nodejs的子產品
nodejs的子產品允許從被引入的檔案中選擇要暴露給我們的函數和變量,如果子產品傳回的函數或變量不止一個,我們可以通過設定exports對象的屬性來指明,如果子產品隻傳回一個函數或變量,則使用module.exports屬性。
CommonJS子產品引入
a子產品下面的index.js
const canadianDollars = .9; //a 子產品的私有變量
function roundTwo (amount) { // a子產品的是有方法,a子產品外面是通路不到
return Math.round(amount * 100) / 100;
}
//對外面暴露的的兩個方法
exports.canadianToUs = canadian => roundTwo(canadian * canadianDollars)
exports.UsToCanadian = us => roundTwo(us / canadianDollars)
我們要require的子產品順序是先查找nodejs的核心子產品,然後是目前的目錄,最後是node_modules,
require是nodejs中的少有的幾個同步的方法,一般都是在檔案頂端引入,保持代碼的整潔。
//nodejs的子產品規範引入CommonJS
const currency = require('./a');
console.log(currency) //{ canadianToUs: [Function], UsToCanadian: [Function] }
ES6(es2015)方式引入
這裡有個坑,nodejs目前還不支援es6的import ,from方法,需要我們做相關的配置來相容
全局安裝es-checker,運作會列出相關的支援情況
nelsen-mac:nodeshizhan mac$ es-checker
ECMAScript 6 Feature Detection (v1.4.1)
Variables
√ let and const
√ TDZ error for too-early access of let or const declarations
√ Redefinition of const declarations not allowed
√ destructuring assignments/declarations for arrays and objects
√ ... operator
Data Types
√ For...of loop
√ Map, Set, WeakMap, WeakSet
√ Symbol
√ Symbols cannot be implicitly coerced
Number
√ Octal (e.g. 0o1 ) and binary (e.g. 0b10 ) literal forms
√ Old octal literal invalid now (e.g. 01 )
√ Static functions added to Math (e.g. Math.hypot(), Math.acosh(), Math.imul() )
√ Static functions added to Number (Number.isNaN(), Number.isInteger() )
String
√ Methods added to String.prototype (String.prototype.includes(), String.prototype.repeat() )
√ Unicode code-point escape form in string literals (e.g. \u{20BB7} )
√ Unicode code-point escape form in identifier names (e.g. var \u{20BB7} = 42; )
√ Unicode code-point escape form in regular expressions (e.g. var regexp = /\u{20BB7}/u; )
√ y flag for sticky regular expressions (e.g. /b/y )
√ Template String Literals
Function
√ arrow function
√ default function parameter values
√ destructuring for function parameters
√ Inferences for function name property for anonymous functions
× Tail-call optimization for function calls and recursion
Array
× Methods added to Array.prototype ([].fill(), [].find(), [].findIndex(), [].entries(), [].keys(), [].values() )
√ Static functions added to Array (Array.from(), Array.of() )
√ TypedArrays like Uint8Array, ArrayBuffer, Int8Array(), Int32Array(), Float64Array()
√ Some Array methods (e.g. Int8Array.prototype.slice(), Int8Array.prototype.join(), Int8Array.prototype.forEach() ) added to the TypedArray prototypes
√ Some Array statics (e.g. Uint32Array.from(), Uint32Array.of() ) added to the TypedArray constructors
Object
√ __proto__ in object literal definition sets [[Prototype]] link
√ Static functions added to Object (Object.getOwnPropertySymbols(), Object.assign() )
√ Object Literal Computed Property
√ Object Literal Property Shorthands
√ Proxies
√ Reflect
Generator and Promise
√ Generator function
√ Promises
Class
√ Class
√ super allowed in object methods
√ class ABC extends Array { .. }
Module
× Module export command //不支援
× Module import command //不支援
全局安裝babel-cli,項目安裝babel-preset-es2015,建立.babelrc檔案
{
"presets": [
"es2015"
],
"plugins": []
}
//es6的方式引入
import { canadianToUs, UsToCanadian } from './a';
console.log(canadianToUs, UsToCanadian) //[Function] [Function]
然後再指令行執行babel-node就可以正常列印
nelsen-mac:nodeshizhan mac$ babel-node module/
[Function] [Function]
b子產品下面的index.js
class Currency {
constructor(dollar) {
this.dollar = dollar
}
roundToDecimals (amount) {
return Math.round(amount * 100) / 100
}
canadianToUs (canadian) {
return this.roundToDecimals(canadian * this.dollar)
}
UsToCanadian (us) {
return this.roundToDecimals(us * this.dollar)
}
}
//exports = Currency 是錯誤的暴露方式
module.exports = Currency; //這是正确的方式
module下面的index.js
const Currency = require('./b');
console.log(Currency) //[Function: Currency]
const curr = new Currency(.9);
console.log(curr.canadianToUs(50)) //45
es6的執行方法 babel-node module/
//es6的導入方法
import Currency from './b';
console.log(Currency) //[Function: Currency]
總結:
- nodejs不支援import from方法,需要配置.babelrc檔案,babel-node的方式執行
- module.exports用在暴露一個函數或變量的情況下,此時不能用exports代替,nodejs不允許對exports重新指派,exports隻是對module.exports的全局引用,重新指派exports會切斷兩者之間的引用,最初被定義為一個可以添加屬性的空對象。exports.done 隻是module.exports.done的簡寫。module.exports = exports = done
- 如果一個程式既有module.exports又有exports,最終暴露的是module.exports。