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

[sinyovercosy]

Fixes #1690

In the prefer-string-starts-ends-with rule, the isEndsWith flag did not check that the startIndex argument of String#slice or String#substring is a negative index (-i or length-i)

typescript-eslint/packages/eslint-plugin/src/rules/prefer-string-starts-ends-with.ts

Lines 538 to 541 in e5db36f

	const isEndsWith =
	callNode.arguments.length === 1 ||
	(callNode.arguments.length === 2 &&
	isLengthExpression(callNode.arguments[1], node.object));

To resolve this issue, an additional constraint has been added:

function isNegativeIndexExpression(node, expectedIndexedNode) {
  return (
    (node.type === AST_NODE_TYPES.UnaryExpression &&
      node.operator === '-') ||
    (node.type === AST_NODE_TYPES.BinaryExpression &&
      node.operator === '-' &&
      isLengthExpression(node.left, expectedIndexedNode))
  );
}

[typescript-eslint]

Thanks for the PR, @sinyovercosy!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #1920 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1920   +/-   ##
=======================================
  Coverage   94.37%   94.37%           
=======================================
  Files         168      168           
  Lines        7644     7646    +2     
  Branches     2195     2196    +1     
=======================================
+ Hits         7214     7216    +2     
  Misses        183      183           
  Partials      247      247           

Flag	Coverage Δ
#unittest	94.37% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
...plugin/src/rules/prefer-string-starts-ends-with.ts	100.00% <100.00%> (ø)

[bradzacher]

LGTM - thanks