Map sequence in Rx.JS

Question

I would like to map correct sequence using Rx.js. A simplest example will be using keyboard. I attach Rx.js to keyboard events and waiting for three A no matter what will be between them, except Q. When sequence is matched I would like to print OK on console. For example:

• AAA -> should print OK
• AsssAqweA -> should print OK
• AAQAAQ -> shouldn't print anything
• AAQAAA -> should print OK
• AAAsssAAAsssAAA -> should print three times OK

Should I use filter function and keep state inside it? Or there is an esier way to do that.

Answer 1

You should be able to use the filter and scan operators to do what you want:

Rx.Observable.from("AsssAqweAbAAQAAA")

  // Only A and Q characters are important:

  .filter(char => (char === "A") || (char === "Q"))

  // Accumulate the A characters until there are three
  // or until a Q is received:

  .scan((acc, char) => {
    if ((char === "Q") || (acc.length === 3)) {
      acc = [];
    }
    if (char === "A") {
      acc.push(char);
    }    
    return acc;
  }, [])

  // Filter the accumulated characters until there
  // are three:

  .filter(acc => acc.length === 3)
  .mapTo("OK")
  .subscribe(value => console.log(value));

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