JSON

The syntax of JSON, JSON.parse and JSON.stringify, the difference from a JavaScript object and the access path to a nested value.

24 cards
3 topics
Free

Study this deck

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Concepts
Questions
Sort and link

A few cards from the deck

JSON: what is it?
Show the answer
A text format for exchanging data, derived from JavaScript's object notation, made of objects, arrays, strings, numbers, booleans and null.
It is readable by every language, not just JavaScript: that is why web APIs use it. It knows nothing of dates, functions or undefined.
What string does this program produce?

```javascript
JSON.stringify({ a: 1, b: undefined, c: [1, undefined], d: () => 1 })
`
- {"a":1,"c":[1,null]}
- {"a":1,"b":null,"c":[1,null]}
- {"a":1,"c":[1]}
- {"a":1,"c":[1,null],"d":null}
Show the answer
{"a":1,"c":[1,null]}
In an object, undefined or function properties are omitted (b and d disappear). In an array, they keep their place and become null.
A valid JSON document is written with … quotes; it accepts neither a … nor a comment.
Show the answer
A valid JSON document is written with double quotes; it accepts neither a trailing comma nor a comment.
Single quotes and unquoted names work in a JavaScript literal, not in JSON. In an object, a trailing comma produces Expected double-quoted property name.
JSON's strict syntax: what is it?
Show the answer
JSON's rules: property names and strings are written in double quotes, with no comma after the last element and no comment.
{ a: 1 }, {'a': 1} and {"a": 1,} are valid JavaScript literals, but JSON.parse rejects them: for the third, it raises Expected double-quoted property name in JSON.
What does JSON.stringify({ n: NaN }) return?
- {"n":NaN}
- {"n":null}
- {"n":"NaN"}
- {}
Show the answer
{"n":null}
JSON has no representation for NaN or infinity: they become null. It is not an omission, unlike undefined in an object.

Study this deck