Add helper function to trim and dedent multiline strings

src/shared/test/trim-and-dedent-test.js

+import { trimAndDedent } from '../trim-and-dedent';
+
+describe('trimAndDedent', () => {
+  [
+    ['Foo', 'Foo'],
+    ['   Foo', 'Foo'],
+    [
+      `First line
+  Second line
+    Third line`,
+      `First line
+  Second line
+    Third line`,
+    ],
+    [
+      `
+      
+        Hello, Jane!
+          Indented line
+        Goodbye, John!
+      
+      `,
+      `Hello, Jane!
+  Indented line
+Goodbye, John!`,
+    ],
+  ].forEach(([str, expectedResult]) => {
+    it('normalizes strings with multiple lines', () => {
+      const result = trimAndDedent(str);
+      assert.equal(result, expectedResult);
+    });
+  });
+});

src/shared/trim-and-dedent.ts

+/**
+ * Remove leading and trailing empty lines from a string.
+ */
+function trimEmptyLines(str: string): string {
+  return str.replace(/^\s*\n|\n\s*$/g, '');
+}
+
+/**
+ * Remove common indentation from each line of a string.
+ */
+function dedent(str: string) {
+  // Match the smallest indentation
+  const match = str.match(/^[ \t]*(?=\S)/gm);
+  const indent = match && Math.min(...match.map(el => el.length));
+
+  if (indent) {
+    const regexp = new RegExp(`^ {${indent}}`, 'gm');
+    return str.replace(regexp, '');
+  }
+
+  return str;
+}
+
+/**
+ * Remove leading and trailing empty lines from a string, then remove common
+ * indentation from each line.
+ *
+ * This is useful to improve readability for multi-line template literals,
+ * where you don't want to
+ *  1) start the first line right after the first backtick.
+ *  2) indent the string inconsistently with the rest of the code.
+ *
+ * This function allows to move from this:
+ *   function foo(arg) {
+ *     if (arg === 3) {
+ *       console.log(`First line
+ *   Second line
+ *   Third line`);
+ *     }
+ *   }
+ *
+ * to this:
+ *   function foo(arg) {
+ *     if (arg === 3) {
+ *       console.log(trimAndDedent(`
+ *         First line
+ *         Second line
+ *         Third line
+ *       `));
+ *     }
+ *   }
+ */
+export function trimAndDedent(str: string): string {
+  return dedent(trimEmptyLines(str));
+}