Fixing 'The body of a for-in should be wrapped in an if statement to filter unwanted properties from the prototype'

Question

I have this code:

data.forEach(item => {
    for (const i in Object.keys(item)) {
      const key = Object.keys(item)[i];
      //..
    }
  });

eslint throwing an error:

error The body of a for-in should be wrapped in an if statement to filter unwanted properties from the prototype guard-for-in

I did research and tried to fix that this way:

data.forEach(item => {
if (pattern.hasOwnProperty(i)) {
    for (const i in Object.keys(item)) {
      const key = Object.keys(item)[i];
      //..
    }
  }
});

So that made the linter happy but the for..in loop now does not work.

that is the value of the item - { 'bla': '42' }

Any ideas how to solve?

Never use [for...in](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in), use [for ... of](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of), or use `.forEach` because that's kind of what it's for (`for ... in` is a legacy construct that does all kinds of things you really don't want). And there is _no_ need to double iterate, the top level iteration already does exactly what you want: `for (const [key, value] of Object.entries(items)) { ... }` or `Object.entries(items).forEach(([key, value]) => {... });` and off you go. — Mike 'Pomax' Kamermans, Sep 22 '21 at 00:06
@Mike'Pomax'Kamermans great, it seems more like an answer, not a comment ;) — Sergino, Sep 22 '21 at 00:12
Oh, no, this is _absolutely_ a comment. No one familiar with iterating over lists would write a double-nested loop on the _same_ list =) — Mike 'Pomax' Kamermans, Sep 22 '21 at 00:13

score 2 · Answer 1 · answered Sep 22 '21 at 00:33

It should be either

for (const key of Object.keys(item)) {
  …
}

or

Object.keys(item).forEach(key => {
  …
});

or

for (const key in item) {
  …
}

but not a mix of for … in with Object.keys. Also no, it should not be necessary to wrap loop bodies in if statements, better disable that linter rule (and rather enable no-prototype-builtins, which would've caught your broken fix).

Fixing 'The body of a for-in should be wrapped in an if statement to filter unwanted properties from the prototype'

1 Answers1