fix(eslint-plugin): [prefer-string-starts-ends-with] check for negative start index in slice (typescript-eslint#1920)

sinyovercosy · web-flow · commit ed2bd6067f74 · 2020-04-26T18:23:46.000-07:00
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,

Original file line number	Diff line number	Diff line change
`@@ -209,6 +209,12 @@ ruleTester.run('prefer-string-starts-ends-with', rule, {`
`209`	`209`	`s.slice(-4, -1) === "bar"`
`210`	`210`	`}`
`211`	`211`	`,
	`212`	`+ // https://github.com/typescript-eslint/typescript-eslint/issues/1690`
	`213`	+ `
	`214`	`+ function f(s: string) {`
	`215`	`+ s.slice(1) === "bar"`
	`216`	`+ }`
	`217`	+ `,
`212`	`218`	`
`213`	`219`	`function f(s: string) {`
`214`	`220`	`pattern.test(s)`
`@@ -872,7 +878,7 @@ ruleTester.run('prefer-string-starts-ends-with', rule, {`
`872`	`878`	`{`
`873`	`879`	code: `
`874`	`880`	`function f(s: string) {`
`875`		`- s.slice(startIndex) === needle // 'startIndex' can be different`
	`881`	`+ s.slice(-length) === needle // 'length' can be different`
`876`	`882`	`}`
`877`	`883`	`,
`878`	`884`	`output: null,`