[New] add nullish coalescing

.babelrc

@@ -3,5 +3,6 @@
   "plugins": [
     ["transform-replace-object-assign", { "moduleSpecifier": "object.assign" }],
     "transform-object-rest-spread",
+    // "@babel/plugin-proposal-nullish-coalescing-operator", // TODO: update to babel 7
   ],
 }

.eslintrc

@@ -1,6 +1,7 @@
 {
-  extends: "airbnb-base",
-  rules: {
-    no-use-before-define: ["error", { functions: false }]
-  }
+  "extends": "airbnb-base",
+  "parser": "babel-eslint",
+  "rules": {
+    "no-use-before-define": ["error", { "functions": false }],
+  },
 }

__tests__/helper.js

@@ -9,7 +9,14 @@ let parserName;
 const babelParser = fallbackToBabylon ? require('babylon') : require('@babel/parser');
 const flowParser = require('flow-parser');
 
-const defaultPlugins = ['jsx', 'functionBind', 'estree', 'objectRestSpread', 'optionalChaining'];
+const defaultPlugins = [
+  'jsx',
+  'functionBind',
+  'estree',
+  'objectRestSpread',
+  'optionalChaining',
+  // 'nullishCoalescing', // TODO: update to babel 7
+];
 let plugins = [...defaultPlugins];
 
 export function setParserName(name) {

__tests__/src/getPropValue-babelparser-test.js

@@ -365,6 +365,60 @@ describe('getPropValue', () => {
 
       assert.equal(expected, actual);
     });
+
+    it('should correctly infer result of ?? logical expression based on derived values', () => {
+      const prop = extractProp('<div foo={bar ?? baz} />');
+
+      const expected = 'bar';
+      const actual = getPropValue(prop);
+
+      assert.equal(expected, actual);
+    });
+
+    it('should correctly infer result of ?? logical expression based on derived values', () => {
+      const prop = extractProp('<div foo={undefined ?? baz} />');
+
+      const expected = 'baz';
+      const actual = getPropValue(prop);
+
+      assert.equal(expected, actual);
+    });
+
+    it('should return undefined when evaluating `undefined ?? undefined` ', () => {
+      const prop = extractProp('<div foo={undefined ?? undefined} />');
+
+      const expected = undefined;
+      const actual = getPropValue(prop);
+
+      assert.equal(expected, actual);
+    });
+
+    it('should return undefined when evaluating `null ?? undefined` ', () => {
+      const prop = extractProp('<div foo={null ?? undefined} />');
+
+      const expected = undefined;
+      const actual = getPropValue(prop);
+
+      assert.equal(expected, actual);
+    });
+
+    it('should return undefined when evaluating `undefined ?? null` ', () => {
+      const prop = extractProp('<div foo={undefined ?? null} />');
+
+      const expected = null;
+      const actual = getPropValue(prop);
+
+      assert.equal(expected, actual);
+    });
+
+    it('should return null when evaluating `null ?? null` ', () => {
+      const prop = extractProp('<div foo={null ?? null} />');
+
+      const expected = null;
+      const actual = getPropValue(prop);
+
+      assert.equal(expected, actual);
+    });
   });
 
   describe('Member expression', () => {

src/values/expressions/LogicalExpression.js

@@ -13,5 +13,12 @@ export default function extractValueFromLogicalExpression(value) {
   const leftVal = getValue(left);
   const rightVal = getValue(right);
 
-  return operator === '&&' ? leftVal && rightVal : leftVal || rightVal;
+  if (operator === '&&') {
+    return leftVal && rightVal;
+  }
+  if (operator === '??') {
+    // return leftVal ?? rightVal; // TODO: update to babel 7
+    return (leftVal === null || typeof leftVal === 'undefined') ? rightVal : leftVal;
+  }
+  return leftVal || rightVal;
 }