Meet sequence expression

January, 2019

You have no idea how fun is to transpiler JavaScript. I’m digging into that last few weeks and there is a step where I have to transform an AST into valid code. There is one tree node which I really like - sequence expression.

Photo by Pixabay

You are actually using sequence expression more often then you think. Here is an example:

console.log((4, 8)); // 8

The result of this line is 8. We define different expressions which are evaluated from left to right. The result of the process is the result of the last one. In the example above that’s just the number 8.

The sequence expression could be helpful in lots of cases and it is useful when the expressions are related to each other. Let’s check the following snippet:

var from = 0;
var to = 10;
var skipIf = function (value) {
  return i === value ? (i = to, true) : false;
}
for(var i = from; skipIf(3), i < to; i++) {
  console.log(i);
}

The output is:

0
1
2

There are two sequence expressions. The first one is in the condition bit of the for loop. The second one is part of the return statement in skipIf function. The idea of the code is to break the loop after the third iteration /For sure there are better ways to break a loop, but this example helps me illustrate the idea/

In skipIf(3), i < to we first run the function and then evaluate the condition. Both, the helper and the condition, are related to each other because the first one modifies a variable used in the second one. skipIf has two actions to perform if i matches the desire value. It should return true and it should stop the loop. (i = to, true) is doing exactly what we want. It changes i and returns the result of the second expression which is just true.

I started using sequence expression where I have to return a boolean based on some actions. For example:

function validate(item) {
  if (42 === 42) {
    item.status = 'valid';
  }
  return item;
}
var item = { status: null };
var isReady = (item = validate(item), item.status === 'valid');

if (isReady) { // true
  // do something ...
}

There is another type of expression which is widely used and it is close to sequence expression - logical expression. It also evaluates its parts from left to right but there are different rules applied.

var a = 'a';
var b = 'b';
(a && b); // returns b
(a || b); // returns a

var a = 0;
var b = 'b';
(a && b); // returns 0
(a || b); // returns b

When we use && the engine starts evaluating the expressions till it finds one that has falsy result. With || the engine continues till it finds truthy expression.

var a = 'a';
var b = 'b';
var c = 0;

(a && c && b); // returns 0
(c || a || b); // returns a

“Talk is cheap. Show me the code.”

Full screen

More by
the same passionate developer