for-expressions.js

// When a for-$X construct is created, two new constructs are allowed within
// the loop body: `continue with` and `break with`. `continue with` is the equivalent
// of `return` in `fn` in something like Array.prototype.map(fn). `break with` is
// similar, but it stop further iterations of the loop.

// When a for-$X construct is given a single expression instead of a block
// as the loop body, the value of the expression is wrapped with an implicit
// `continue with`

const cases = {};

cases["for-map-of/block/continue-with-"] = () => {
  const value = for map (const item of [1, 2, 3]) {
    if (item % 2 === 0) {
      continue with item * 2;
    }
    continue with `${item * item}`;
  };
  
  assert.deepEqual(value, [`1`, 4, `9`];
};

cases["for-map/block/break-and-lack-continue"] = () => {
  const value = for map (let i = 0; i < 5; i++) {
    if (i === 3) {
      break with 3;
    }
    if (i % 2 === 0) {
      continue with `${i} is even`;
    }
  };
  
  assert.deepEqual(value, ["0 is even", undefined, "2 is even", 3]);
};

cases["for-reduce-of/block"] = () => {
  const value = for reduce (const item of [1,2,3,4]; let acc = 0) {
    continue with acc + item;
  };
  
  assert.equal(value, 10);
};

cases["for-reduce-of/expression"] = () => {
  const value = for reduce (const item of [1,2,3,4]; let acc = 0) acc + item;
  assert.equal(value, 10);
};

cases["for-reduce/expression/range"] = () => {
  const value = for reduce (let i = 1; i <= 4; i++; let acc = 0) acc + i;
  assert.equal(value, 10);
};


// Note that this still allows for the rarely-used feature of `continue :label` and
// `break :label`
cases["for-map/block/break-label"] = () => {
  let value = null;
  outerLoop: for (const item of [1,2,3]) {
    innerLoop: const loopVal = for map (let i = 0; i < item; i++) {
      value = i;
      break outerLoop;
    };
  }
  
  assert.equal(value, 0);
};