New rule no-if-statement. Requested in #54.

README.md

@@ -46,6 +46,7 @@ In addition to immutable rules this project also contains a few rules for enforc
   * [no-class](#no-this-no-class)
   * [no-mixed-interface](#no-mixed-interface)
   * [no-expression-statement](#no-expression-statement)
+  * [no-if-statement](#no-if-statement)
 * [Recommended built-in rules](#recommended-built-in-rules)
 
 ## Immutability rules
@@ -304,6 +305,26 @@ This rule checks that the value of an expression is assigned to a variable and t
 "no-expression-statement": [true, {"ignore-prefix": ["console.log", "console.error"]}]
 ```
 
+### no-if-statement
+If statements is not a good fit for functional style programming as they are not expresssions and do not return a value. This rule disallows if statements.
+
+```typescript
+let x;
+if(i === 1) {
+  x = 2;
+} else {
+  x = 3;
+}
+```
+
+Instead consider using the [tenary operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Conditional_Operator) which is an expression that returns a value:
+
+```typescript
+const x = i === 1 ? 2 : 3;
+```
+
+For more background see this [blog post](https://hackernoon.com/rethinking-javascript-the-if-statement-b158a61cd6cb) and discussion in [#54](https://github.com/jonaskello/tslint-immutable/issues/54).
+
 ## Options
 
 ### Using the `ignore-local` option

src/noIfStatementRule.ts

@@ -0,0 +1,26 @@
+import * as ts from "typescript";
+import * as Lint from "tslint";
+import {
+  createInvalidNode,
+  CheckNodeResult,
+  createCheckNodeRule
+} from "./shared/check-node";
+
+type Options = {};
+
+// tslint:disable-next-line:variable-name
+export const Rule = createCheckNodeRule(
+  checkNode,
+  "Unexpected if, use tenary operator instead."
+);
+
+function checkNode(
+  node: ts.Node,
+  _ctx: Lint.WalkContext<Options>
+): CheckNodeResult {
+  return node && node.kind === ts.SyntaxKind.IfStatement
+    ? { invalidNodes: [createInvalidNode(node)] }
+    : {
+        invalidNodes: []
+      };
+}

test/rules/no-if-statement/default/test.ts.lint

@@ -0,0 +1,10 @@
+var x = 0;
+
+if(i === 1) {
+~~~~~~~~~~~~~
+    x = 2;
+~~~~~~~~~~
+}
+~ [failure]
+
+[failure]: Unexpected if, use tenary operator instead.

test/rules/no-if-statement/default/tslint.json

@@ -0,0 +1,6 @@
+{
+  "rulesDirectory": ["../../../../rules"],
+  "rules": {
+    "no-if-statement": true
+  }
+}

tslint-immutable.json

@@ -4,6 +4,7 @@
     "no-class": false,
     "no-delete": false,
     "no-expression-statement": false,
+    "no-if-statement": false,
     "no-let": false,
     "no-method-signature": false,
     "no-mixed-interface": false,