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.

Below is the translated version of the Go code example into JavaScript.

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