Merge pull request #345 from knockout/fix/nullish-coalescing-earlyout

Fix ?? (nullish coalescing) earlyOut to not short-circuit on falsy values

Author: brianmhunt

packages/utils.parser/spec/nodeBehaviors.ts

@@ -52,6 +52,30 @@ describe('Operators', function () {
     test([null, op['??'], false], false)
   })
 
+  it('?? evaluates rhs iff lhs is null/undefined', () => {
+    let rhsCalls = 0
+    const rhs = () => {
+      rhsCalls++
+      return 'right'
+    }
+    const parser = new Parser()
+    const args = new Arguments(null, [])
+
+    for (const [lhs, expected, expectedCalls] of [
+      [0, 0, 0],
+      ['', '', 0],
+      [false, false, 0],
+      [null, 'right', 1],
+      [undefined, 'right', 1]
+    ] as const) {
+      rhsCalls = 0
+      const context = ctxStub({ a: lhs, rhs })
+      const root = nodes_to_tree([new Identifier(parser, 'a'), op['??'], new Identifier(parser, 'rhs', [args])])
+      assert.strictEqual(root.get_value(null, context), expected, `${String(lhs)} ?? rhs() value`)
+      assert.strictEqual(rhsCalls, expectedCalls, `${String(lhs)} ?? rhs() calls`)
+    }
+  })
+
   it('performs ?. optional chaining', () => {
     test([{}, op['?.'], 'a'], undefined)
     test([null, op['?.'], 'a'], undefined)

packages/utils.parser/src/operators.ts

@@ -216,7 +216,7 @@ operators['??'].precedence = 3
 
 operators['&&'].earlyOut = a => !a
 operators['||'].earlyOut = a => a
-operators['??'].earlyOut = a => a
+operators['??'].earlyOut = a => a !== null && a !== undefined
 
 // Assignment and miscellaneous (lamda)
 operators['=>'].precedence = 2