Parse structured values without guessing types

Parse structured values without guessing types — guided practice

Distinguish JSON strings, numbers, arrays and missing fields.

Save api-viewer.html from the download and open it in a modern browser. Open its source in a text editor. The complete original file supplies every fixture and function. It simulates GET /hours locally; it does not contact a real library. Reload after edits; restore the supplied file before each lesson unless told otherwise.

1. 1. Inspect a number

   Find opens:12 in the success East record. In JSON text it is written "opens":12.

   You should see: The value is numeric.

   Number operations require a deliberate type choice.

2. 2. Inspect the wrong type

   Choose wrong-type and click Load hours.

   You should see: The viewer reports Invalid branch record.

   The string "12" is not the integer 12 in this contract.

3. 3. Run a parser example

   In the browser console run the exact expression.

   typeof JSON.parse('{"opens":12}').opens

   You should see: The returned value is number.

   typeof reveals the parsed value’s JavaScript type.

4. 4. Compare a missing value

   Run the second expression.

   JSON.parse('{"name":"West"}').opens

   You should see: It returns undefined because the field is absent.

   Missing is different from an explicit zero.