Defer

To ensure a function call is performed later in a program’s execution, usually for purposes of cleanup, we can use constructs in JavaScript that mimic similar behavior. In JavaScript, a common pattern is to use finally blocks for cleanup.

Suppose we wanted to create a file, write to it, and then close it when we’re done. Here’s how we can achieve that in JavaScript using the fs module and finally for cleanup.

To ensure a function call is performed later in a program’s execution, usually for purposes of cleanup, we can use constructs in JavaScript that mimic similar behavior. In JavaScript, a common pattern is to use `finally` blocks for cleanup.

Suppose we wanted to create a file, write to it, and then close it when we’re done. Here's how we can achieve that in JavaScript using the `fs` module and `finally` for cleanup.

Immediately after getting a file descriptor with `fs.openSync`, we ensure the closing of that file with `fs.closeSync` inside a `finally` block. This will be executed at the end of the try block, after `fs.writeFileSync` has finished.

```javascript
const fs = require('fs');
const path = '/tmp/defer.txt';

function createFile(path) {
    console.log('creating');
    let fd;
    try {
        fd = fs.openSync(path, 'w');
    } catch (err) {
        console.error('Error creating file:', err);
        throw err;
    }
    return fd;
}

function writeFile(fd) {
    console.log('writing');
    fs.writeFileSync(fd, 'data');
}

function closeFile(fd) {
    console.log('closing');
    try {
        fs.closeSync(fd);
    } catch (err) {
        console.error('Error closing file:', err);
        process.exit(1);
    }
}

function main() {
    const fd = createFile(path);
    try {
        writeFile(fd);
    } finally {
        closeFile(fd);
    }
}

main();

It’s important to check for errors when closing a file, even in a deferred function. Running the program confirms that the file is closed after being written.

$ node defer.js
creating
writing
closing

Now that we can run and build basic JavaScript programs, let’s learn more about the language.

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Defer