Today I gonna describe conditional module loading feature that SystemJS added couple weeks ago. This feature helps you to load your ES2015 (ES6) modules depend on the conditions that you specify.

Keep in mind this feature is still not stable but I am using it in my Employee Scheduling application already without any problem.

Use Case

In my application I am using ES6 modules and creating production bundle with SystemJS Builder. I have a case where I want to load different configuration files depend on environment and I also want to include mock data only for test environment and this is all possible now with new feature introduced in SystemJS.

Conditional Syntax

SystemJS supports two conditional syntaxes Extension Conditions and Boolean Conditions at the time of writing this blog. Before I start describing these syntaxes let's assume I create global env variables that look like this:

env.conditions.js

export var mock = true ; export var environment = 'prod' ;

and then I add this file to jspm.config.js like this:

System . config ({ map : { "ENV" : "env.conditions.js" } });

Extension Conditions

The extension conditions syntax allows a condition module to alter the resolution of an import so I can use variables from previous section in my condition extension import:

import './config.#{ENV|environment}.js' ;

and SystemJS will replace this variable with string value:

import './config.prod.js' ;

Boolean Conditions

The boolean conditions syntax allows a module not to be loaded if it's not needed:

import './employee.mock.js#?ENV|mock' ;

In above example SystemJS will not load this module as ENV.mock is set to false . There is also support for negated conditions via ~ symbol:

import './employee.mock.js#?~ENV|mock' ;

In this case module is loaded because value ENV.mock is negated to true .

There are not so many condition options that you can use but even with these few options I find SystemJS conditional import feature really powerful in my projects.

Real Use Case

I have described conditional syntax but let's have look on some real example and how to get these conditions running with SystemJS and SystemJS Builder in real project.

In my projects I like to use npm scripts and hide all the logic there so when I want to run application I use follow commands:

npm start -- --env = DEV npm start -- --env = TEST npm start -- --env = PROD

The command npm start start the application and env arguments are passed through CLI into gulp or other build tool of your choice. These env values are important because we need to get them to SystemJS so here is my development flow.

1.

I create gulp task that inject arguments passing through cli into file that we gonna create in second step. The reason why I inject these values instead manually add them is because these values are changing depends on arguments passing from cli .

gulp . task ( 'config' , () => { return gulp . src ( path . app . config . conditions ) . pipe ( inject ( gulp . src ( '.' ), { starttag : '/* inject:env */' , endtag : '/* endinject */' , transform : () => ` export var mock = $ { ENV . toLowerCase () === 'test' }; \ nexport var environment = '${env.toLowerCase()}' ; ` })) . pipe ( gulp . dest ( path . root )); });

NOTE: It's up to you what approach you decide to use to dynamically inject env variables to env.conditions.js file. In my projects I use gulp-inject.

2.

I create env.conditions.js file where I inject all conditions via gulp task that we created in step 1.

'use strict' ; /* inject:env */ export var mock = false ; export var environment = 'prod' ; /* endinject */

I also commit this file to git and then stop tracking changes in this file because you can chance environments quit often during development.

# ask git to stop tracking any changes to env.conditions.js git update-index --assume-unchanged env.conditions.js # if you want to start tracking changes again to env.conditions.js git update-index --no-assume-unchanged env.conditions.js

3.

I add env.conditions.js file to jspm.config.js and register ENV module with SystemJS.

System . config ({ map : { "ENV" : "src/app/core/config/env.conditions.js" } });

4.

Now you can start using these conditions in the project e.g:

config.js

'use strict' ; import './config.#{ENV|environment}.js' ; import { ACCESS_LEVELS } from '../constants/constants.js' ; import { Config , Run , Inject } from '../../ng-decorators' ;

employee.js

'use strict' ; import './employee.mock.js#?ENV|mock' ; import AbstractResource from '../abstract-resource' ; import { Service , Inject } from '../../../ng-decorators' ;

All previous steps describe development workflow when you are developing your application but at the end of day you want to create production code which mean create a bundle with SystemJS Builder. SFX bundles do support conditional builds but they work little a bit differently than conditions in development workflow. There is open ticket #311 to support multiple conditional variations. So for now you need to add conditions like is show in below gulp bundle task:

gulp . task ( 'bundle' , [ 'jshint' ], ( cb ) => { const Builder = require ( 'systemjs-builder' ); const builder = new Builder (); const inputPath = 'src/app/app' ; const outputFile = ` $ { path . tmp . scripts } build . js ` ; const outputOptions = { conditions : { 'src/app/core/config/env.conditions.js|mock.js' : ENV . toLowerCase () === 'test' , 'src/app/core/config/env.conditions.js|environment' : ENV . toLowerCase () }, sourceMaps : true , config : { sourceRoot : path . tmp . scripts } }; builder . loadConfig ( ` $ { path . root } /jspm.conf.js` ) . then (() => { builder . buildStatic ( inputPath , outputFile , outputOptions ) . then (() => cb ()) . catch (( ex ) => cb ( new Error ( ex ))); }); });

Conclusion

The real use case of SystemJS conditional import can be found in my Employee Scheduling project. Keep in mind this feature is still in development and it might change in the future. Currently you probably won't find any documentation for conditional import so I would suggest to look at SystemJS source code or SystemJS test cases for more examples.