diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index 81d4857ad073a..af3cf3b022dca 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -2532,7 +2532,9 @@ namespace ts {
                         return symbol;
                     }
                     else {
-                        return declareSymbol(parent ? parent.exports! : container.locals!, parent, id, flags, excludeFlags);
+                        const table = parent ? parent.exports! :
+                            file.jsGlobalAugmentations || (file.jsGlobalAugmentations = createSymbolTable());
+                        return declareSymbol(table, parent, id, flags, excludeFlags);
                     }
                 });
             }
@@ -2901,6 +2903,9 @@ namespace ts {
         if (local) {
             return local.exportSymbol || local;
         }
+        if (isSourceFile(container) && container.jsGlobalAugmentations && container.jsGlobalAugmentations.has(name)) {
+            return container.jsGlobalAugmentations.get(name);
+        }
         return container.symbol && container.symbol.exports && container.symbol.exports.get(name);
     }
 
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 2a8b0ce43147e..11f279d27d531 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -850,7 +850,7 @@ namespace ts {
                 target.flags |= source.flags;
                 if (source.valueDeclaration &&
                     (!target.valueDeclaration ||
-                     isAssignmentDeclaration(target.valueDeclaration) ||
+                     isAssignmentDeclaration(target.valueDeclaration) && !isAssignmentDeclaration(source.valueDeclaration) ||
                      isEffectiveModuleDeclaration(target.valueDeclaration) && !isEffectiveModuleDeclaration(source.valueDeclaration))) {
                     // other kinds of value declarations take precedence over modules and assignment declarations
                     target.valueDeclaration = source.valueDeclaration;
@@ -28611,6 +28611,9 @@ namespace ts {
                 if (!isExternalOrCommonJsModule(file)) {
                     mergeSymbolTable(globals, file.locals!);
                 }
+                if (file.jsGlobalAugmentations) {
+                    mergeSymbolTable(globals, file.jsGlobalAugmentations);
+                }
                 if (file.patternAmbientModules && file.patternAmbientModules.length) {
                     patternAmbientModules = concatenate(patternAmbientModules, file.patternAmbientModules);
                 }
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 56ddc1de896ef..a5e06c7304bc8 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -2593,6 +2593,8 @@ namespace ts {
         /* @internal */ externalModuleIndicator?: Node;
         // The first node that causes this file to be a CommonJS module
         /* @internal */ commonJsModuleIndicator?: Node;
+        // JS identifier-declarations that are intended to merge with globals
+        /* @internal */ jsGlobalAugmentations?: SymbolTable;
 
         /* @internal */ identifiers: Map<string>; // Map from a string to an interned string
         /* @internal */ nodeCount: number;
diff --git a/tests/baselines/reference/globalMergeWithCommonJSAssignmentDeclaration.errors.txt b/tests/baselines/reference/globalMergeWithCommonJSAssignmentDeclaration.errors.txt
new file mode 100644
index 0000000000000..bc67e012a8618
--- /dev/null
+++ b/tests/baselines/reference/globalMergeWithCommonJSAssignmentDeclaration.errors.txt
@@ -0,0 +1,11 @@
+tests/cases/conformance/salsa/bug27099.js(1,1): error TS2322: Type '1' is not assignable to type 'string'.
+
+
+==== tests/cases/conformance/salsa/bug27099.js (1 errors) ====
+    window.name = 1;
+    ~~~~~~~~~~~
+!!! error TS2322: Type '1' is not assignable to type 'string'.
+    window.console; // should not have error: Property 'console' does not exist on type 'typeof window'.
+    module.exports = 'anything';
+    
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/globalMergeWithCommonJSAssignmentDeclaration.symbols b/tests/baselines/reference/globalMergeWithCommonJSAssignmentDeclaration.symbols
new file mode 100644
index 0000000000000..dd9b8a1bab9cc
--- /dev/null
+++ b/tests/baselines/reference/globalMergeWithCommonJSAssignmentDeclaration.symbols
@@ -0,0 +1,17 @@
+=== tests/cases/conformance/salsa/bug27099.js ===
+window.name = 1;
+>window.name : Symbol(Window.name, Decl(lib.dom.d.ts, --, --))
+>window : Symbol(window, Decl(lib.dom.d.ts, --, --), Decl(bug27099.js, 0, 0))
+>name : Symbol(Window.name, Decl(lib.dom.d.ts, --, --))
+
+window.console; // should not have error: Property 'console' does not exist on type 'typeof window'.
