Modify Babel React JSX Duplicate Children Fix (#17101)

If a JSX element has both a children prop and children (ie. <div children={childOne}>{childTwo}</div>), IE throws an Multiple definitions of a property not allowed in strict mode. This modifies the previous fix (which used an Object.assign) by making the duplicate children a sequence expression on the next prop/child instead so that ordering is preserved. For example:

```
<Component children={useA()} foo={useB()} children={useC()}>{useD()}</Component>
```
should compile to
```
React.jsx(Component, {foo: (useA(), useB()), children: (useC(), useD)})
```

packages/babel-plugin-react-jsx/__tests__/TransformJSXToReactJSX-test.js

@@ -481,9 +481,24 @@ describe('transform react to jsx', () => {
     ).toMatchSnapshot();
   });
 
-  it('should not contain duplicate children key in props object', () => {
+  it('duplicate children prop should transform into sequence expression with actual children', () => {
     expect(
       transform(`<Component children={1}>2</Component>`)
     ).toMatchSnapshot();
   });
+  it('duplicate children prop should transform into sequence expression with next prop', () => {
+    expect(
+      transform(`<Component children={1} foo={3}>2</Component>`)
+    ).toMatchSnapshot();
+  });
+  it('duplicate children props should transform into sequence expression with next prop', () => {
+    expect(
+      transform(`<Component children={1} children={4} foo={3}>2</Component>`)
+    ).toMatchSnapshot();
+  });
+  it('duplicate children prop should transform into sequence expression with spread', () => {
+    expect(
+      transform(`<Component children={1} {...x}>2</Component>`)
+    ).toMatchSnapshot();
+  });
 });

packages/babel-plugin-react-jsx/__tests__/__snapshots__/TransformJSXToReactJSX-test.js.snap

@@ -46,6 +46,32 @@ var x = React.jsxs("div", {
 });
 `;
 
+exports[`transform react to jsx duplicate children prop should transform into sequence expression with actual children 1`] = `
+React.jsx(Component, {
+  children: (1, "2")
+});
+`;
+
+exports[`transform react to jsx duplicate children prop should transform into sequence expression with next prop 1`] = `
+React.jsx(Component, {
+  foo: (1, 3),
+  children: "2"
+});
+`;
+
+exports[`transform react to jsx duplicate children prop should transform into sequence expression with spread 1`] = `
+React.jsx(Component, Object.assign({}, (1, x), {
+  children: "2"
+}));
+`;
+
+exports[`transform react to jsx duplicate children props should transform into sequence expression with next prop 1`] = `
+React.jsx(Component, {
+  foo: (1, 4, 3),
+  children: "2"
+});
+`;
+
 exports[`transform react to jsx fragment with no children 1`] = `var x = React.jsx(React.Fragment, {});`;
 
 exports[`transform react to jsx fragments 1`] = `
@@ -250,14 +276,6 @@ var e = React.jsx(F, {
 });
 `;
 
-exports[`transform react to jsx should not contain duplicate children key in props object 1`] = `
-React.jsx(Component, Object.assign({
-  children: 1
-}, {
-  children: "2"
-}));
-`;
-
 exports[`transform react to jsx should not strip nbsp even couple with other whitespace 1`] = `
 React.jsx("div", {
   children: "\\xA0 "

packages/babel-plugin-react-jsx/src/TransformJSXToReactBabelPlugin.js

@@ -119,8 +119,8 @@ You can turn on the 'throwIfNamespace' flag to bypass this warning.`,
     }
   }
 
-  function convertAttribute(node) {
-    const value = convertAttributeValue(node.value || t.booleanLiteral(true));
+  function convertAttribute(node, duplicateChildren) {
+    let value = convertAttributeValue(node.value || t.booleanLiteral(true));
 
     if (t.isStringLiteral(value) && !t.isJSXExpressionContainer(node.value)) {
       value.value = value.value.replace(/\n\s+/g, ' ');
@@ -130,6 +130,9 @@ You can turn on the 'throwIfNamespace' flag to bypass this warning.`,
         delete value.extra.raw;
       }
     }
+    if (duplicateChildren && duplicateChildren.length > 0) {
+      value = t.sequenceExpression([...duplicateChildren, value]);
+    }
 
     if (t.isJSXNamespacedName(node.name)) {
       node.name = t.stringLiteral(
@@ -281,6 +284,17 @@ You can turn on the 'throwIfNamespace' flag to bypass this warning.`,
   function buildJSXOpeningElementAttributes(attribs, file, children) {
     let _props = [];
     const objs = [];
+
+    // In order to avoid having duplicate "children" keys, we avoid
+    // pushing the "children" prop if we have actual children. However,
+    // the children prop may have side effects, so to be certain
+    // these side effects are evaluated, we add them to the following prop
+    // as a sequence expression to preserve order. So:
+    // <div children={x++} foo={y}>{child}</div> becomes
+    // React.jsx('div', {foo: (x++, y), children: child});
+    // duplicateChildren contains the extra children prop values
+    let duplicateChildren = [];
+
     const hasChildren = children && children.length > 0;
 
     const useBuiltIns = file.opts.useBuiltIns || false;
@@ -293,32 +307,48 @@ You can turn on the 'throwIfNamespace' flag to bypass this warning.`,
 
     while (attribs.length) {
       const prop = attribs.shift();
-      if (t.isJSXSpreadAttribute(prop)) {
+      if (hasChildren && isChildrenProp(prop)) {
+        duplicateChildren.push(convertAttributeValue(prop.value));
+      } else if (t.isJSXSpreadAttribute(prop)) {
         _props = pushProps(_props, objs);
-        objs.push(prop.argument);
-      } else if (hasChildren && isChildrenProp(prop)) {
-        // In order to avoid having duplicate "children" keys, we avoid
-        // pushing the "children" prop if we have actual children. Instead
-        // we put the children into a separate object and then rely on
-        // the Object.assign logic below to ensure the correct object is
-        // formed.
-        _props = pushProps(_props, objs);
-        objs.push(t.objectExpression([convertAttribute(prop)]));
+        if (duplicateChildren.length > 0) {
+          objs.push(
+            t.sequenceExpression([...duplicateChildren, prop.argument]),
+          );
+          duplicateChildren = [];
+        } else {
+          objs.push(prop.argument);
+        }
       } else {
-        _props.push(convertAttribute(prop));
+        _props.push(convertAttribute(prop, duplicateChildren));
+        if (duplicateChildren.length > 0) {
+          duplicateChildren = [];
+        }
       }
     }
 
     // In React.JSX, children is no longer a separate argument, but passed in
     // through the argument object
     if (hasChildren) {
       if (children.length === 1) {
-        _props.push(t.objectProperty(t.identifier('children'), children[0]));
+        _props.push(
+          t.objectProperty(
+            t.identifier('children'),
+            duplicateChildren.length > 0
+              ? t.sequenceExpression([...duplicateChildren, children[0]])
+              : children[0],
+          ),
+        );
       } else {
         _props.push(
           t.objectProperty(
             t.identifier('children'),
-            t.arrayExpression(children),
+            duplicateChildren.length > 0
+              ? t.sequenceExpression([
+                  ...duplicateChildren,
+                  t.arrayExpression(children),
+                ])
+              : t.arrayExpression(children),
           ),
         );
       }