) collapses whitespace in concatenated \`class\` attributes 7`] = `
+Program {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@foo",
+                  "parts": Array [
+                    "foo",
+                  ],
+                  "this": false,
+                },
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) collapses whitespace in concatenated \`class\` attributes 8`] = `
+"<button class=\\"   {{@foo}}   \\"/>
 ---
-{{foo}}"
+<button class=\\"{{@foo}}\\"></button>"
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) collapses whitespace in regular \`class\` attributes 1`] = `
+Program {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": TextNode {
+            "chars": "btn btn--primary btn--blue",
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) collapses whitespace in regular \`class\` attributes 2`] = `
+"
+        <button
+          class=\\"
+            btn
+            btn--primary
+            btn--blue
+          \\"
+        />
+      
+---
+<button class=\\"btn btn--primary btn--blue\\"></button>"
 `;
 
 exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) collapses whitespace into single space character 1`] = `
@@ -1974,6 +2576,307 @@ exports[`HBS Minifier plugin (with @glimmer/syntax v0.37.1) strips leading and t
 {{foo}}"
 `;
 
+exports[`HBS Minifier plugin collapses whitespace in concatenated \`class\` attributes 1`] = `
+Template {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              TextNode {
+                "chars": "btn ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [
+                  PathExpression {
+                    "data": false,
+                    "original": "this.isPrimary",
+                    "parts": Array [
+                      "isPrimary",
+                    ],
+                    "this": true,
+                  },
+                  StringLiteral {
+                    "original": "btn--primary",
+                    "value": "btn--primary",
+                  },
+                ],
+                "path": PathExpression {
+                  "data": false,
+                  "original": "if",
+                  "parts": Array [
+                    "if",
+                  ],
+                  "this": false,
+                },
+              },
+              TextNode {
+                "chars": " ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [
+                  PathExpression {
+                    "data": true,
+                    "original": "@stretch",
+                    "parts": Array [
+                      "stretch",
+                    ],
+                    "this": false,
+                  },
+                  StringLiteral {
+                    "original": "btn--stretch",
+                    "value": "btn--stretch",
+                  },
+                ],
+                "path": PathExpression {
+                  "data": false,
+                  "original": "if",
+                  "parts": Array [
+                    "if",
+                  ],
+                  "this": false,
+                },
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin collapses whitespace in concatenated \`class\` attributes 2`] = `
+"
+        <button
+          class=\\"
+            btn
+            {{if this.isPrimary \\"btn--primary\\"}}
+            {{if @stretch \\"btn--stretch\\"}}
+          \\"
+        />
+      
+---
+<button class=\\"btn {{if this.isPrimary \\"btn--primary\\"}} {{if @stretch \\"btn--stretch\\"}}\\"></button>"
+`;
+
+exports[`HBS Minifier plugin collapses whitespace in concatenated \`class\` attributes 3`] = `
+Template {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              TextNode {
+                "chars": "foo ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@bar",
+                  "parts": Array [
+                    "bar",
+                  ],
+                  "this": false,
+                },
+              },
+              TextNode {
+                "chars": " baz",
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin collapses whitespace in concatenated \`class\` attributes 4`] = `
+"<button class=\\"   foo   {{@bar}}  baz  \\"/>
+---
+<button class=\\"foo {{@bar}} baz\\"></button>"
+`;
+
+exports[`HBS Minifier plugin collapses whitespace in concatenated \`class\` attributes 5`] = `
+Template {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@foo",
+                  "parts": Array [
+                    "foo",
+                  ],
+                  "this": false,
+                },
+              },
+              TextNode {
+                "chars": " bar ",
+              },
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@baz",