+>window.console : Symbol(WindowConsole.console, Decl(lib.dom.d.ts, --, --))
+>window : Symbol(window, Decl(lib.dom.d.ts, --, --), Decl(bug27099.js, 0, 0))
+>console : Symbol(WindowConsole.console, Decl(lib.dom.d.ts, --, --))
+
+module.exports = 'anything';
+>module.exports : Symbol("tests/cases/conformance/salsa/bug27099", Decl(bug27099.js, 0, 0))
+>module : Symbol(export=, Decl(bug27099.js, 1, 15))
+>exports : Symbol(export=, Decl(bug27099.js, 1, 15))
+
+
diff --git a/tests/baselines/reference/globalMergeWithCommonJSAssignmentDeclaration.types b/tests/baselines/reference/globalMergeWithCommonJSAssignmentDeclaration.types
new file mode 100644
index 0000000000000..485c90f6100cf
--- /dev/null
+++ b/tests/baselines/reference/globalMergeWithCommonJSAssignmentDeclaration.types
@@ -0,0 +1,21 @@
+=== tests/cases/conformance/salsa/bug27099.js ===
+window.name = 1;
+>window.name = 1 : 1
+>window.name : string
+>window : Window
+>name : string
+>1 : 1
+
+window.console; // should not have error: Property 'console' does not exist on type 'typeof window'.
+>window.console : Console
+>window : Window
+>console : Console
+
+module.exports = 'anything';
+>module.exports = 'anything' : string
+>module.exports : string
+>module : { "tests/cases/conformance/salsa/bug27099": string; }
+>exports : string
+>'anything' : "anything"
+
+
diff --git a/tests/baselines/reference/jsContainerMergeJsContainer.types b/tests/baselines/reference/jsContainerMergeJsContainer.types
index 75d2152c57190..73e61aded0df9 100644
--- a/tests/baselines/reference/jsContainerMergeJsContainer.types
+++ b/tests/baselines/reference/jsContainerMergeJsContainer.types
@@ -6,18 +6,18 @@ const a = {};
 
 a.d = function() {};
 >a.d = function() {} : { (): void; prototype: {}; }
->a.d : typeof a.d
+>a.d : { (): void; prototype: {}; }
 >a : typeof a
->d : typeof a.d
+>d : { (): void; prototype: {}; }
 >function() {} : { (): void; prototype: {}; }
 
 === tests/cases/conformance/salsa/b.js ===
 a.d.prototype = {};
 >a.d.prototype = {} : {}
 >a.d.prototype : {}
->a.d : typeof a.d
+>a.d : { (): void; prototype: {}; }
 >a : typeof a
->d : typeof a.d
+>d : { (): void; prototype: {}; }
 >prototype : {}
 >{} : {}
 
diff --git a/tests/baselines/reference/typeFromPropertyAssignment35.symbols b/tests/baselines/reference/typeFromPropertyAssignment35.symbols
new file mode 100644
index 0000000000000..d651748d1cc42
--- /dev/null
+++ b/tests/baselines/reference/typeFromPropertyAssignment35.symbols
@@ -0,0 +1,51 @@
+=== tests/cases/conformance/salsa/bug26877.js ===
+/** @param {Emu.D} x */
+function ollKorrect(x) {
+>ollKorrect : Symbol(ollKorrect, Decl(bug26877.js, 0, 0))
+>x : Symbol(x, Decl(bug26877.js, 1, 20))
+
+    x._model
+>x._model : Symbol(D._model, Decl(bug26877.js, 7, 19))
+>x : Symbol(x, Decl(bug26877.js, 1, 20))
+>_model : Symbol(D._model, Decl(bug26877.js, 7, 19))
+
+    const y = new Emu.D()
+>y : Symbol(y, Decl(bug26877.js, 3, 9))
+>Emu.D : Symbol(Emu.D, Decl(bug26877.js, 5, 1), Decl(second.js, 2, 4))
+>Emu : Symbol(Emu, Decl(bug26877.js, 5, 1), Decl(second.js, 0, 3), Decl(second.js, 0, 12))
+>D : Symbol(Emu.D, Decl(bug26877.js, 5, 1), Decl(second.js, 2, 4))
+
+    const z = Emu.D._wrapperInstance;
+>z : Symbol(z, Decl(bug26877.js, 4, 9))
+>Emu.D._wrapperInstance : Symbol(Emu.D._wrapperInstance, Decl(second.js, 0, 12))
+>Emu.D : Symbol(Emu.D, Decl(bug26877.js, 5, 1), Decl(second.js, 2, 4))
+>Emu : Symbol(Emu, Decl(bug26877.js, 5, 1), Decl(second.js, 0, 3), Decl(second.js, 0, 12))
+>D : Symbol(Emu.D, Decl(bug26877.js, 5, 1), Decl(second.js, 2, 4))
+>_wrapperInstance : Symbol(Emu.D._wrapperInstance, Decl(second.js, 0, 12))
+}
+Emu.D = class {
