initial version

Author: zardoy

.gitignore

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+node_modules
+build

package.json

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+{
+  "name": "modify-json-file",
+  "version": "0.0.1",
+  "main": "build/index.js",
+  "types": "build/index.d.ts",
+  "description": "Simple and modern way to modify JSON files",
+  "repository": "https://github.com/zardoy/modify-json-file",
+  "keywords": [
+    "json",
+    "files",
+    "modify",
+    "change",
+    "update",
+    "replace",
+    "fields",
+    "package.json",
+    "tsconfig.json"
+  ],
+  "author": "Vitaly <vital2580@icloud.com>",
+  "license": "MIT",
+  "scripts": {
+    "prepublishOnly": "yarn tsc",
+    "test": "echo they're definitely needed"
+  },
+  "devDependencies": {
+    "@types/node": "^15.12.2",
+    "@zardoy/tsconfig": "^1.0.2",
+    "ts-node": "^10.0.0",
+    "typescript": "^4.3.2"
+  },
+  "dependencies": {
+    "detect-indent": "^6.1.0",
+    "load-json-file": "^6.2.0",
+    "type-fest": "^1.2.0"
+  }
+}

readme.md

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+# Modify JSON
+
+Simplest way to modify JSON files
+
+[API](https://paka.dev/npm/@zardoy/modify-json)
+
+## Usage
+
+- Only async use
+
+Let's say we *package.json* file:
+```json
+// 📁package.json
+{
+    "name": "package",
+    "main": "index.js",
+    "author": "eldar",
+    "dependencies": {
+        "type-fest": "*",
+        "fdir": ">=2"
+    }
+}
+```
+
+And this code:
+
+```ts
+import { modifyJsonFile } from "@zardoy/modify-json";
+
+// of course, you should use path module here
+await modifyJsonFile("package.json", {
+    name: s => `super ${s}`,
+    main: "build/electron.js",
+    dependencies: {
+        "type-fest": "^1.0.0"
+    }
+})
+```
+After running this code, we'll get this package.json:
+
+```json
+{
+    "name": "super package",
+    "main": "build/electron.js",
+    "author": "eldar",
+    "dependencies": {
+        "type-fest": "^1.0.0"
+    }
+}
+```
+
+As you can see above, `modifyJsonFile` only merges fields on 1 level depth. Currently, we don't support merging in nested fields.
+
+We're using [detect-indent](https://www.npmjs.com/package/detect-indent) to preserve the tab size in `.json` files.
+
+
+## TODO
+
+- [ ] Fix. Currently, to preserve the tab size, json file readed twice
+- [ ] tests. should be easy but I'm extremely lazy
+- [ ] transformer for paths (most likely babel plugin): 
+
+```ts
+await fs.promises.readFile(./package.json, "utf8");
+```
+
+Into this:
+
+```ts
+await fs.promises.readFile(path.join(__dirname, "package.json"), "utf8");
+```
+
+- [ ] find a way to use FS in builder-way (like [fdir](https://www.npmjs.com/package/fdir) does) and deprecate this module
+
+## Related
+
+<!-- With *jsonfile*, you need to read / write objects. 1 function is simpler. That's super important for me, because I need to work with JSON files a lot. -->
+
+- [jsonfile](https://npmjs.com/jsonfile): simple reading / writing for json files
+- immer ?
+
+[Other cool modules for working with JSON](https://github.com/search?q=user%3Asindresorhus+json)

