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NOTE: Elements may change whether they are allowed to repeat or not between versions, even once normative (see [cardinality rules for backward compatibility)](versions#f-compat-cardianality)).
Processors should be prepared to manage such changes.
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### JSON representation of primitive elements
FHIR elements with primitive datatypes are represented in two parts:
- A JSON property with the name of the element, which has a JSON type of number, boolean, or string
- a JSON property with `_` prepended to the name of the element, which, if present, contains the value's id and/or extensions
The FHIR types [integer](datatypes#integer), [unsignedInt](datatypes#unsignedInt), [positiveInt](datatypes#positiveInt) and [decimal](datatypes#decimal) are represented as a JSON number, the FHIR type [boolean](datatypes#boolean) as a JSON boolean, and all other types (including [integer64](datatypes#integer64)) are represented as a JSON string which has the same content as that specified for the relevant datatype. Whitespace is always significant (i.e., no leading and trailing spaces for non-strings).
"code" : "abc",
"birthDate" : "1972-11-30",
"deceased" : false,
"count" : 23
}
For comparison, this is represented in XML as
<**code** value="abc"/>
<**birthDate** value="1972-11-30"/>
<**deceased** value="false" />
<**count** value="23" />
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When using a JavaScript JSON.parse() implementation, note that JavaScript natively supports only one numeric datatype, which is a floating point number. This can cause loss of precision for FHIR numbers. In particular, trailing 0s after a decimal point will be lost e.g., 2.00 will be converted to 2. The FHIR decimal datatype is defined such that precision, including trailing zeros, is preserved for presentation purposes, and this is widely regarded as critical for correct presentation of clinical measurements. Implementations should consider using a custom parser and big number library (e.g., [https://github.com/jtobey/javascript-bignum](https://github.com/jtobey/javascript-bignum)) to meet these requirements. See also [the precision extension]([%extensions-location%]StructureDefinition-quantity-precision).
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If the value has an id attribute, or extensions, then this is represented as follows:
"birthDate": "1970-03-30",
"\_birthDate": {
"id": "314159",
"extension" : \[ {
"url" : "http://example.org/fhir/StructureDefinition/text",
"valueString" : "Easter 1970"
}\]
}
Note: If the primitive has an id attribute or extension, but no value, only the property with the `_` is rendered.
This is represented in XML as:
<**birthDate** id="314159" value="1970-03-30" >
...
...
...