用ES6读取文件 [英] reading file with ES6 promises

问题描述

let arr = [];

function getData(fileName, type) {
    return fs.readFile(fileName,'utf8', (err, data) => {
        if (err) throw err;

        return new Promise(function(resolve, reject) {
            for (let i = 0; i < data.length; i++) {
                arr.push(data[i]);
            }

            resolve();
        });
    });
}

getData('./file.txt', 'sample').then((data) => {
    console.log(data);
});

当我使用上面的代码并使用nodejs在命令行中运行它时，出现以下错误.

When I use above code and run it in command line using nodejs I get following error.

getData('./file.txt', 'sample').then((data) => {
                               ^

TypeError: Cannot read property 'then' of undefined

我该如何解决?

推荐答案

您将希望将整个fs.readFile调用包装在新的Promise中，然后根据回调结果拒绝或解决承诺:

You'll want to wrap the entire fs.readFile invocation inside a new Promise, and then reject or resolve the promise depending on the callback result:

function getData(fileName, type) {
  return new Promise(function(resolve, reject){
    fs.readFile(fileName, type, (err, data) => {
        err ? reject(err) : resolve(data);
    });
  });
}

[更新]从Node.js v10开始，您可以选择使用fs.promises.<API>使用fs模块的内置Promise实现.对于我们的readFile示例，我们将更新解决方案以使用fs.promises这样:

[UPDATE] As of Node.js v10, you can optionally use the built-in Promise implementations of the fs module by using fs.promises.<API>. In the case of our readFile example, we would update our solution to use fs.promises like this:

function getData(fileName, type) {
  return fs.promises.readFile(fileName, {encoding: type});
}

这篇关于用ES6读取文件的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！