Finding and querying documents in Express - Step-by-Step Execution

Concept Flow - Finding and querying documents

Start Express app

↓

Receive HTTP GET request

↓

Parse query parameters

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Build query object

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Call database find method

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Database returns matching documents

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Send documents as HTTP response

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End request

The Express app receives a GET request, extracts query parameters, builds a query, fetches matching documents from the database, and sends them back as a response.

Execution Sample

Express

app.get('/users', async (req, res) => {
  const { age } = req.query;
  const users = await User.find({ age: Number(age) });
  res.json(users);
});

This code listens for GET requests on '/users', queries users by age, and returns matching users as JSON.

Execution Table

Step	Action	Input/Condition	Database Query	Result	Response Sent
1	Receive GET request	GET /users?age=30	N/A	N/A	N/A
2	Parse query parameters	age=30	N/A	N/A	N/A
3	Build query object	{ age: 30 }	N/A	N/A	N/A
4	Call User.find()	{ age: 30 }	Find users with age 30	[{name: 'Alice', age: 30}, {name: 'Bob', age: 30}]	N/A
5	Send response	N/A	N/A	N/A	JSON array of users with age 30
6	End request	N/A	N/A	N/A	Response complete

💡 Request ends after sending JSON response with matching documents.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
req.query.age	undefined	'30'	'30'	'30'	'30'
queryObject	{}	{}	{ age: 30 }	{ age: 30 }	{ age: 30 }
users	undefined	undefined	undefined	[{name: 'Alice', age: 30}, {name: 'Bob', age: 30}]	[{name: 'Alice', age: 30}, {name: 'Bob', age: 30}]

Key Moments - 2 Insights

Why is the age value a string ('30') in the query object instead of a number?

Query parameters from the URL are always strings. To query by number, you must convert the string to a number before building the query object. See Step 3 in execution_table where age is still a string.

What happens if no documents match the query?

The database returns an empty array. The response will send an empty JSON array. This is shown in Step 4 where the result could be [].

Visual Quiz - 3 Questions

Look at the execution_table at Step 4, what does the database return?

A A single user object

B An empty object

C An array of user objects with age 30

D An error message

💡 Hint

Check the 'Result' column in Step 4 of execution_table

At which step is the query object created?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' column in execution_table where the query object is built

If the URL query parameter was age=25, how would the 'users' variable change after Step 4?

A It would contain users with age 25

B It would contain users with age 30

C It would be undefined

D It would contain all users

💡 Hint

Refer to variable_tracker and execution_table Step 4 for how query affects users

Concept Snapshot

Express GET route listens for requests
Extract query parameters from req.query
Build a query object for database
Use Model.find(query) to get matching documents
Send results as JSON response
Query params are strings; convert if needed

Full Transcript

This example shows how an Express app handles finding and querying documents. When a GET request arrives at '/users' with a query parameter like age=30, the app extracts the age from req.query. It builds a query object with this age and calls User.find() to get matching users from the database. The database returns an array of user objects matching the age. Finally, the app sends this array as a JSON response. Note that query parameters are strings, so if you want to query by number, you should convert the string to a number before querying. If no users match, an empty array is returned and sent. This flow helps you understand how to find and query documents in Express apps.