+                  "parts": Array [
+                    "baz",
+                  ],
+                  "this": false,
+                },
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin collapses whitespace in concatenated \`class\` attributes 6`] = `
+"<button class=\\"{{@foo}}   bar   {{@baz}}\\"/>
+---
+<button class=\\"{{@foo}} bar {{@baz}}\\"></button>"
+`;
+
+exports[`HBS Minifier plugin collapses whitespace in concatenated \`class\` attributes 7`] = `
+Template {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": ConcatStatement {
+            "parts": Array [
+              MustacheStatement {
+                "escaped": true,
+                "hash": Hash {
+                  "pairs": Array [],
+                },
+                "params": Array [],
+                "path": PathExpression {
+                  "data": true,
+                  "original": "@foo",
+                  "parts": Array [
+                    "foo",
+                  ],
+                  "this": false,
+                },
+              },
+            ],
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin collapses whitespace in concatenated \`class\` attributes 8`] = `
+"<button class=\\"   {{@foo}}   \\"/>
+---
+<button class=\\"{{@foo}}\\"></button>"
+`;
+
+exports[`HBS Minifier plugin collapses whitespace in regular \`class\` attributes 1`] = `
+Template {
+  "blockParams": Array [],
+  "body": Array [
+    ElementNode {
+      "attributes": Array [
+        AttrNode {
+          "name": "class",
+          "value": TextNode {
+            "chars": "btn btn--primary btn--blue",
+          },
+        },
+      ],
+      "blockParams": Array [],
+      "children": Array [],
+      "comments": Array [],
+      "modifiers": Array [],
+      "selfClosing": true,
+      "tag": "button",
+    },
+  ],
+}
+`;
+
+exports[`HBS Minifier plugin collapses whitespace in regular \`class\` attributes 2`] = `
+"
+        <button
+          class=\\"
+            btn
+            btn--primary
+            btn--blue
+          \\"
+        />
+      
+---
+<button class=\\"btn btn--primary btn--blue\\"></button>"
+`;
+
 exports[`HBS Minifier plugin collapses whitespace into single space character 1`] = `
 Template {
   "blockParams": Array [],
diff --git a/hbs-minifier-plugin.js b/hbs-minifier-plugin.js
index d1807e16..2bb725e7 100644
--- a/hbs-minifier-plugin.js
+++ b/hbs-minifier-plugin.js
@@ -32,6 +32,32 @@ function createGlimmerPlugin(config) {
       AttrNode: {
         enter(node) {
           skipStack.push(node);
+
+          if (node.name === 'class') {
+            if (node.value.type === 'TextNode') {
+              node.value.chars = node.value.chars
+                .replace(leadingWhiteSpace, '')
+                .replace(trailingWhiteSpace, '')
+                .replace(/[ \t\r\n]+/g, ' ');
+            } else if (node.value.type === 'ConcatStatement') {
+              let { parts } = node.value;
+
+              parts.forEach((part, i) => {
+                if (part.type === 'TextNode') {
+                  let isFirst = i === 0;
+                  let isLast = i === parts.length - 1;
+
+                  part.chars = part.chars
+                    .replace(leadingWhiteSpace, isFirst ? '' : ' ')
+                    .replace(trailingWhiteSpace, isLast ? '' : ' ')
+                    .replace(/[ \t\r\n]+/g, ' ');
+                }
+              });
+
+              node.value.parts = node.value.parts
+                .filter(part => part.type !== 'TextNode' || part.chars !== '');
+            }
+          }
         },
 
         exit(node) {
diff --git a/hbs-minifier-plugin.test.js b/hbs-minifier-plugin.test.js
index 237b5836..a7648b50 100644
--- a/hbs-minifier-plugin.test.js
+++ b/hbs-minifier-plugin.test.js
@@ -164,6 +164,36 @@ for (let version of versions) {
       assert(`<div title="    foo  {{if this.bar "BAR"}}  " />`);
     });
 
+    it('collapses whitespace in regular `class` attributes', function() {
+      assert(`
+        <button
+          class="
+            btn
+            btn--primary
+            btn--blue
+          "
+        />
+      `);
+    });
+
+    it('collapses whitespace in concatenated `class` attributes', function() {
+      assert(`
+        <button
+          class="
+            btn
+            {{if this.isPrimary "btn--primary"}}
+            {{if @stretch "btn--stretch"}}
+          "
+        />
+      `);
+
+      assert(`<button class="   foo   {{@bar}}  baz  "/>`);
+
+      assert(`<button class="{{@foo}}   bar   {{@baz}}"/>`);
+
+      assert(`<button class="   {{@foo}}   "/>`);
+    });
+
     function assert(template, config = {}) {
       let ast = process(template, config);
       expect(ast).toMatchSnapshot();