Get all keys of object. Contribute to coderaiser/all-object-keys development by creating an account on GitHub. Object.keys(obj) Parameters obj The object of which the enumerable's own properties are to be returned. Return value. An array of strings that represent all the enumerable properties of the given object. Object.keys returns an array whose elements are strings corresponding to the enumerable properties found directly upon object. The ordering of the properties is the same as that given. The Object.entries method returns an array of a given object's own enumerable string-keyed property key, value pairs, in the same order as that provided by a for.in loop. (The only important difference is that a for.in loop enumerates properties in the prototype chain as well). So I noticed a PHP RFC today which suggests implementing objects as array keys via a magic toHash method. I'll be addressing it in today's PHP newsletter to some extent (if you haven't signed up.

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Table 9-40 shows the operators that are available for use with the two JSON data types (see Section 8.14).

Table 9-40. json and jsonb Operators

Note: There are parallel variants of these operators for both the json and jsonb types. The field/element/path extraction operators return the same type as their left-hand input (either json or jsonb), except for those specified as returning text, which coerce the value to text. The field/element/path extraction operators return NULL, rather than failing, if the JSON input does not have the right structure to match the request; for example if no such element exists. The field/element/path extraction operators that accept integer JSON array subscripts all support negative subscripting from the end of arrays.

The standard comparison operators shown in Table 9-1 are available for jsonb, but not for json. They follow the ordering rules for B-tree operations outlined at Section 8.14.4.

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Some further operators also exist only for jsonb, as shown in Table 9-41. Many of these operators can be indexed by jsonb operator classes. For a full description of jsonb containment and existence semantics, see Section 8.14.3. Section 8.14.4 describes how these operators can be used to effectively index jsonb data.

Table 9-41. Additional jsonb Operators

Note: The operator concatenates the elements at the top level of each of its operands. It does not operate recursively. For example, if both operands are objects with a common key field name, the value of the field in the result will just be the value from the right hand operand.

Table 9-42 shows the functions that are available for creating json and jsonb values. (There are no equivalent functions for jsonb, of the row_to_json and array_to_json functions. However, the to_jsonb function supplies much the same functionality as these functions would.)

Table 9-42. JSON Creation Functions

Note:array_to_json and row_to_json have the same behavior as to_json except for offering a pretty-printing option. The behavior described for to_json likewise applies to each individual value converted by the other JSON creation functions.

Note: The hstore extension has a cast from hstore to json, so that hstore values converted via the JSON creation functions will be represented as JSON objects, not as primitive string values.

Table 9-43 shows the functions that are available for processing json and jsonb values.

Table 9-43. JSON Processing Functions

Note: Many of these functions and operators will convert Unicode escapes in JSON strings to the appropriate single character. This is a non-issue if the input is type jsonb, because the conversion was already done; but for json input, this may result in throwing an error, as noted in Section 8.14.

Note: While the examples for the functions json_populate_record, json_populate_recordset, json_to_record and json_to_recordset use constants, the typical use would be to reference a table in the FROM clause and use one of its json or jsonb columns as an argument to the function. Extracted key values can then be referenced in other parts of the query, like WHERE clauses and target lists. Extracting multiple values in this way can improve performance over extracting them separately with per-key operators.

JSON keys are matched to identical column names in the target row type. JSON type coercion for these functions is 'best effort' and may not result in desired values for some types. JSON fields that do not appear in the target row type will be omitted from the output, and target columns that do not match any JSON field will simply be NULL.

Note: All the items of the path parameter of jsonb_set must be present in the target, unless create_missing is true, in which case all but the last item must be present. If these conditions are not met the target is returned unchanged.

If the last path item is an object key, it will be created if it is absent and given the new value. If the last path item is an array index, if it is positive the item to set is found by counting from the left, and if negative by counting from the right - -1 designates the rightmost element, and so on. If the item is out of the range -array_length . array_length -1, and create_missing is true, the new value is added at the beginning of the array if the item is negative, and at the end of the array if it is positive.

Note: The json_typeof function's null return value should not be confused with a SQL NULL. While calling json_typeof('null'::json) will return null, calling json_typeof(NULL::json) will return a SQL NULL.

Note: If the argument to json_strip_nulls contains duplicate field names in any object, the result could be semantically somewhat different, depending on the order in which they occur. This is not an issue for jsonb_strip_nulls since jsonb values never have duplicate object field names.

See also Section 9.20 for the aggregate function json_agg which aggregates record values as JSON, and the aggregate function json_object_agg which aggregates pairs of values into a JSON object, and their jsonb equivalents, jsonb_agg and jsonb_object_agg.

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Let’s step away from the individual data structures and talk about the iterations over them.

In the previous chapter we saw methods map.keys(), map.values(), map.entries().

These methods are generic, there is a common agreement to use them for data structures. If we ever create a data structure of our own, we should implement them too.

They are supported for:

• Map
• Set
• Array

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Plain objects also support similar methods, but the syntax is a bit different.

Object.keys, values, entries

For plain objects, the following methods are available:

• Object.keys(obj) – returns an array of keys.
• Object.values(obj) – returns an array of values.
• Object.entries(obj) – returns an array of [key, value] pairs.

Please note the distinctions (compared to map for example):

The first difference is that we have to call Object.keys(obj), and not obj.keys().

Why so? The main reason is flexibility. Remember, objects are a base of all complex structures in JavaScript. So we may have an object of our own like data that implements its own data.values() method. And we still can call Object.values(data) on it.

The second difference is that Object.* methods return “real” array objects, not just an iterable. That’s mainly for historical reasons.

For instance:

• Object.keys(user) = ['name', 'age']
• Object.values(user) = ['John', 30]
• Object.entries(user) = [ ['name','John'], ['age',30] ]

Here’s an example of using Object.values to loop over property values:

Just like a for.in loop, these methods ignore properties that use Symbol(..) as keys.

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Usually that’s convenient. But if we want symbolic keys too, then there’s a separate method Object.getOwnPropertySymbols that returns an array of only symbolic keys. Also, there exist a method Reflect.ownKeys(obj) that returns all keys.

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Transforming objects

Objects lack many methods that exist for arrays, e.g. map, filter and others.

If we’d like to apply them, then we can use Object.entries followed Object.fromEntries:

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1. Use Object.entries(obj) to get an array of key/value pairs from obj.
2. Use array methods on that array, e.g. map.
3. Use Object.fromEntries(array) on the resulting array to turn it back into an object.

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For example, we have an object with prices, and would like to double them:

It may look difficult from the first sight, but becomes easy to understand after you use it once or twice. We can make powerful chains of transforms this way.