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Transform ID: indexof_to_includes Language: TypeScript

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What it does

Rewrites the indexOf “contains” idiom built on Array.prototype.indexOf / String.prototype.indexOf to the boolean .includes(...) form added in ES2016. Five comparison shapes qualify:
BeforeAfter
arr.indexOf(x) !== -1arr.includes(x)
arr.indexOf(x) >= 0arr.includes(x)
arr.indexOf(x) > -1arr.includes(x)
arr.indexOf(x) === -1!arr.includes(x)
arr.indexOf(x) < 0!arr.includes(x)
The mirrored LHS forms (-1 !== arr.indexOf(x), 0 <= arr.indexOf(x), etc.) are also handled. Position comparisons like arr.indexOf(x) > 5 or === 3 are NOT rewritten — .includes returns a boolean and the position information would be lost. Receiver-type checking goes through ts-morph’s type system. Transform at src/transform/transforms/typescript/indexof-to-includes.ts.

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Detector pattern

The detector at src/analyze/detectors/typescript/indexof-comparison.ts walks ts-morph BinaryExpression nodes whose operator is one of the five recognised shapes and where exactly one side is an indexOf call expression with exactly one argument.

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Preconditions

  1. non_es2016 — refuses when the resolved tsconfig target predates ES2016. Array.prototype.includes and String.prototype.includes are ES2016 additions; rewriting on an older target would introduce a runtime TypeError on legacy platforms.
  2. non_callable_indexof — refuses when the indexOf receiver’s ts-morph type is neither string nor array (nor ReadonlyArray). DOM NodeList, jQuery objects, Buffer, and custom classes have indexOf methods but no guaranteed .includes — and where they do, signatures may differ.
  3. The comparison shape is exactly one of the five recognised contains / not-contains idioms above. Position comparisons (> 5, === 3) are silently ignored.
  4. Exactly one side of the binary expression is an indexOf call — a.indexOf(x) !== b.indexOf(y) does not qualify.

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Before / after

const items: string[] = ['a', 'b', 'c'];
const name: string = 'hello';

if (items.indexOf('b') !== -1) {
  /* found */
}
if (items.indexOf('z') === -1) {
  /* missing */
}
if (name.indexOf('ell') >= 0) {
  /* substring */
}

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Edge cases NOT handled (skip via precondition)

  • Position comparisons (arr.indexOf(x) > 5, === 3, etc.) — .includes returns a boolean and discards position.
  • Non-string / non-array receivers (Buffer, NodeList, jQuery, custom classes) — non_callable_indexof. .includes is not guaranteed.
  • tsconfig target < ES2016 — non_es2016; .includes doesn’t exist on those engines.
  • Symmetric two-sided forms (a.indexOf(x) !== b.indexOf(y)) — not a contains idiom.
Type safety here comes from ts-morph’s getType() checks on the indexOf receiver. A union type like string | string[] is accepted only when every constituent is safe; anything else (object | string) refuses cleanly.