Example of wrong solution:

export function stringToNumber(str: string): number {
  return  str as any as number

The error exist in TypeScript test.

assert.equal use non-strict equality (==) and assert.equal(3, '3') does not throw error.

assert.strictEqual use strict equality (===) and assert.strictEqual(3, '3') throw error.

Current test:

import { stringToNumber } from "./solution";
import { assert } from "chai";

describe("stringToNumber", function() {
  it("should work for the examples", function() {
    assert.equal(stringToNumber("1234"),1234);
    assert.equal(stringToNumber("605"), 605);
    assert.equal(stringToNumber("1405"),1405);
    assert.equal(stringToNumber("-7"),  -7);
  });
  it("should work for random strings", function() {
    var i, t;
    for(i = 0; i < 100; ++i){
      t = Math.round(Math.random() * 1e6 - 5e5);
      assert.equal(stringToNumber(t.toString(10)), t);
    }
  });
});

My porposal:

import { stringToNumber } from "./solution";
import { assert } from "chai";

describe("stringToNumber", function() {
  it("should work for the examples", function() {
    assert.strictEqual(stringToNumber("1234"),1234);
    assert.strictEqual(stringToNumber("605"), 605);
    assert.strictEqual(stringToNumber("1405"),1405);
    assert.strictEqual(stringToNumber("-7"),  -7);
  });
  it("should work for random strings", function() {
    var i, t;
    for(i = 0; i < 100; ++i){
      t = Math.round(Math.random() * 1e6 - 5e5);
      assert.strictEqual(stringToNumber(t.toString(10)), t);
    }
  });
});

Doc assert.equal:
https://www.chaijs.com/api/assert/#method_equal

Doc assert.strictEqual
https://www.chaijs.com/api/assert/#method_strictequal

In TypeScript tests use assert.strictEqual insted assert.equal