JavaScript Strange Array Definition Syntax

Question

A few coworkers and I have come across some more strange JavaScript syntax. We are having trouble explaining the following behaviour (I am using the Chrome console):

> {}[1]

Yields

[1]

Essentially, it is valid syntax to include any object (not just empty) before the array, and the result is always just the array. Is there any explanation for this? Any case where this doesn't behave this way?

Also, this question is sort of hard to search for, since it consists of characters that don't play well with search engines. Please let me know if this is a duplicate question.

Can you give an example with a non-empty object before the array? — Sara Fuerst
– Sara Fuerst, Commented Oct 31, 2016 at 19:50
@JamieCounsell that's a block that contains a labelled statement (a is the label, 'b' is a String literal expression statement). AST explorer. — joews
– joews, Commented Oct 31, 2016 at 20:07
Thanks! I've never seen that explorer before. Will definitely refer to that going forward. — Jamie Counsell
– Jamie Counsell, Commented Nov 1, 2016 at 3:18

joews · Accepted Answer · 2016-10-31 20:22:07Z

7

{} is an empty code block statement. It's followed by an Array literal [1], which is the value that your program {}[1] evaluates to.

it's pretty much equivalent to:

if (true) {
  // empty block!
}
[1];

If you wanted to get the value with key 1 in an empty object literal, use parentheses:

({})[1] // undefined

You can use AST Explorer to see the JavaScript parser's view of your code.

edited Oct 31, 2016 at 20:22

answered Oct 31, 2016 at 19:50

joews

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epascarello · Accepted Answer · 2016-10-31 19:50:44Z

4

A block statement (or compound statement in other languages) is used to group zero or more statements. The block is delimited by a pair of curly brackets.

{} <-- block statement
[1] <-- array

so basically typed > [1]

edited Oct 31, 2016 at 19:50

answered Oct 31, 2016 at 19:49

epascarello

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3 Comments

Jamie Counsell Over a year ago

Ahh, that makes sense! Thanks!

MinusFour Over a year ago

The chrome console not always evaluate it as it were a statement though, if you introduce { } alone it will evaluate it as an expression of an object literal.

joews Over a year ago

@MinusFour the console implicitly wraps your input in parentheses. { }: block, ({ }): object literal.

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