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Docs: Amend rule document to correct and complete it (refs #6251). (#…

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jrpool authored and gyandeeps committed Oct 27, 2017
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# enforce placing object properties on separate lines (object-property-newline)
While formatting preferences are very personal, a number of style guides require that object properties be placed on separate lines for better readability.
This rule permits you to restrict the locations of property specifications in object literals. You may prohibit any part of any property specification from appearing on the same line as any part of any other property specification. You may make this prohibition absolute, or, by invoking an object option, you may allow an exception, permitting an object literal to have all parts of all of its property specifications on a single line.
Another argument in favor of this style is that it improves the readability of diffs when a property is changed:
## Rule Details
### Motivations
This rule makes it possible to ensure, as some style guides require, that property specifications appear on separate lines for better readability. For example, you can prohibit all of these:
```js
const newObject = {a: 1, b: [2, {a: 3, b: 4}]};
const newObject = {
a: 1, b: [2, {a: 3, b: 4}]
};
const newObject = {
a: 1,
b: [2, {a: 3, b: 4}]
};
const newObject = {
a: 1,
b: [
2,
{a: 3, b: 4}
]
};
```
Instead of those, you can comply with the rule by writing
```js
const newObject = {
a: 1,
b: [2, {
a: 3,
b: 4
}]
};
```
or
```js
const newObject = {
a: 1,
b: [
2,
{
a: 3,
b: 4
}
]
};
```
Another benefit of this rule is specificity of diffs when a property is changed:
```diff
// More readable
// More specific
var obj = {
foo: "foo",
- bar: "bar",
@@ -15,51 +67,164 @@ Another argument in favor of this style is that it improves the readability of d
```
```diff
// Less readable
// Less specific
-var obj = { foo: "foo", bar: "bar", baz: "baz" };
+var obj = { foo: "foo", bar: "bazz", baz: "baz" };
```
## Rule Details
### Optional Exception
The rule offers one object option, `allowMultiplePropertiesPerLine`. If you set it to `true`, object literals such as the first two above, with all property specifications on the same line, will be permitted, but one like
```js
const newObject = {
a: 'a.m.', b: 'p.m.',
c: 'daylight saving time'
};
```
will be prohibited, because two properties, but not all properties, appear on the same line.
### Notations
This rule applies equally to all property specifications, regardless of notation, including:
- `a: 1` (ES5)
- `a` (ES2015 shorthand property)
- ``[`prop${a}`]`` (computed property name)
Thus, the rule (without the object option) prohibits both of these:
```js
const newObject = {
a: 1, [
process.argv[4]
]: '01'
};
const newObject = {
a: 1, [process.argv[4]]: '01'
};
```
(This behavior differs from that of the JSCS rule cited below, which does not treat the leading `[` of a computed property name as part of that property specification. The JSCS rule prohibits the second of these formats but permits the first.)
This rule aims to maintain consistency of newlines between object properties.
### Multiline Properties
Examples of **incorrect** code for this rule:
The rule prohibits the colocation on any line of at least 1 character of one property specification with at least 1 character of any other property specification. For example, the rule prohibits
```js
const newObject = {a: [
'Officiële website van de Europese Unie',
'Официален уебсайт на Европейския съюз'
], b: 2};
```
because 1 character of the specification of `a` (i.e. the trailing `]` of its value) is on the same line as the specification of `b`.
The optional exception does not excuse this case, because the entire collection of property specifications spans 4 lines, not 1.
### Inter-property Delimiters
The comma and any whitespace that delimit property specifications are not considered parts of them. Therefore, the rule permits both of these formats:
```js
const newFunction = multiplier => ({
a: 2 * multiplier,
b: 4 * multiplier,
c: 8 * multiplier
});
const newFunction = multiplier => ({
a: 2 * multiplier
, b: 4 * multiplier
, c: 8 * multiplier
});
```
(This behavior differs from that of the JSCS rule cited below, which permits the first but prohibits the second format.)
### --fix
If this rule is invoked with the command-line `--fix` option, object literals that violate the rule are generally modified to comply with it. The modification in each case is to move a property specification to the next line whenever there is part or all of a previous property specification on the same line. For example,
```js
const newObject = {
a: 'a.m.', b: 'p.m.',
c: 'daylight saving time'
};
```
is converted to
```js
const newObject = {
a: 'a.m.',
b: 'p.m.',
c: 'daylight saving time'
};
```
The modification does not depend on whether the object option is set to `true`. In other words, ESLint never collects all the property specifications onto a single line, even when the object option would permit that.
ESLint does not correct a violation of this rule if a comment immediately precedes the second or subsequent property specification on a line, since ESLint cannot determine which line to put the comment onto.
As illustrated above, the `--fix` option, applied to this rule, does not comply with other rules, such as `indent`, but, if those other rules are also in effect, the option applies them, too.
## Examples
Examples of **incorrect** code for this rule, with no object option or with `allowMultiplePropertiesPerLine` set to `false`:
```js
/*eslint object-property-newline: "error"*/
var obj = { foo: "foo", bar: "bar", baz: "baz" };
const obj0 = { foo: "foo", bar: "bar", baz: "baz" };
var obj2 = {
const obj1 = {
foo: "foo", bar: "bar", baz: "baz"
};
var obj3 = {
const obj2 = {
foo: "foo", bar: "bar",
baz: "baz"
};
const obj3 = {
[process.argv[3] ? "foo" : "bar"]: 0, baz: [
1,
2,
4,
8
]
};
const a = "antidisestablishmentarianistically";
const b = "yugoslavyalılaştırabildiklerimizdenmişsiniz";
const obj4 = {a, b};
```
Examples of **correct** code for this rule:
Examples of **correct** code for this rule, with no object option or with `allowMultiplePropertiesPerLine` set to `false`:
```js
/*eslint object-property-newline: "error"*/
var obj = {
const obj = {
foo: "foo",
bar: "bar",
baz: "baz"
};
```
## Options
This rule has an object option:
* `"allowMultiplePropertiesPerLine"`: `true` allows all keys and values to be on the same line
### allowMultiplePropertiesPerLine
const user = process.argv[2];
const obj3 = {
user,
[process.argv[3] ? "foo" : "bar"]: 0,
baz: [
1,
2,
4,
8
]
};
```
Examples of additional **correct** code for this rule with the `{ "allowMultiplePropertiesPerLine": true }` option:
@@ -71,19 +236,23 @@ var obj = { foo: "foo", bar: "bar", baz: "baz" };
var obj2 = {
foo: "foo", bar: "bar", baz: "baz"
};
const user = process.argv[2];
const obj3 = {
user, [process.argv[3] ? "foo" : "bar"]: 0, baz: [1, 2, 4, 8]
};
```
## When Not To Use It
You can turn this rule off if you are not concerned with the consistency of newlines between object properties.
You can turn this rule off if you want to decide, case-by-case, whether to place property specifications on separate lines.
## Compatibility
* **JSCS**: [requireObjectKeysOnNewLine](http://jscs.info/rule/requireObjectKeysOnNewLine)
- **JSCS**: This rule provides partial compatibility with [requireObjectKeysOnNewLine](http://jscs.info/rule/requireObjectKeysOnNewLine).
## Related Rules
* [brace-style](brace-style.md)
* [comma-dangle](comma-dangle.md)
* [key-spacing](key-spacing.md)
* [object-curly-spacing](object-curly-spacing.md)
- [brace-style](brace-style.md)
- [comma-dangle](comma-dangle.md)
- [key-spacing](key-spacing.md)
- [object-curly-spacing](object-curly-spacing.md)

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