[Solved] Detect alternating key events

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You only need one event listener, and you need to define the a key press state variable (x) outside of the listener function so that it can be referenced by subsequent executions of the listener function.

You also need to make sure that you reset the a key press variable after the b key press.

it is also generally a good idea to cache your references to elements, rather than selecting the element from the DOM each time your listener function runs, and using textContent instead of innerHTML bypasses the HTML parser.

const target = document.getElementById('points');
var points = 0, x;

document.addEventListener('keydown', function(event) {
    if(event.key === 'a') x = true; // If this is an `a` key event, set x to true
    if(event.key === 'b') {
        // if this is a `b` key event and a was not pressed, return early
        if(!x) return; 
        // otherwise increment the points variable and assign the result to 
        // the textContent property of the target element
        target.textContent = ++points; 
        // remember to set x to false again
        x = false;
    }
});
<p>Points: <span id="points">0</span></p>

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solved Detect alternating key events