css_keyframes_values_at – Get property values from keyframes at specific time https://codescreens.com

css_keyframes_values_at.html

<div id="app">
    Get property values from keyframes at specific time.
    <br>
    <textarea v-model="keyframes" cols="50" rows="5"></textarea>
    <br>
    <input v-model="time" type="number" min="0" max="1" step="0.01">
    <pre>{{ values }}</pre>
</div>

<script src="https://unpkg.com/vue@2.6.10/dist/vue.js"></script>
<script>
new Vue({
    el: '#app',
    data: {
        time: 0.25,
        keyframes: `0% { opacity: 0; animation-timing-function: ease-in-out; }
100% { opacity: 1; }`,
    },
    computed: {
        values: function () {
            try {
            return css_keyframe_values_at(this.keyframes, this.time);
            }
            catch (error) {
                return `Error: ${error}`;
            }
        }
    }
});
function css_keyframe_values_at(keyframes, time)
{
    // 1. Parse **keyframes** using STYLE element
    // 2. Get a list of CSSKeyframeRule from STYLE element

    const div = document.createElement('DIV');
    const style = document.createElement('STYLE');
    try {
        style.innerHTML = `@keyframes __KEYFRAMES_VALUES_AT__ { ${keyframes} }`;
        document.body.append(div);
        document.body.append(style);

        // 3. Create an array of keyframes for Element.animate
        const rows = [];
        const rules = style.sheet.rules[0].cssRules;
        const props = [];
        for (let i = 0, end = rules.length; i < end; ++i) {
            const row = {};
            row.offset = rules[i].keyText.replace('%', '')/100;
            for (let j = 0, jj = rules[i].style.length; j < jj; ++j) {
                const prop = rules[i].style[j];
                if (prop == 'animation-timing-function') {
                    row.easing = rules[i].style[prop];
                }
                else {
                    row[prop] = rules[i].style[prop];
                    if (!props.includes(prop)) {
                        props.push(prop);
                    }
                }
            }
            rows.push(row);
        }

        // 4. Set animation to **time**
        // https://developer.mozilla.org/en-US/docs/Web/API/Web_Animations_API/Keyframe_Formats
        const anim = div.animate(rows, {duration: 1});
        anim.pause();
        anim.currentTime = time;

        // 5. Extract values
        const styles = getComputedStyle(div);
        const ret = {};
        for (let i = 0, end = props.length; i < end; ++i) {
            ret[props[i]] = styles[props[i]];
        }
        return ret;
    }
    finally {
        div.remove();
        style.remove();
    }
}
</script>