A brief analysis of how nodejs loads json files in the ECMAScript module

ECMAScript modulenodeHow to load json files》The following article will introduce to you how nodejs loads json files in the ECMAScript module. I hope it will be helpful to everyone!

After reading this article, you will learn:

1,nodejsHow to load and parse json files

2. How to read json files in fs module

3. Learn about import.meta.url

3. Learn about new URL()

4. Learn the load-json-file library

As we all know, if you load ajsonfile in theCommonJS module, just passrequire ()The function can be loaded directly, and thejsonobject can be obtained.

But when loading the json file directly in theECMAScript module, an error will be reported. The error is as follows:

First, enableESMmode, in fact, the official document (http://nodejs.cn/api/esm.html#introduction) also explains:

Node.jswill be used by default JavaScript code is treated as a CommonJS module. Authors can tell Node via the.mjsfile extension,package.json"type"field, or--input-typeflag .js treats JavaScript code as an ECMAScript module

So how can I load ajsonfile in theECMAScript module? In fact, there are two solutions:

Assume there is a json file now:test.json

The content of the file is as follows:

{ "name": "project" }

Next, Introducetest.json:into

index.js

1. Read thejsonfile through thefsfile system

import { readFile } from "fs/promises"; // 以promise的方式引入 readFile API const json = JSON.parse( await readFile(new URL('./test.json', import.meta.url)) ) console.log('[json1]:', json); // 输出： { "name": "project" }

Explanation:

await: According toECMAScript top-levelawaitproposal,awaitkeyword Can be used at the top level within the module (outside the asynchronous function);

import.meta.url：nodejsReturns the module’s localfile:/ /The absolute path of the protocol, for example:file://home/user/main.js, if there is another filetest.jsin the module, then ## The path of #test.jsisnew URL('test.js', import.meta.url);

new URL: Generatefile:An object of the protocol (for mostfsmodule functions, thepathorfilenameparameter can be used asfile:The object of the protocol is passed in).

2. Implementation through the

createRequiremethod of thenodejsbuilt-inmodulemodule

import { createRequire } from "module"; const require = createRequire(import.meta.url); const json = require('./test.json'); console.log('[json2]:', json); // 输出： { "name": "project" }

This method is based on

ThecreateRequiremethod provided by nodejsis implemented.

3. Third-party library with 24 lines of source code

load-json-file

load-json-fileI accidentally discovered it on the npm website. The source code only has 24 lines, as follows:

import {readFileSync, promises as fs} from 'node:fs'; const {readFile} = fs; const parse = (buffer, {beforeParse, reviver} = {}) => { // Unlike `buffer.toString()` and `fs.readFile(path, 'utf8')`, `TextDecoder`` will remove BOM. // 这里对buffer进行转义，没有用`buffer.toString()`和`fs.readFile(path, 'utf8')`，是因为`new TextDecoder().decode(buffer)`这种方式可以删除字节顺序标记（BOM） // 解码 buffer 并返回字符串 let data = new TextDecoder().decode(buffer); // 在parse解析之前对字符串进行处理 if (typeof beforeParse === 'function') { data = beforeParse(data); } return JSON.parse(data, reviver); }; // 导出异步方法 export async function loadJsonFile(filePath, options) { // 如果未指定编码，则返回原始缓冲区。 const buffer = await readFile(filePath); return parse(buffer, options); } // 导出同步方法 export function loadJsonFileSync(filePath, options) { // 如果未指定编码，则返回原始缓冲区。 const buffer = readFileSync(filePath); return parse(buffer, options); }

load-json-file source codeOverall, it is relatively simple, but there are many things that can be done Learn to dig deep into the knowledge points.

For more node-related knowledge, please visit:

nodejs tutorial! !

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