fix(eslint-plugin): [prefer-string-starts-ends-with] check for negati…

…ve start index in slice (#1920)
typescript-eslint · Apr 27, 2020 · ed2bd60 · ed2bd60
1 parent a85c3e1
commit ed2bd60
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Showing 2 changed files with 35 additions and 4 deletions.
diff --git a/packages/eslint-plugin/src/rules/prefer-string-starts-ends-with.ts b/packages/eslint-plugin/src/rules/prefer-string-starts-ends-with.ts
@@ -166,6 +166,27 @@ export default createRule({
       );
     }
 
+    /**
+     * Check if a given node is a negative index expression
+     *
+     * E.g. `s.slice(- <expr>)`, `s.substring(s.length - <expr>)`
+     *
+     * @param node The node to check.
+     * @param expectedIndexedNode The node which is expected as the receiver of index expression.
+     */
+    function isNegativeIndexExpression(
+      node: TSESTree.Node,
+      expectedIndexedNode: TSESTree.Node,
+    ): boolean {
+      return (
+        (node.type === AST_NODE_TYPES.UnaryExpression &&
+          node.operator === '-') ||
+        (node.type === AST_NODE_TYPES.BinaryExpression &&
+          node.operator === '-' &&
+          isLengthExpression(node.left, expectedIndexedNode))
+      );
+    }
+
     /**
      * Check if a given node is the expression of the last index.
      *
@@ -538,9 +559,10 @@ export default createRule({
         }
 
         const isEndsWith =
-          callNode.arguments.length === 1 ||
-          (callNode.arguments.length === 2 &&
-            isLengthExpression(callNode.arguments[1], node.object));
+          (callNode.arguments.length === 1 ||
+            (callNode.arguments.length === 2 &&
+              isLengthExpression(callNode.arguments[1], node.object))) &&
+          isNegativeIndexExpression(callNode.arguments[0], node.object);
         const isStartsWith =
           !isEndsWith &&
           callNode.arguments.length === 2 &&
@@ -564,6 +586,9 @@ export default createRule({
             ) {
               return null;
             }
+            // code being checked is likely mistake:
+            // unequal length of strings being checked for equality
+            // or reliant on behavior of substring (negative indices interpreted as 0)
             if (isStartsWith) {
               if (!isLengthExpression(callNode.arguments[1], eqNode.right)) {
                 return null;

diff --git a/packages/eslint-plugin/tests/rules/prefer-string-starts-ends-with.test.ts b/packages/eslint-plugin/tests/rules/prefer-string-starts-ends-with.test.ts
@@ -209,6 +209,12 @@ ruleTester.run('prefer-string-starts-ends-with', rule, {
         s.slice(-4, -1) === "bar"
       }
     `,
+    // https://github.com/typescript-eslint/typescript-eslint/issues/1690
+    `
+      function f(s: string) {
+        s.slice(1) === "bar"
+      }
+    `,
     `
       function f(s: string) {
         pattern.test(s)
@@ -872,7 +878,7 @@ ruleTester.run('prefer-string-starts-ends-with', rule, {
     {
       code: `
         function f(s: string) {
-          s.slice(startIndex) === needle // 'startIndex' can be different
+          s.slice(-length) === needle // 'length' can be different
         }
       `,
       output: null,