typescript-eslint · bradzacher · Aug 24, 2020 · Aug 10, 2020 · Aug 10, 2020 · Aug 11, 2020
diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts b/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts
@@ -166,6 +166,14 @@ export default createRule<Options, MessageId>({
      * if the type of the node is always true or always false, it's not necessary.
      */
     function checkNode(node: TSESTree.Expression): void {
+      // Check if the node is Unary Negation expression and handle it
+      if (
+        node.type === AST_NODE_TYPES.UnaryExpression &&
+        node.operator === '!'
+      ) {
+        return checkIfUnaryNegationExpressionIsNecessaryConditional(node);
+      }
+
       // Since typescript array index signature types don't represent the
       //  possibility of out-of-bounds access, if we're indexing into an array
       //  just skip the check, to avoid false positives
@@ -213,6 +221,30 @@ export default createRule<Options, MessageId>({
       }
     }
 
+    /**
+     * If it is Unary Negation Expression, check the argument for alwaysTruthy / alwaysFalsy
+     */
+    function checkIfUnaryNegationExpressionIsNecessaryConditional(
+      node: TSESTree.UnaryExpression,
+    ): void {
+      if (isArrayIndexExpression(node.argument)) {
+        return;
+      }
+      const type = getNodeType(node.argument);
+      const messageId = isTypeFlagSet(type, ts.TypeFlags.Never)
+        ? 'never'
+        : isPossiblyTruthy(type)
+        ? 'alwaysFalsy'
+        : isPossiblyFalsy(type)
+        ? 'alwaysTruthy'
+        : undefined;
+
+      if (messageId) {
+        context.report({ node, messageId });
+      }
+      return;
+    }
+
     function checkNodeForNullish(node: TSESTree.Expression): void {
       // Since typescript array index signature types don't represent the
       //  possibility of out-of-bounds access, if we're indexing into an array

diff --git a/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts b/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts
@@ -445,7 +445,26 @@ declare const key: Key;
 
 foo?.[key]?.trim();
     `,
-    // https://github.com/typescript-eslint/typescript-eslint/issues/2384
+    `
+let latencies: number[][] = [];
+
+function recordData(): void {
+  if (!latencies[0]) latencies[0] = [];
+  latencies[0].push(4);
+}
+
+recordData();
+    `,
+    `
+let latencies: number[][] = [];
+
+function recordData(): void {
+  if (latencies[0]) latencies[0] = [];
+  latencies[0].push(4);
+}
+
+recordData();
+    `,
     `
 function test(testVal?: boolean) {
   if (testVal ?? true) {
@@ -458,6 +477,34 @@ function test(testVal?: boolean) {
     // Ensure that it's checking in all the right places
     {
       code: `
+const a = null;
+if (!a) {
+}
+      `,
+      errors: [ruleError(3, 5, 'alwaysTruthy')],
+    },
+    {
+      code: `
+const a = true;
+if (!a) {
+}
+      `,
+      errors: [ruleError(3, 5, 'alwaysFalsy')],
+    },
+    {
+      code: `
+function sayHi(): void {
+  console.log('Hi!');
+}
+
+let speech: never = sayHi();
+if (!speech) {
+}
+      `,
+      errors: [ruleError(7, 5, 'never')],
+    },
+    {
+      code: `
 const b1 = true;
 declare const b2: boolean;
 const t1 = b1 && b2;