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Created March 8, 2024 15:31
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Builder pattern with async execution, demoing how to create a tiny shell script utility
Raw
README.md

Async builder

DO NOT COPY THIS CODE INTO PRODUCTION!

For this specific example, you should use packages like zx or bun shell which are battle-tested shell script runners for javascript environments

This is meant to showcase what a builder pattern for await operators can do.

Example usage

// Does nothing
const myScript = $`ls -al`;

// Executes here
await myScript;
await $`ls -al`;
await $`ls -al`.setEnv({ key: 'value' });
await $`cd ${'./folder name'}`;

How it works

This works by creating a pseudo promise (called PromiseLike in Typescript) which creates a block of code that only executes when the developer calls await.

Raw
file.ts
import { SpawnOptions, spawn } from 'child_process';
type $Result = PromiseLike<void> & {
setEnv: (newEnv: NodeJS.ProcessEnv) => $Result;
};
/**
* Run a shell command, printing its output to stdout and stderr accordingly
* @param text The command to execute with its arguments
* @param arguments Arguments to pass to the shell command
* @see Inspired by https://bun.sh/docs/runtime/shell
* @see Inspired by https://github.com/google/zx
*/
export const $ = (
text: TemplateStringsArray,
...values: (string | number)[]
): $Result => {
let opts: SpawnOptions = {};
const args = getScriptParts(text, ...values);
const cmd = args[0];
const cmdArgs: string[] = args.slice(1);
const sh: $Result = {
setEnv: (newEnv) => {
opts = {
...opts,
env: newEnv,
};
return sh;
},
then: (resolve, reject) => {
return new Promise<void>((resolve, reject) => {
console.info(`$ ${cmd} ${cmdArgs.join(' ')}`);
const process = spawn(cmd, cmdArgs, {
stdio: 'inherit',
...(opts ?? {}),
});
process.on('close', (code) => {
if (code) reject(new Error(`Command failed with exit code ${code}`));
else resolve();
});
}).then(resolve, reject);
},
};
return sh;
};
const getScriptParts = (
text: TemplateStringsArray,
...values: (string | number)[]
) => {
const args: string[] = [];
let curArg = '';
for (let i = 0; i < text.length; i++) {
let curText = text[i];
let argEnd = curText.indexOf(' ');
while (argEnd >= 0) {
curArg += curText.substring(0, argEnd);
curText = curText.substring(argEnd + 1);
argEnd = curText.indexOf(' ');
args.push(curArg);
curArg = '';
}
curArg += curText;
if (values[i]) {
curArg += `${values[i]}`;
}
}
if (curArg) {
args.push(curArg);
}
return args;
};
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