src/index.ts

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+import fs from "fs";
+import { PackageJson, TsConfigJson } from "type-fest";
+import detectIndent from "detect-indent"
+import loadJsonFile from "load-json-file"
+
+interface Options {
+    /** @default utf-8 */
+    encoding: BufferEncoding
+    /** 
+     * Will throw in case of FS error or invalid JSON
+     * @default true
+     *  */
+    throws: boolean
+    // ideally this lib should integrate with json validator
+    /** @default "throw" (even if throws is false) */
+    ifFieldIsMissing: "throw" | "skip" | "add"
+    /** 
+     * - throw - throws (even if throws param is false) 
+     *-  skip - won't call the function
+     * - pass - pass the `undefined` value
+     * @default "throw" 
+     * */
+    ifFieldIsMissingForSetter: "throw" | "skip" | "pass"
+    /** 
+     * - null - disable formatting
+     * - hard - one hard tab \t
+     * - number - number of spaces
+     * @default "preserve"
+     *  */
+    tabSize: null | number | "preserve" | "hard"
+}
+
+type GettersDeep<T extends object> = {
+    [K in keyof T]: (oldValue: T[K], json: T) => unknown
+    //T[K] extends object ? ((oldValue: T[K]) => unknown)/*  | GettersDeep<T[K]> */ : (oldValue: T[K]) => unknown
+}
+
+export type ModifyJsonFileFunction<T extends object> = (
+    path: string,
+    fields: Partial<T | GettersDeep<T>>, 
+    options?: Options
+) => Promise<void>;
+
+type ModifyJsonFileGenericFunction = <T extends object>(
+    path: string,
+    fields: Partial<T | GettersDeep<T>>, 
+    options?: Partial<Options>
+) => Promise<void>;
+
+/** It's just Error */
+class InnerError extends Error {
+    innerError = true;
+}
+
+/**
+ * modifies **original** JSON file
+ * You can pass generic, that reflects the structure of original JSON file
+ * 
+ * Fields, that are functions will be skipped if they're not preset in original JSON file
+ */
+export const modifyJsonFile: ModifyJsonFileGenericFunction = async (
+    path,
+    fields, 
+    options
+) => {
+    const { 
+        encoding = "utf-8",
+        throws = true ,
+        ifFieldIsMissing = "throw",
+        ifFieldIsMissingForSetter = "throw",
+        tabSize: tabSizeOption = "preserve"
+    } = options || {};
+    try {
+        const json = await loadJsonFile(path);
+        // todo remove restriction or not?
+        if (!json || typeof json !== "object" || Array.isArray(json)) throw new TypeError(`${path}: JSON root type must be object`);
+        // todo we don't need to read the file twice
+        const rawText = await fs.promises.readFile(path, encoding);
+        const indent = tabSizeOption === "preserve" ? detectIndent(rawText).indent : tabSizeOption === "hard" ? "\t" : tabSizeOption === null ? undefined : " ".repeat(tabSizeOption);
+        
+        for (const [name, value] of Object.entries(fields)) {
+            if (!(name in json)) {
+                const isSetter = typeof value === "function";
+                const generalAction = isSetter ? ifFieldIsMissingForSetter : ifFieldIsMissing;
+                if (generalAction === "throw") throw new InnerError(`Property to modify "${name}" is missing in ${path}`);
+                if (generalAction === "skip") continue;
+            }
+            json[name as string] = typeof value === "function" ? value(json[name as string], json) : value;
+        }
+
+        await fs.promises.writeFile(
+            path, 
+            JSON.stringify(json, undefined, indent)
+        );
+    } catch(err) {
+        if (err.innerError) throw new Error(err.message);
+        if (throws) throw err;
+    }
+}
+
+// todo: use read-pkg / write-pkg for normalization
+
+/**
+ * Almost the same is sindresorhus/write-pkg, but with proper typing support and setters for fields
+ */
+export const modifyPackageJson: ModifyJsonFileFunction<PackageJson> = modifyJsonFile;
+
+export const modifyTsConfigJson: ModifyJsonFileFunction<TsConfigJson> = modifyJsonFile;
+

tsconfig.json

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+{
+    "extends": "@zardoy/tsconfig/node",
+    "compilerOptions": {
+        "module": "CommonJS",
+        "esModuleInterop": true,
+        "declaration": true
+    }
+}