152 lines
4 KiB
Markdown
152 lines
4 KiB
Markdown
---
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categories:
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- Programming Languages
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tags:
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- backend
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- node-js
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---
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# Modules
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> Modules are partitioned files where we define our variables and functions. Values defined in modules are scoped to that specific module, constituting a unique name space. This avoids name clashes in large programs.
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- Every file in a Node application is considered a module.
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- The variables and methods in a module are equivalent to `private` properties and methods in object-oriented programming.
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- If you wish to use a function or variable defined in a module outside of its modular container you need to explicitly export it and make it public.
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## Structure of a module
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Node keeps an internal record of the properties of a module. To see this we can log the property `module` to the console.
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```js
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// index.js
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console.log(module);
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```
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This gives us:
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```plaintext
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Module {
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id: '.',
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path: '/home/thomas/repos/node-learning',
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exports: {},
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filename: '/home/thomas/repos/node-learning/index.js',
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loaded: false,
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children: [],
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paths: [
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'/home/thomas/repos/node-learning/node_modules',
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'/home/thomas/repos/node_modules',
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'/home/thomas/node_modules',
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'/home/node_modules',
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'/node_modules'
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]
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}
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```
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## Exports
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- Whenever we export a property or method from a module we are directly targeting the `exports` property of the module object.
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- Once we add exports to a file they will be displayed under that property of the module object.
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- We can export the entire module itself as the export (typically used when the module is a single function or class) or individual properties.
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### Exporting a whole module
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_The example below is a module file that consists in a single function_
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```js
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module.exports = function (...params) {
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// function body
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};
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```
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Note the module is unnamed. We would name it when we import:
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```js
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const myFunction = require("./filenme");
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```
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### Exporting sub-components from a module
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In the example below we export a variable and function from the same module. Note only those values prefixed with `exports` are exported.
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```js
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exports.myFunc = (...params) => {
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// function bod[]()y
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};
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exports.aVar = 321.3;
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var nonExportedVar = true;
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```
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This time the exports are already name so we would import with the following:
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```js
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const { myFunc, aVar } = require("./filename");
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```
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We can also do the exporting at the bottom when the individual components are named:
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```js
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const myNamedFunc = (val) => {
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return val + 1;
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};
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function anotherNamedFunc(val) {
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return val * 2;
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}
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// This time we export at the bottom
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exports.myNamedFunc = myNamedFunc;
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exports.differentName = anotherNamedFunc; // We can use different names
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// Or we could export them together
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module.exports = { myNamedFunc, anotherNamedFunc };
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```
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The import is the same:
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```js
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const { myNamedFunc, anotherNamedFunc } = require("./modules/multiExports");
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```
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## Structuring modules
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The techniques above are useful to know but generally you would want to enforce a stricter structure than a mix of exported and private values in the one file. The best way to do this is with a single default export.
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Here the thing exported could be a composite function or an object that basically acts like a class with methods and properties.
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_Export a composite single function_
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```js
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module.exports = () => {
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foo() {...}
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bar() {...}
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}
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```
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_Export an object_
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```js
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module.exports = {
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foo : () => {...},
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bar: () => {...}
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}
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```
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**Both of these structures would be referred to in the same way when importing and using them.**
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Or you could export an actual class as the default. This is practically the same as the two above other than that you would have to use `new` to initiate an instance of the class.
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```js
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export default class {
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foo() {}
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bar() {}
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}
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```
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Every method and property within the export will be public by default, whether it is an object, class or function. If you wanted to keep certain methods/properties private, the best approach is to define them as variables and functions within the module file but outside of the `export` block.
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