Improve quoted string flexibility

index.d.ts

@@ -162,6 +162,12 @@ export namespace scalarOptions {
      * Default: `'PLAIN'`
      */
     defaultType: Scalar.Type
+    /**
+     * Use 'single quote' rather than "double quote" by default
+     *
+     * Default: `false`
+     */
+    defaultQuoteSingle: boolean
     doubleQuoted: {
       /**
        * Whether to restrict double-quoted strings to use JSON-compatible syntax.

src/stringify/stringifyString.js

@@ -219,16 +219,26 @@ function plainString(item, ctx, onComment, onChompKeep) {
       value
     )
   ) {
+    const hasDouble = value.indexOf('"') !== -1
+    const hasSingle = value.indexOf("'") !== -1
+    let quotedString
+    if (hasDouble && !hasSingle) {
+      quotedString = singleQuotedString
+    } else if (hasSingle && !hasDouble) {
+      quotedString = doubleQuotedString
+    } else if (strOptions.defaultQuoteSingle) {
+      quotedString = singleQuotedString
+    } else {
+      quotedString = doubleQuotedString
+    }
     // not allowed:
     // - empty string, '-' or '?'
     // - start with an indicator character (except [?:-]) or /[?-] /
     // - '\n ', ': ' or ' \n' anywhere
     // - '#' not preceded by a non-space char
     // - end with ' ' or ':'
     return implicitKey || inFlow || value.indexOf('\n') === -1
-      ? value.indexOf('"') !== -1 && value.indexOf("'") === -1
-        ? singleQuotedString(value, ctx)
-        : doubleQuotedString(value, ctx)
+      ? quotedString(value, ctx)
       : blockString(item, ctx, onComment, onChompKeep)
   }
   if (

src/tags/options.js

@@ -13,6 +13,7 @@ export const nullOptions = { nullStr: 'null' }
 
 export const strOptions = {
   defaultType: Type.PLAIN,
+  defaultQuoteSingle: false,
   doubleQuoted: {
     jsonEncoding: false,
     minMultiLineLength: 40

tests/doc/stringify.js

@@ -622,6 +622,62 @@ describe('indentSeq: false', () => {
   })
 })
 
+describe('default quote option', () => {
+  let origDefaultQuoteOption
+  beforeAll(() => {
+    origDefaultQuoteOption = YAML.scalarOptions.str.defaultQuoteSingle
+  })
+  afterAll(() => {
+    YAML.scalarOptions.str.defaultQuoteSingle = origDefaultQuoteOption
+  })
+
+  const testSingleQuote = str => {
+    const expected = `'${str}'\n`
+    const actual = YAML.stringify(str)
+    expect(actual).toBe(expected)
+    expect(YAML.parse(actual)).toBe(str)
+  }
+  const testDoubleQuote = str => {
+    const expected = `"${str}"\n`
+    const actual = YAML.stringify(str)
+    expect(actual).toBe(expected)
+    expect(YAML.parse(actual)).toBe(str)
+  }
+
+  const testPlainStyle = () => {
+    const str = YAML.stringify('foo bar')
+    expect(str).toBe('foo bar\n')
+  }
+  const testForcedQuotes = () => {
+    let str = YAML.stringify('foo: "bar"')
+    expect(str).toBe(`'foo: "bar"'\n`)
+    str = YAML.stringify("foo: 'bar'")
+    expect(str).toBe(`"foo: 'bar'"\n`)
+  }
+
+  test('default', () => {
+    YAML.scalarOptions.str.defaultQuoteSingle = origDefaultQuoteOption
+    testPlainStyle()
+    testForcedQuotes()
+    testDoubleQuote('123')
+    testDoubleQuote('foo #bar')
+  })
+  test("'", () => {
+    YAML.scalarOptions.str.defaultQuoteSingle = true
+    testPlainStyle()
+    testForcedQuotes()
+    testDoubleQuote('123') // number-as-string is double-quoted
+    testSingleQuote('foo #bar')
+  })
+  test('"', () => {
+    YAML.scalarOptions.str.defaultQuoteSingle = false
+    testPlainStyle()
+    testForcedQuotes()
+    testDoubleQuote('123')
+    testDoubleQuote('foo #bar')
+  })
+})
+
 describe('Document markers in top-level scalars', () => {
   let origDoubleQuotedOptions
   beforeAll(() => {