Arrow function with multiple statements in one line

Question

Recently I found out that a simple arrow function, for example callback in this map

[1, 2, 3, 4, 5, 6].map(item => {
    console.log(item) // or anything else
    return item - 1;
})

I can re-write to one line command like this

[1, 2, 3, 4, 5, 6].map(item => (console.log(item), item - 1))

I can use as much statements I want to devided by , and last argument will always be a return value. It looks kind of cool to me, but cant find anything about this syntax in arrow function documentation. Can anyone explain this syntax or just point to place where I found a docs?

[Comma Operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator). But please don't actually do this. Only minifiers should ever output something that unreadable. — Patrick Roberts, Aug 17 '20 at 18:23
[Also related](https://stackoverflow.com/a/33050635/1541563) — Patrick Roberts, Aug 17 '20 at 18:27
@PatrickRoberts sure, it didn't occur to me it is just comma operator :) — l2ysho, Aug 17 '20 at 18:27

score 0 · Accepted Answer · answered Aug 17 '20 at 18:27

0

Essentially it allows you to do multiple operations in a single statement. It's used commonly in for loops and to assign multiple variables in a single statment, eg.: var a = 1, b = 2;.

Just because it works though, doesn't mean it's a good idea. It makes your code less readable and harder to debug.

See the MDN docs on the Comma Operator.

The comma operator (,) evaluates each of its operands (from left to right) and returns the value of the last operand. This lets you create a compound expression in which multiple expressions are evaluated, with the compound expression's final value being the value of the rightmost of its member expressions. This is commonly used to provide multiple parameters to a for loop.

answered Aug 17 '20 at 18:27

I wrestled a bear once.

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Sure it is not good Idea :) I just never saw a comma operator used in this way, so I have been little confused – l2ysho Aug 17 '20 at 18:30
3

_"It's used commonly... to assign multiple variables in a single statement"_ That's technically wrong. The `,` token in a [declaration statement](https://www.ecma-international.org/ecma-262/#sec-declarations-and-the-variable-statement) is not the [comma operator](https://www.ecma-international.org/ecma-262/#sec-comma-operator), they're actually defined separately in two different parts of the grammar. – Patrick Roberts Aug 17 '20 at 18:35

score -1 · Answer 2 · answered Aug 17 '20 at 18:27

-1

You are using the comma operator to evaluate multiple expressions: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator

answered Aug 17 '20 at 18:27

Nils Schwebel

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Arrow function with multiple statements in one line

2 Answers2