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How to modify files?


When we need to modify files is more complex than add or modify files. Because, when you modify a file you need to know where you want to add, modify, delete code. In this section, we will know to do it.

JSON files

To modify json files is not complex. Let me show you. For this example, we will update the package.json:

import { Rule, SchematicContext, Tree } from '@angular-devkit/schematics';
export function updatePackageJson(_options: any): Rule {
return (tree: Tree, _context: SchematicContext) => {
const filePath = '/package.json';
// Here we'll check if the file exists:
if (!tree.exists(filePath)) {
_context.logger.warn(`The file ${filePath} doesn't exist.`);
return tree;
// Then we'll read the file
const content =;
if (!content) {
_context.logger.warn(`Failed to read the file ${filePath}.`);
return tree;
// Parse the JSON content of package.json
const jsonStr = content.toString('utf-8');
const packageJson = JSON.parse(jsonStr);
// Modify the package.json object
// Example: Add a dependency
const dependencyName = '@example/dependency';
const dependencyVersion = '^1.0.0';
packageJson.dependencies = packageJson.dependencies || {};
packageJson.dependencies[dependencyName] = dependencyVersion;
// Serialize the modified object and write it back
tree.overwrite(filePath, JSON.stringify(packageJson, null, 2));

Typescript/Javascript files

Will modify this file:

export const tags = [
"Typescript", "Javascript", "CSS"
export function printTags() {
tags.forEach( t => console.log);

Problem to solve

We need to add a new tags to the code, for this reason we need to modify the line 2.

Code to solve the problem

import { Tree } from '@angular-devkit/schematics';
import {
} from 'typescript';
export function filesDemoFactory() {
return (tree: Tree) => {
const filePath = 'my-file.ts'; // Adjust path as necessary
if (!tree.exists(filePath)) {
throw new Error(`File ${filePath} does not exist.`);
const fileContent = tree.readText(filePath);
const sourceFile = createSourceFile(filePath, fileContent, ScriptTarget.Latest, true);
// Find the array node to modify
const listArrayNode = findArrayNodeByName(sourceFile, 'tags');
if (!listArrayNode) {
throw new Error(`Could not find the 'tags' array in ${filePath}`);
// Modify the array by adding a new element
const modifiedArrayText = addElementToArrayLiteral(listArrayNode, `"Scss"`, sourceFile);
const modifiedContent = replaceNodeTextInFileContent(fileContent, listArrayNode, modifiedArrayText);
// Update the file with the modified content
tree.overwrite(filePath, modifiedContent);
* Finds an array literal node by its variable name in a source file.
function findArrayNodeByName(sourceFile: SourceFile, arrayName: string): ArrayLiteralExpression | undefined {
let targetNode: ArrayLiteralExpression | undefined;
const visitNode = (node: Node) => {
if (isVariableDeclaration(node) && node.initializer && isArrayLiteralExpression(node.initializer)) {
const identifier =;
if (isIdentifier(identifier) && identifier.text === arrayName) {
targetNode = node.initializer;
forEachChild(node, visitNode);
return targetNode;
* Adds a new element to an array literal and returns the modified array as a string.
function addElementToArrayLiteral(node: ArrayLiteralExpression, newElement: string, sourceFile: SourceFile): string {
const elementsText = => element.getText(sourceFile));
elementsText.push(newElement); // Add new element
return `[${elementsText.join(', ')}]`;
* Replaces the text of a node in the file content with new text.
function replaceNodeTextInFileContent(fileContent: string, node: Node, newText: string): string {
return fileContent.substring(0, node.pos) + newText + fileContent.substring(node.end);


When you are modifying a file, you will use NodeJS to treat this file as a array of Node. This will help you to find the exact point where are the code to insert or modify.