const source = { my: { nested: [1, 2, 3] } }
digx(source, "my.nested[1]") //=> 2

mkdir digx
cd digx
npm init --yes

git init
echo "node_modules" >> .gitignore
echo "dist" >> .gitignore
git add .
git commit -m "initial"

npm i -D typescript

{
  "files": ["src/index.ts"],
  "compilerOptions": {
    "target": "es2015",
    "module": "es2015",
    "declaration": true,
    "outDir": "./dist",
    "noEmit": false,
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictBindCallApply": true,
    "strictPropertyInitialization": true,
    "noImplicitThis": true,
    "alwaysStrict": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitOverride": true,
    "noPropertyAccessFromIndexSignature": true,
    "esModuleInterop": true,
    "forceConsistentCasingInFileNames": true,
    "skipLibCheck": true
  }
}

"scripts": {
  "build": "tsc"
}

npm i -D jest @types/jest ts-jest

./node_modules/.bin/jest --init

{
  // ...
  preset: "ts-jest",
  // ...
}

import dg from "./index";
test("works with a shallow object", () => {
  expect(dg({ param: 1 }, "param")).toBe(1);
});
test("works with a shallow array", () => {
  expect(dg([1, 2, 3], "[2]")).toBe(3);
});
test("works with a shallow array when shouldThrow is true", () => {
  expect(dg([1, 2, 3], "[2]", true)).toBe(3);
});
test("works with a nested object", () => {
  const source = { param: [{}, { test: "A" }] };
  expect(dg(source, "param[1].test")).toBe("A");
});
test("returns undefined when source is null", () => {
  expect(dg(null, "param[1].test")).toBeUndefined();
});
test("returns undefined when path is wrong", () => {
  expect(dg({ param: [] }, "param[1].test")).toBeUndefined();
});
test("throws an exception when path is wrong and shouldThrow is true", () => {
  expect(() => dg({ param: [] }, "param[1].test", true)).toThrow();
});
test("works tranparently with Sets and Maps", () => {
  const source = new Map([
    ["param", new Set()],
    ["innerSet", new Set([new Map(), new Map([["innerKey", "value"]])])],
  ]);
  expect(dg(source, "innerSet[1].innerKey")).toBe("value");
});

export default dig;
/**
 * A dig function that takes any object with a nested structure and a path,
 * and returns the value under that path or undefined when no value is found.
 *
 * @param {any}     source - A nested objects.
 * @param {string}  path - A path string, for example `my[1].test.field`
 * @param {boolean} [shouldThrow=false] - Optionally throw an exception when nothing found
 *
 */
function dig(source: any, path: string, shouldThrow: boolean = false) {
  if (source === null || source === undefined) {
    return undefined;
  }
  // split path: "param[3].test" => ["param", 3, "test"]
  const parts = splitPath(path);
  return parts.reduce((acc, el) => {
    if (acc === undefined) {
      if (shouldThrow) {
        throw new Error(`Could not dig the value using path: ${path}`);
      } else {
        return undefined;
      }
    }
    if (isNum(el)) {
      // array getter [3]
      const arrIndex = parseInt(el);
      if (acc instanceof Set) {
        return Array.from(acc)[arrIndex];
      } else {
        return acc[arrIndex];
      }
    } else {
      // object getter
      if (acc instanceof Map) {
        return acc.get(el);
      } else {
        return acc[el];
      }
    }
  }, source);
}
const ALL_DIGITS_REGEX = /^\d+$/;
function isNum(str: string) {
  return str.match(ALL_DIGITS_REGEX);
}
const PATH_SPLIT_REGEX = /\.|\]|\[/;
function splitPath(str: string) {
  return (
    str
      .split(PATH_SPLIT_REGEX)
      // remove empty strings
      .filter((x) => !!x)
  );
}

{
  // ...
  "files": ["dist", "LICENSE", "README.md", "package.json"],
  // ...
}

npm publish --dry-run

npm publish

npm install --save digx

import dg from "digx"
console.log(dg({ test: [1, 2, 3] }, "test[0]"))