How to add/set environment Angular 6 angular.json file

Open angular.json file. we can see the configurations by default it will be shown for production add code snippet for your respective environments. add environment.dev.ts file in environment for dev, add environment.qa.ts for qa. Name as you prefered.
use

 ng serve --configuration=environment_name

environment_name – (dev,qa,prod) same process can be followed for ng build

"configurations": {
        "production": {
          "fileReplacements": [
            {
              "replace": "src/environments/environment.ts",
              "with": "src/environments/environment.prod.ts"
            }
          ],
          "optimization": true,
          "outputHashing": "all",
          "sourceMap": false,
          "extractCss": true,
          "namedChunks": false,
          "aot": true,
          "extractLicenses": true,
          "vendorChunk": false,
          "buildOptimizer": true
        },
        "dev": {
          "fileReplacements": [
            {
              "replace": "src/environments/environment.ts",
              "with": "src/environments/environment.dev.ts"
            }
          ],
          "optimization": true,
          "outputHashing": "all",
          "sourceMap": true,
          "extractCss": true,
          "namedChunks": false,
          "aot": true,
          "extractLicenses": true,
          "vendorChunk": false,
          "buildOptimizer": true
        },
        "qa": {
          "fileReplacements": [
            {
              "replace": "src/environments/environment.ts",
              "with": "src/environments/environment.qa.ts"
            }
          ],
          "optimization": true,
          "outputHashing": "all",
          "sourceMap": false,
          "extractCss": true,
          "namedChunks": false,
          "aot": true,
          "extractLicenses": true,
          "vendorChunk": false,
          "buildOptimizer": true
        }
      }