fix: Apply minFractionDigits only to decimal strings (#676)

Author: eemeli

src/nodes/Scalar.ts

@@ -43,7 +43,11 @@ export class Scalar<T = unknown> extends NodeBase {
    */
   declare format?: string
 
-  /** If `value` is a number, use this value when stringifying this node. */
+  /**
+   * If `value` is a number that is serialized as a decimal string
+   * (i.e. not using exponential notation),
+   * use this value when stringifying this node.
+   */
   declare minFractionDigits?: number
 
   /** Set during parsing to the source string value */

src/stringify/stringifyNumber.ts

@@ -14,7 +14,8 @@ export function stringifyNumber({
     !format &&
     minFractionDigits &&
     (!tag || tag === 'tag:yaml.org,2002:float') &&
-    /^\d/.test(n)
+    /^-?\d/.test(n) &&
+    !n.includes('e')
   ) {
     let i = n.indexOf('.')
     if (i < 0) {

tests/doc/stringify.ts

@@ -47,16 +47,30 @@ for (const [name, version] of [
         expect(YAML.stringify(-0, { version })).toBe('-0\n')
       })
 
-      test('float with trailing zeros', () => {
-        const doc = new YAML.Document<YAML.Scalar, false>(3, { version })
-        doc.contents.minFractionDigits = 2
-        expect(String(doc)).toBe('3.00\n')
-      })
-      test('scientific float ignores minFractionDigits', () => {
-        const doc = new YAML.Document<YAML.Scalar, false>(3, { version })
-        doc.contents.format = 'EXP'
-        doc.contents.minFractionDigits = 2
-        expect(String(doc)).toBe('3e+0\n')
+      describe('minFractionDigits', () => {
+        for (const [n, exp] of [
+          [3, '3.00\n'],
+          [-3, '-3.00\n'],
+          [4.2, '4.20\n'],
+          [4.21, '4.21\n'],
+          [4.215, '4.215\n'],
+          [0, '0.00\n'],
+          [-0, '-0.00\n'],
+          [1e32, '1e+32\n'],
+        ]) {
+          test(`number (${n}) with trailing zeros`, () => {
+            const doc = new YAML.Document<YAML.Scalar, false>(n, { version })
+            doc.contents.minFractionDigits = 2
+            expect(doc.toString()).toBe(exp)
+          })
+        }
+
+        test('scientific float ignores minFractionDigits', () => {
+          const doc = new YAML.Document<YAML.Scalar, false>(3, { version })
+          doc.contents.format = 'EXP'
+          doc.contents.minFractionDigits = 2
+          expect(String(doc)).toBe('3e+0\n')
+        })
       })
 
       test('integer with HEX format', () => {