27 Maret 2021
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Selection and Range

In this chapter we’ll cover selection in the document, as well as selection in form fields, such as <input>.

JavaScript can access an existing selection, select/deselect DOM nodes as a whole or partially, remove the selected content from the document, wrap it into a tag, and so on.

You can find some recipes for common tasks at the end of the chapter, in “Summary” section. Maybe that covers your current needs, but you’ll get much more if you read the whole text.

The underlying Range and Selection objects are easy to grasp, and then you’ll need no recipes to make them do what you want.

Range

The basic concept of selection is Range, that is essentially a pair of “boundary points”: range start and range end.

A Range object is created without parameters:

let range = new Range();

Then we can set the selection boundaries using range.setStart(node, offset) and range.setEnd(node, offset).

As you might guess, further we’ll use the Range objects for selection, but first let’s create few such objects.

Selecting the text partially

The interesting thing is that the first argument node in both methods can be either a text node or an element node, and the meaning of the second argument depends on that.

If node is a text node, then offset must be the position in its text.

For example, given the element <p>Hello</p>, we can create the range containing the letters “ll” as follows:

<p id="p">Hello</p>
<script>
  let range = new Range();
  range.setStart(p.firstChild, 2);
  range.setEnd(p.firstChild, 4);

  // toString of a range returns its content as text
  console.log(range); // ll
</script>

Here we take the first child of <p> (that’s the text node) and specify the text positions inside it:

Selecting element nodes

Alternatively, if node is an element node, then offset must be the child number.

That’s handy for making ranges that contain nodes as a whole, not stop somewhere inside their text.

For example, we have a more complex document fragment:

<p id="p">Example: <i>italic</i> and <b>bold</b></p>

Here’s its DOM structure with both element and text nodes:

Let’s make a range for "Example: <i>italic</i>".

As we can see, this phrase consists of exactly two children of <p>, with indexes 0 and 1:

  • The starting point has <p> as the parent node, and 0 as the offset.

    So we can set it as range.setStart(p, 0).

  • The ending point also has <p> as the parent node, but 2 as the offset (it specifies the range up to, but not including offset).

    So we can set it as range.setEnd(p, 2).

Here’s the demo. If you run it, you can see that the text gets selected:

<p id="p">Example: <i>italic</i> and <b>bold</b></p>

<script>
  let range = new Range();

  range.setStart(p, 0);
  range.setEnd(p, 2);

  // toString of a range returns its content as text, without tags
  console.log(range); // Example: italic

  // apply this range for document selection (explained later below)
  document.getSelection().addRange(range);
</script>

Here’s a more flexible test stand where you can set range start/end numbers and explore other variants:

<p id="p">Example: <i>italic</i> and <b>bold</b></p>

From <input id="start" type="number" value=1> – To <input id="end" type="number" value=4>
<button id="button">Click to select</button>
<script>
  button.onclick = () => {
    let range = new Range();

    range.setStart(p, start.value);
    range.setEnd(p, end.value);

    // apply the selection, explained later below
    document.getSelection().removeAllRanges();
    document.getSelection().addRange(range);
  };
</script>

E.g. selecting in the same <p> from offset 1 to 4 gives us the range <i>italic</i> and <b>bold</b>:

Starting and ending nodes can be different

We don’t have to use the same node in setStart and setEnd. A range may span across many unrelated nodes. It’s only important that the end is after the start in the document.

Selecting a bigger fragment

Let’s make a bigger selection in our example, like this:

We already know how to do that. We just need to set the start and the end as a relative offset in text nodes.

We need to create a range, that:

  • starts from position 2 in <p> first child (taking all but two first letters of "Example: ")
  • ends at the position 3 in <b> first child (taking first three letters of “bold”, but no more):
<p id="p">Example: <i>italic</i> and <b>bold</b></p>

<script>
  let range = new Range();

  range.setStart(p.firstChild, 2);
  range.setEnd(p.querySelector('b').firstChild, 3);

  console.log(range); // ample: italic and bol

  // use this range for selection (explained later)
  window.getSelection().addRange(range);
</script>

As you can see, it’s fairly easy to make a range of whatever we want.

