Allow Boolean() to be used to perform a null check

Forbes Lindesay · Forbes Lindesay · commit d3c572c1873e · 2019-02-18T13:20:19.000Z
diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
@@ -513,7 +513,7 @@ interface Boolean {
 
 interface BooleanConstructor {
     new(value?: any): Boolean;
-    (value?: any): boolean;
+    <T>(value?: T): value is Exclude<T, false | null | undefined | '' | 0>;
     readonly prototype: Boolean;
 }
 
diff --git a/tests/cases/conformance/expressions/typeGuards/typeGuardBoolean.ts b/tests/cases/conformance/expressions/typeGuards/typeGuardBoolean.ts
@@ -0,0 +1,14 @@
+// @strictNullChecks: true
+function test(strOrNull: string | null, strOrUndefined: string | undefined) {
+  var str: string = "original";
+  var nil: null;
+  if (!Boolean(strOrNull)) {
+      nil = strOrNull;
+  }
+  else {
+      str = strOrNull;
+  }
+  if (Boolean(strOrUndefined)) {
+      str = strOrUndefined;
+  }
+}

Original file line number	Diff line number	Diff line change
`@@ -513,7 +513,7 @@ interface Boolean {`
`513`	`513`
`514`	`514`	`interface BooleanConstructor {`
`515`	`515`	`new(value?: any): Boolean;`
`516`		`- (value?: any): boolean;`
	`516`	`+ <T>(value?: T): value is Exclude<T, false \| null \| undefined \| '' \| 0>;`
`517`	`517`	`readonly prototype: Boolean;`
`518`	`518`	`}`
`519`	`519`