cors middleware setup in Express - Step-by-Step Execution

Concept Flow - cors middleware setup

Start Express App

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Import cors Middleware

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Configure cors Options

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Use cors Middleware in App

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Handle Incoming Requests

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cors Checks Request Origin

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Allow or Block Request

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Send Response

This flow shows how the Express app imports and uses the cors middleware to check and allow cross-origin requests before sending responses.

Execution Sample

Express

import express from 'express';
import cors from 'cors';

const app = express();
app.use(cors({ origin: 'https://example.com' }));

app.get('/', (req, res) => res.send('Hello'));
app.listen(3000);

This code sets up an Express server that uses cors middleware to allow requests only from https://example.com.

Execution Table

Step	Action	cors Middleware Behavior	Request Origin	Allowed?	Response Sent
1	Server starts and listens on port 3000	cors middleware ready	N/A	N/A	N/A
2	Incoming GET request to '/' from https://example.com	Checks origin header	https://example.com	Yes	Hello
3	Incoming GET request to '/' from https://notallowed.com	Checks origin header	https://notallowed.com	No	Blocked by CORS
4	Incoming GET request to '/' with no origin header (same origin)	No origin, allowed by default	None	Yes	Hello
5	Server continues listening for requests	cors middleware active	N/A	N/A	N/A

💡 Requests blocked if origin is not https://example.com; otherwise allowed and responded.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
app	Express instance	Express instance	Express instance	Express instance	Express instance
corsOptions.origin	'https://example.com'	'https://example.com'	'https://example.com'	'https://example.com'	'https://example.com'
request.origin	N/A	'https://example.com'	'https://notallowed.com'	None	N/A
request.allowed	N/A	true	false	true	N/A

Key Moments - 3 Insights

Why does the request from https://notallowed.com get blocked?

Because the cors middleware checks the origin against the allowed origin 'https://example.com' and blocks any origin not matching, as shown in execution_table row 3.

What happens if the request has no origin header?

Requests without an origin header are treated as same-origin and allowed by default, as shown in execution_table row 4.

Where in the code is the allowed origin set?

The allowed origin is set in the cors middleware options passed to app.use(cors({ origin: 'https://example.com' })), as tracked in variable_tracker under corsOptions.origin.

Visual Quiz - 3 Questions

Look at the execution table, what is the 'Allowed?' value at Step 3?

A N/A

B Yes

C No

D Depends on method

💡 Hint

Check the 'Allowed?' column in execution_table row 3 for the request from https://notallowed.com.

At which step does the cors middleware allow a request with no origin header?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at execution_table row 4 where the origin is 'None' and 'Allowed?' is 'Yes'.

If we change the allowed origin to '*' in cors options, what changes in the execution table?

A No origins are allowed, so all requests blocked

B All origins are allowed, so Step 3 'Allowed?' becomes Yes

C Only requests from localhost are allowed

D No change, only https://example.com allowed

💡 Hint

Changing origin to '*' means all origins pass the check, affecting the 'Allowed?' column in execution_table.

Concept Snapshot

Express CORS Middleware Setup:
1. Import cors and express.
2. Create app with express().
3. Use app.use(cors({ origin: 'allowed-origin' })) to set allowed origins.
4. cors middleware checks incoming request origins.
5. Requests from allowed origins proceed; others blocked.
6. Helps secure cross-origin resource sharing easily.

Full Transcript

This visual execution trace shows how to set up CORS middleware in an Express app. The app imports the cors package and uses it with options specifying allowed origins. When requests come in, the middleware checks the origin header. If the origin matches the allowed origin, the request is allowed and the response is sent. If not, the request is blocked by CORS. Requests without an origin header are treated as same-origin and allowed. The execution table tracks each step, showing how requests from different origins are handled. The variable tracker follows key variables like the app instance, cors options, request origin, and whether the request is allowed. Key moments clarify common confusions about origin checking and default behaviors. The quiz tests understanding of how the middleware behaves at different steps and how changing options affects request allowance. This setup helps developers control which external sites can access their server resources safely.