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b-thenable.js
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46 lines (39 loc) · 821 Bytes
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'use strict';
const fs = require('node:fs');
class Thenable {
next = null;
then(fn) {
this.fn = fn;
this.next = new Thenable();
return this.next;
}
resolve(value) {
if (!this.fn) return;
const next = this.fn(value);
if (!next) return;
next.then((value) => {
this.next.resolve(value);
});
}
}
// Usage
const readFile = (filename) => {
const thenable = new Thenable();
fs.readFile(filename, 'utf8', (err, data) => {
if (err) throw err;
thenable.resolve(data);
});
return thenable;
};
readFile('file1.txt')
.then((data) => {
console.dir({ file1: data });
return readFile('file2.txt');
})
.then((data) => {
console.dir({ file2: data });
return readFile('file3.txt');
})
.then((data) => {
console.dir({ file3: data });
});