Question: ```
var statusCodes = {};

statusCodes[exports.ACCEPTED = 202] = "Accepted";

```

what does the second line mean? More specifically, the part in the squared brackets.

Answer: 
Complex things are made of multiple simple things. Just look at it one operation at a time.

First this operation is performed:

```
exports.ACCEPTED = 202

```

Which sets `exports.ACCEPTED` to the value `202`, and the operation evaluates to the value `202`. Then this operation is performed:

```
statusCodes[202] = "Accepted"

```

Which sets `statusCodes[202]` to the value `"Accepted"`.
It's just a shorter version for

```
var statusCodes = {};
exports.ACCEPTED = 202;
statusCodes[exports.ACCEPTED] = "Accepted";

```

Check this question to get the idea

[Value returned by the assignment](https://stackoverflow.com/questions/16027247/value-returned-by-the-assignment)