Add a --fix option for prefer-arrow-callback

I’m using Node v6.4.0 and ESLint v3.4.0.

It would be useful to have an autofix option for the prefer-arrow-callback rule. The fixer would attempt to convert regular callback functions to arrow functions.

• If the function does not use this, super, arguments, or new.target, and it is not explicitly bound, it can be replaced naively:

/* eslint prefer-arrow-callback: ["error"] */

[1, 2, 3].map(function(a, b, c) {
  return a * 2;
});

// gets fixed to:

[1, 2, 3].map((a, b, c) => {
  return a * 2;
});

• If the function is explicitly bound with .bind(this), and it does not use super, arguments, or new.target, the binding can be removed and the function can be replaced:

/* eslint prefer-arrow-callback: ["error"] */

[1, 2, 3].map(function(a, b, c) {
  return a * 2;
}.bind(this));

// gets fixed to:

[1, 2, 3].map((a, b, c) => {
  return a * 2;
});

// ---

[1, 2, 3].map(function(a, b, c) {
  return a * this.foo;
}.bind(this));

// gets fixed to:

[1, 2, 3].map((a, b, c) => {
  return a * this.foo;
});

• If the option {allowUnboundThis: false} is used, and the function includes this without being explicitly bound, the problem should not get fixed, since there is no way to be sure of the correct this value:

/* eslint prefer-arrow-callback: ["error", {allowUnboundThis: false}] */
[1, 2, 3].map(function(a, b, c) {
  return a * this.foo;
});

// (Reports an error, but is not autofixed)

• If the function uses super, arguments, or new.target, the rule does not report any problems, so nothing gets fixed.

/* eslint prefer-arrow-callback: ["error"] */

[1, 2, 3].map(function(a, b, c) {
  return arguments[0] * 5;
});

// (no errors are reported, so no fixes are performed)

• To avoid SyntaxErrors, the fixer should not fix anything unless ES6 parsing is enabled.

---

Other things to consider:

• If the {allowNamedFunctions: false} option is used and a function has a name, should the function still be changed when autofixing? (This doesn’t break recursive functions, since those are ignored by prefer-arrow-callback anyway.) I think the fixer should still fix these functions; if people want to allow named functions, then they should set {allowNamedFunctions: true}.

---

I would be willing to add this if the issue is accepted.

[not-an-aardvark]

but this would be a massive fix (in terms of characters of source code affected per fix)

Note that this is only a massive fix if .bind(this) is used; otherwise, the fix would just replace function (a, b, c) with (a, b, c) => and leave the rest of the function untouched.

[vitorbal]

I’ll 👍 this!