在Angular/TypeScript项目中，可以使用ESLint规则来确保代码中的函数不会产生副作用，即只返回新数据而不修改现有数据。这样可以提高代码的可测试性和可维护性。

具体实现的做法是在ESLint配置文件中添加如下规则：

{
  "rules": {
    "@typescript-eslint/explicit-module-boundary-types": "off",
    "@typescript-eslint/no-explicit-any": "off",
    "@typescript-eslint/ban-types": [
      "error",
      {
        "types": {
          "{}": false
        }
      }
    ],
    "import/no-extraneous-dependencies": [
      "error",
      {
        "devDependencies": ["**/*.test.ts", "**/*.spec.ts"]
      }
    ],
    "complexity": ["warn", 7],
    "max-lines-per-function": ["warn", 25],
    "max-nested-callbacks": ["warn", 3],
    "max-params": ["warn", 4],
    "max-depth": ["warn", 3],
    "max-statements": ["warn", 15],
    "max-lines": ["warn", 120],
    "no-console": ["error"],
    "no-param-reassign": [
      "error",
      { "props": true, "ignorePropertyModificationsFor": ["memo"] }
    ],
    "no-plusplus": ["error", { "allowForLoopAfterthoughts": true }],
    "no-underscore-dangle": [
      "error",
      {
        "allow": [
          "__",
          "_id"
        ]
      }
    ],
    "id-length": [
      "error",
      {
        "min": 2,
        "max": 40,
        "properties": "never",
        "exceptions": ["i", "j", "x", "y", "z"]
      }
    ],
    "prefer-const": ["warn"],
    "camelcase": [
      "error",
      {
        "properties": "always",
        "ignoreDestructuring": false,
        "ignoreImports": false,
        "allow": []
      }
    ],
    "valid-jsdoc