modify package json via schematics

index.ts

import { Rule, SchematicContext, Tree, chain } from '@angular-devkit/schematics';
import {
  addPackageJsonDependency,
  NodeDependency,
  NodeDependencyType,
} from '@schematics/angular/utility/dependencies';
import { NodePackageInstallTask } from '@angular-devkit/schematics/tasks';
import { dependencies } from '../dependencies';

function addDependencies(): Rule {
  return (host: Tree, _context: SchematicContext) => {
    for (let pkg in dependencies) {
      const nodeDependency: NodeDependency = _nodeDependencyFactory(pkg, dependencies[pkg]);

      addPackageJsonDependency(host, nodeDependency);
    }

    _context.addTask(new NodePackageInstallTask());
  };
}

// You don't have to export the function as default. You can also have more than one rule factory
// per file.
export function mySchematic(_options: any): Rule {
  return (tree: Tree, _context: SchematicContext) => {
    return chain([
      addDependencies(),
    ])(tree, _context);
  };
}

function _nodeDependencyFactory(packageName: string, version: string): NodeDependency {
  return {
    type: NodeDependencyType.Default,
    name: packageName,
    version: version,
    overwrite: true
  };
}