no-implicit-coercion autofixer can change logic

Tell us about your environment

• ESLint Version: 3.6.1
• Node Version: 6.7.0
• npm Version: 3.10.3

What parser (default, Babel-ESLint, etc.) are you using?

default

Please show your full configuration:

rules:
  no-implicit-coercion: error

What did you do? Please include the actual source code causing the issue.

Ran eslint --fix on:

const catNoises = ["meow", "hiss", "purr"];

if (!~catNoises.indexOf("purr")) {
  console.log("no purr");
} else {
  console.log("yes purr");
}

What did you expect to happen?

Expected ESLint to autofix the code to:

const catNoises = ["meow", "hiss", "purr"];

if (catNoises.indexOf("purr") === -1) {
  console.log("no purr");
} else {
  console.log("yes purr");
}

Or just report the problem without trying to autofix it.

What actually happened? Please include the actual, raw output from ESLint.

ESLint autofixed the code to:

const catNoises = ["meow", "hiss", "purr"];

if (!catNoises.indexOf("purr") !== -1) {
  console.log("no purr");
} else {
  console.log("yes purr");
}

This autofix changes the logic of the code significantly.

[meeber]

@not-an-aardvark makes a good point, although his example contains a concerning lack of animal noises.

On a similar note:

if (~catNoises.indexOf("purr") === 0) { ... }

// gets fixed to:

if (catNoises.indexOf("purr") !== -1 === 0) { ... }

[nzakas]

Lets just not auto fix ~. Our policy is not to auto fix unless it can be done safely, and in this case we obviously can’t. (Trying to detect methods that return numbers is a step beyond what we should be doing.)