+>Emu.D : Symbol(Emu.D, Decl(bug26877.js, 5, 1), Decl(second.js, 2, 4))
+>Emu : Symbol(Emu, Decl(bug26877.js, 5, 1), Decl(second.js, 0, 3), Decl(second.js, 0, 12))
+>D : Symbol(Emu.D, Decl(bug26877.js, 5, 1), Decl(second.js, 2, 4))
+
+    constructor() {
+        this._model = 1
+>this._model : Symbol(D._model, Decl(bug26877.js, 7, 19))
+>this : Symbol(D, Decl(bug26877.js, 6, 7))
+>_model : Symbol(D._model, Decl(bug26877.js, 7, 19))
+    }
+}
+
+=== tests/cases/conformance/salsa/second.js ===
+var Emu = {}
+>Emu : Symbol(Emu, Decl(bug26877.js, 5, 1), Decl(second.js, 0, 3), Decl(second.js, 0, 12))
+
+/** @type {string} */
+Emu.D._wrapperInstance;
+>Emu.D._wrapperInstance : Symbol(Emu.D._wrapperInstance, Decl(second.js, 0, 12))
+>Emu.D : Symbol(Emu.D, Decl(bug26877.js, 5, 1), Decl(second.js, 2, 4))
+>Emu : Symbol(Emu, Decl(bug26877.js, 5, 1), Decl(second.js, 0, 3), Decl(second.js, 0, 12))
+>D : Symbol(Emu.D, Decl(bug26877.js, 5, 1), Decl(second.js, 2, 4))
+>_wrapperInstance : Symbol(Emu.D._wrapperInstance, Decl(second.js, 0, 12))
+
+
diff --git a/tests/baselines/reference/typeFromPropertyAssignment35.types b/tests/baselines/reference/typeFromPropertyAssignment35.types
new file mode 100644
index 0000000000000..678adb1ba5346
--- /dev/null
+++ b/tests/baselines/reference/typeFromPropertyAssignment35.types
@@ -0,0 +1,57 @@
+=== tests/cases/conformance/salsa/bug26877.js ===
+/** @param {Emu.D} x */
+function ollKorrect(x) {
+>ollKorrect : (x: D) => void
+>x : D
+
+    x._model
+>x._model : number
+>x : D
+>_model : number
+
+    const y = new Emu.D()
+>y : D
+>new Emu.D() : D
+>Emu.D : typeof D
+>Emu : typeof Emu
+>D : typeof D
+
+    const z = Emu.D._wrapperInstance;
+>z : string
+>Emu.D._wrapperInstance : string
+>Emu.D : typeof D
+>Emu : typeof Emu
+>D : typeof D
+>_wrapperInstance : string
+}
+Emu.D = class {
+>Emu.D = class {    constructor() {        this._model = 1    }} : typeof D
+>Emu.D : typeof D
+>Emu : typeof Emu
+>D : typeof D
+>class {    constructor() {        this._model = 1    }} : typeof D
+
+    constructor() {
+        this._model = 1
+>this._model = 1 : 1
+>this._model : number
+>this : this
+>_model : number
+>1 : 1
+    }
+}
+
+=== tests/cases/conformance/salsa/second.js ===
+var Emu = {}
+>Emu : typeof Emu
+>{} : {}
+
+/** @type {string} */
+Emu.D._wrapperInstance;
+>Emu.D._wrapperInstance : string
+>Emu.D : typeof D
+>Emu : typeof Emu
+>D : typeof D
+>_wrapperInstance : string
+
+
diff --git a/tests/cases/conformance/salsa/globalMergeWithCommonJSAssignmentDeclaration.ts b/tests/cases/conformance/salsa/globalMergeWithCommonJSAssignmentDeclaration.ts
new file mode 100644
index 0000000000000..f1435eea87e04
--- /dev/null
+++ b/tests/cases/conformance/salsa/globalMergeWithCommonJSAssignmentDeclaration.ts
@@ -0,0 +1,8 @@
+// @noEmit: true
+// @allowJs: true
+// @checkJs: true
+// @Filename: bug27099.js
+window.name = 1;
+window.console; // should not have error: Property 'console' does not exist on type 'typeof window'.
+module.exports = 'anything';
+
diff --git a/tests/cases/conformance/salsa/typeFromPropertyAssignment35.ts b/tests/cases/conformance/salsa/typeFromPropertyAssignment35.ts
new file mode 100644
index 0000000000000..952fdd7c9c02d
--- /dev/null
+++ b/tests/cases/conformance/salsa/typeFromPropertyAssignment35.ts
@@ -0,0 +1,22 @@
+// @noEmit: true
+// @allowJs: true
+// @checkJs: true
+
+// @Filename: bug26877.js
+/** @param {Emu.D} x */
+function ollKorrect(x) {
+    x._model
+    const y = new Emu.D()
+    const z = Emu.D._wrapperInstance;
+}
+Emu.D = class {
+    constructor() {
+        this._model = 1
+    }
+}
+
+// @Filename: second.js
+var Emu = {}
+/** @type {string} */
+Emu.D._wrapperInstance;
+