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

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;
 }
 

tests/baselines/reference/typeGuardBoolean.js

@@ -0,0 +1,30 @@
+//// [typeGuardBoolean.ts]
+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;
+  }
+}
+
+
+//// [typeGuardBoolean.js]
+function test(strOrNull, strOrUndefined) {
+    var str = "original";
+    var nil;
+    if (!Boolean(strOrNull)) {
+        nil = strOrNull;
+    }
+    else {
+        str = strOrNull;
+    }
+    if (Boolean(strOrUndefined)) {
+        str = strOrUndefined;
+    }
+}

tests/baselines/reference/typeGuardBoolean.symbols

@@ -0,0 +1,35 @@
+=== tests/cases/conformance/expressions/typeGuards/typeGuardBoolean.ts ===
+function test(strOrNull: string | null, strOrUndefined: string | undefined) {
+>test : Symbol(test, Decl(typeGuardBoolean.ts, 0, 0))
+>strOrNull : Symbol(strOrNull, Decl(typeGuardBoolean.ts, 0, 14))
+>strOrUndefined : Symbol(strOrUndefined, Decl(typeGuardBoolean.ts, 0, 39))
+
+  var str: string = "original";
+>str : Symbol(str, Decl(typeGuardBoolean.ts, 1, 5))
+
+  var nil: null;
+>nil : Symbol(nil, Decl(typeGuardBoolean.ts, 2, 5))
+
+  if (!Boolean(strOrNull)) {
+>Boolean : Symbol(Boolean, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>strOrNull : Symbol(strOrNull, Decl(typeGuardBoolean.ts, 0, 14))
+
+      nil = strOrNull;
+>nil : Symbol(nil, Decl(typeGuardBoolean.ts, 2, 5))
+>strOrNull : Symbol(strOrNull, Decl(typeGuardBoolean.ts, 0, 14))
+  }
+  else {
+      str = strOrNull;
+>str : Symbol(str, Decl(typeGuardBoolean.ts, 1, 5))
+>strOrNull : Symbol(strOrNull, Decl(typeGuardBoolean.ts, 0, 14))
+  }
+  if (Boolean(strOrUndefined)) {
+>Boolean : Symbol(Boolean, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+>strOrUndefined : Symbol(strOrUndefined, Decl(typeGuardBoolean.ts, 0, 39))
+
+      str = strOrUndefined;
+>str : Symbol(str, Decl(typeGuardBoolean.ts, 1, 5))
+>strOrUndefined : Symbol(strOrUndefined, Decl(typeGuardBoolean.ts, 0, 39))
+  }
+}
+

tests/baselines/reference/typeGuardBoolean.types

@@ -0,0 +1,44 @@
+=== tests/cases/conformance/expressions/typeGuards/typeGuardBoolean.ts ===
+function test(strOrNull: string | null, strOrUndefined: string | undefined) {
+>test : (strOrNull: string | null, strOrUndefined: string | undefined) => void
+>strOrNull : string | null
+>null : null
+>strOrUndefined : string | undefined
+
+  var str: string = "original";
+>str : string
+>"original" : "original"
+
+  var nil: null;
+>nil : null
+>null : null
+
+  if (!Boolean(strOrNull)) {
+>!Boolean(strOrNull) : boolean
+>Boolean(strOrNull) : boolean
+>Boolean : BooleanConstructor
+>strOrNull : string | null
+
+      nil = strOrNull;
+>nil = strOrNull : null
+>nil : null
+>strOrNull : null
+  }
+  else {
+      str = strOrNull;
+>str = strOrNull : string
+>str : string
+>strOrNull : string
+  }
+  if (Boolean(strOrUndefined)) {
+>Boolean(strOrUndefined) : boolean
+>Boolean : BooleanConstructor
+>strOrUndefined : string | undefined
+
+      str = strOrUndefined;
+>str = strOrUndefined : string
+>str : string
+>strOrUndefined : string
+  }
+}
+

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;
+  }
+}