ES6 katas for Block scope - http://es6katas.org/

es6-block-scope.js

// 7: block scope - let

describe('`let` restricts the scope of the variable to the current block', () => {

  describe('`let` vs. `var`', () => {

    it('`var` works as usual', () => {
      if (true) {
        var varX = true;
      }
      assert.equal(varX, true);
    });
    
    it('`let` restricts scope to inside the block', () => {
      if (true) {
        let letX = true;
      }
      assert.throws(() => console.log(letX));
    });
    
  });

  describe('`let` usage', () => {
    
    it('`let` use in `for` loops', () => {
      let obj = {x: 1};
      for (let key in obj) {}
      assert.throws(() => console.log(key));
    });
    
    it('create artifical scope, using curly braces', () => {
      {
        let letX = true;
      }
      assert.throws(() => console.log(letX));
    });
    
  });
  
});

// 8: block scope - const

describe('`const` is like `let` plus read-only', () => {

  describe('scalar values are read-only', () => {

    it('number', () => {
      const constNum = 0;
      // constNum = 1;
      assert.equal(constNum, 0);
    });

    it('string', () => {
      const constString = 'I am a const';
      // constString = 'Cant change you?';
      assert.equal(constString, 'I am a const');
    });

  });
  
  const notChangeable = 23;

  it('const scope leaks too', () => {
    assert.equal(notChangeable, 23);
  });
  
  describe('complex types are NOT fully read-only', () => {

    it('array', () => {
      const arr = [42, 23];
      arr[0] = 42;
      assert.equal(arr[0], 42);
    });
    
    it('object', () => {
      const obj = {x: 1};
      obj.x = 3;
      assert.equal(obj.x, 3);
    });
    
  });

});