'expected variable-declaration to have a typedef' lint error on for loops

[janez-svetin]

Bug Report

• TSLint version: 5.10.0
• TypeScript version: 2.7.2
• Running TSLint via: CLI

TypeScript code being linted

// code snippet (for-loop.ts file)
const split = [ 1, 2, 3, 4, 5 ];
for (let i = 0; i < split.length; i += 1) {
  console.log("Foo Bar!");
}

with tslint.json configuration:

{
  "extends": [
    "tslint-eslint-rules"
  ],
  "rulesDirectory": [
    "node_modules/codelyzer"
  ],
  "rules": {
    "typedef": [
      true,
      "variable-declaration"
    ]
  }
}

Actual behavior

ERROR: test-for-loop.ts[1, 7]: expected variable-declaration: 'split' to have a typedef
ERROR: test-for-loop.ts[2, 10]: expected variable-declaration: 'i' to have a typedef

Expected behavior

The typedef rule should ignore variables declared within for loop declarations.

Looking at the source, typedefRule.ts line 176, there is condition written:

if (parent.kind === ts.SyntaxKind.CatchClause
    || parent.parent.kind === ts.SyntaxKind.ForInStatement
    || parent.parent.kind === ts.SyntaxKind.ForOfStatement) {
  return;
}

I think it should also include basic ForStatement kind:

if (parent.kind === ts.SyntaxKind.CatchClause
    || parent.parent.kind === ts.SyntaxKind.ForInStatement
    || parent.parent.kind === ts.SyntaxKind.ForOfStatement
    || parent.parent.kind === ts.SyntaxKind.ForStatement) {
  return;
}

In this case it does not falesly reports on 'i' to have typedef.

Any specific reason it is left out?

[jkillian]

TypeScript actually doesn't allow a type annotation on the LHS of for-in or for-of loops, so the typedef rule must not check for one there. Since a regular for loop can contain an arbitrary initializer, it makes sense to check for a type declaration there.

I think the real issue here is that the typedef rule isn't great and often leads to overly-verbose code. See #711 for somewhat related discussion. What'd be best here may be to let typedef ignore simple declarations like let i = 0 all together.