If we’d like to take nodes as a whole, we can pass elements in setStart/setEnd. Otherwise, we can work on the text level.

Range properties

The range object that we created in the example above has following properties:

  • startContainer, startOffset – node and offset of the start,
    • in the example above: first text node inside <p> and 2.
  • endContainer, endOffset – node and offset of the end,
    • in the example above: first text node inside <b> and 3.
  • collapsed – boolean, true if the range starts and ends on the same point (so there’s no content inside the range),
    • in the example above: false
  • commonAncestorContainer – the nearest common ancestor of all nodes within the range,
    • in the example above: <p>

Range selection methods

There are many convenience methods to manipulate ranges.

We’ve already seen setStart and setEnd, here are other similar methods.

Set range start:

  • setStart(node, offset) set start at: position offset in node
  • setStartBefore(node) set start at: right before node
  • setStartAfter(node) set start at: right after node

Set range end (similar methods):

  • setEnd(node, offset) set end at: position offset in node
  • setEndBefore(node) set end at: right before node
  • setEndAfter(node) set end at: right after node

Technically, setStart/setEnd can do anything, but more methods provide more convenience.

In all these methods, node can be both a text or element node: for text nodes offset skips that many of characters, while for element nodes that many child nodes.

Even more methods to create ranges:

  • selectNode(node) set range to select the whole node
  • selectNodeContents(node) set range to select the whole node contents
  • collapse(toStart) if toStart=true set end=start, otherwise set start=end, thus collapsing the range
  • cloneRange() creates a new range with the same start/end

Range editing methods

Once the range is created, we can manipulate its content using these methods:

  • deleteContents() – remove range content from the document
  • extractContents() – remove range content from the document and return as DocumentFragment
  • cloneContents() – clone range content and return as DocumentFragment
  • insertNode(node) – insert node into the document at the beginning of the range
  • surroundContents(node) – wrap node around range content. For this to work, the range must contain both opening and closing tags for all elements inside it: no partial ranges like <i>abc.

With these methods we can do basically anything with selected nodes.

Here’s the test stand to see them in action:

Click buttons to run methods on the selection, "resetExample" to reset it.

<p id="p">Example: <i>italic</i> and <b>bold</b></p>

<p id="result"></p>
<script>
  let range = new Range();

  // Each demonstrated method is represented here:
  let methods = {
    deleteContents() {
      range.deleteContents()
    },
    extractContents() {
      let content = range.extractContents();
      result.innerHTML = "";
      result.append("extracted: ", content);
    },
    cloneContents() {
      let content = range.cloneContents();
      result.innerHTML = "";
      result.append("cloned: ", content);
    },
    insertNode() {
      let newNode = document.createElement('u');
      newNode.innerHTML = "NEW NODE";
      range.insertNode(newNode);
    },
    surroundContents() {
      let newNode = document.createElement('u');
      try {
        range.surroundContents(newNode);
      } catch(e) { console.log(e) }
    },
    resetExample() {
      p.innerHTML = `Example: <i>italic</i> and <b>bold</b>`;
      result.innerHTML = "";

      range.setStart(p.firstChild, 2);
      range.setEnd(p.querySelector('b').firstChild, 3);

      window.getSelection().removeAllRanges();
      window.getSelection().addRange(range);
    }
  };

  for(let method in methods) {
    document.write(`<div><button onclick="methods.${method}()">${method}</button></div>`);
  }

  methods.resetExample();
</script>

There also exist methods to compare ranges, but these are rarely used. When you need them, please refer to the spec or MDN manual.

Selection

Range is a generic object for managing selection ranges. Although, creating a Range doesn’t mean that we see a selection on screen.

We may create Range objects, pass them around – they do not visually select anything on their own.

The document selection is represented by Selection object, that can be obtained as window.getSelection() or document.getSelection(). A selection may include zero or more ranges. At least, the Selection API specification says so. In practice though, only Firefox allows to select multiple ranges in the document by using Ctrl+click (Cmd+click for Mac).

