したいこと

Flexで並べた要素の間に境界を設置して、そこをドラッグすることでそれぞれの幅を変えられるようにする。
例えるならVSCodeの分割ウィンドウみたいな感じ。

コード

HTML

<div id="wrapper">
  <div id="one"></div>
  <div id="two"></div>
</div>

CSS

# wrapper {
  display: flex;
  height: 200px;
  width: 300px;
}
# one, #two {
  height: 100%;
}
```

### JavaScript
````javascript
window.onload = () => {
    let parent = document.getElementById("wrapper");
    colResizeable(parent, 300);
};

let colResizeable = (element, options) => {
    let {totalWidth, borderWidth, borderColor, postProcess} = {
        totalWidth: element.clientWidth,
        borderWidth: 10,
        borderColor: "#ddd",
        postProcess: () => {},
        ...options
    }

    let w1;
    let p1;

    // setup
    let children = Array.prototype.filter.call((element.childNodes), (e) => e.nodeName != "#text");
    let child1 = children[0];
    let child2 = children[1];
    children.forEach((e) => { e.style.width = (totalWidth - borderWidth) / 2 + "px" });

    // insert border between children
    let border = document.createElement("div");
    let borderStyle = {
        width: borderWidth + "px",
        backgroundColor: borderColor,
        cursor: "col-resize"
    }
    Object.keys(borderStyle).forEach((key) => { border.style[key] = borderStyle[key] });
    element.insertBefore(border, child1.nextSibling);

    let resizeChildren = (event) => {
        let w2 = Math.min(Math.max(0, w1 + event.pageX - p1), totalWidth - borderWidth);
        child1.style.width = w2 + "px";
        child2.style.width = totalWidth - w2 - borderWidth + "px";
    };

    border.addEventListener("mousedown", (event) => {
        w1 = Number(/.*?(?=px)/.exec(child1.style.width));
        p1 = event.pageX;

        element.addEventListener("mousemove", resizeChildren);
    });
    window.addEventListener("mouseup", () => {
        element.removeEventListener("mousemove", resizeChildren)
        postProcess;
    });
}

ポイント

Array.prototype.filter.call((element.childNodes), (e) => e.nodeName != "#text")で空のテキスト要素を弾ける
ドラッグはmousedownしたときのカーソルの座標とmousemoveのときの座標の相対値を取る
面積の小さい要素を動かすときはwindow.addEventListener("mouseup", func)がおすすめ