Here’s a screenshot of a selection with 3 ranges, made in Firefox:

Other browsers support at maximum 1 range. As we’ll see, some of Selection methods imply that there may be many ranges, but again, in all browsers except Firefox, there’s at maximum 1.

Here’s a small demo that shows the current selection (select something and click) as text:

Selection properties

As said, a selection may in theory contain multiple ranges. We can get these range objects using the method:

  • getRangeAt(i) – get i-th range, starting from 0. In all browsers except Firefox, only 0 is used.

Also, there exist properties that often provide better convenience.

Similar to a range, a selection object has a start, called “anchor”, and the end, called “focus”.

The main selection properties are:

  • anchorNode – the node where the selection starts,
  • anchorOffset – the offset in anchorNode where the selection starts,
  • focusNode – the node where the selection ends,
  • focusOffset – the offset in focusNode where the selection ends,
  • isCollapsedtrue if selection selects nothing (empty range), or doesn’t exist.
  • rangeCount – count of ranges in the selection, maximum 1 in all browsers except Firefox.
Selection end/start vs Range

There’s an important differences of a selection anchor/focus compared with a Range start/end.

As we know, Range objects always have their start before the end.

For selections, that’s not always the case.

Selecting something with a mouse can be done in both directions: either “left-to-right” or “right-to-left”.

In other words, when the mouse button is pressed, and then it moves forward in the document, then its end (focus) will be after its start (anchor).

E.g. if the user starts selecting with mouse and goes from “Example” to “italic”:

…But the same selection could be done backwards: starting from “italic” to “Example” (backward direction), then its end (focus) will be before the start (anchor):

Selection events

There are events on to keep track of selection:

  • elem.onselectstart – when a selection starts specifically on element elem (or inside it). For instance, when the user presses the mouse button on it and starts to move the pointer.
    • Preventing the default action cancels the selection start. So starting a selection from this element becomes impossible, but the element is still selectable. The visitor just needs to start the selection from elsewhere.
  • document.onselectionchange – whenever a selection changes or starts.
    • Please note: this handler can be set only on document, it tracks all selections in it.

Selection tracking demo

Here’s a small demo. It tracks the current selection on the document and shows its boundaries:

<p id="p">Select me: <i>italic</i> and <b>bold</b></p>

From <input id="from" disabled> – To <input id="to" disabled>
<script>
  document.onselectionchange = function() {
    let selection = document.getSelection();

    let {anchorNode, anchorOffset, focusNode, focusOffset} = selection;

    // anchorNode and focusNode are text nodes usually
    from.value = `${anchorNode?.data}, offset ${anchorOffset}`;
    to.value = `${focusNode?.data}, offset ${focusOffset}`;
  };
</script>

Selection copying demo

There are two approaches to copying the selected content:

  1. We can use document.getSelection().toString() to get it as text.
  2. Otherwise, to copy the full DOM, e.g. if we need to keep formatting, we can get the underlying ranges with getRangesAt(...). A Range object, in turn, has cloneContents() method that clones its content and returns as DocumentFragment object, that we can insert elsewhere.

Here’s the demo of copying the selected content both as text and as DOM nodes:

<p id="p">Select me: <i>italic</i> and <b>bold</b></p>

Cloned: <span id="cloned"></span>
<br>
As text: <span id="astext"></span>

<script>
  document.onselectionchange = function() {
    let selection = document.getSelection();

    cloned.innerHTML = astext.innerHTML = "";

    // Clone DOM nodes from ranges (we support multiselect here)
    for (let i = 0; i < selection.rangeCount; i++) {
      cloned.append(selection.getRangeAt(i).cloneContents());
    }

    // Get as text
    astext.innerHTML += selection;
  };
</script>

Selection methods

We can work with the selection by addding/removing ranges:

  • getRangeAt(i) – get i-th range, starting from 0. In all browsers except Firefox, only 0 is used.
  • addRange(range) – add range to selection. All browsers except Firefox ignore the call, if the selection already has an associated range.
  • removeRange(range) – remove range from the selection.
  • removeAllRanges() – remove all ranges.
  • empty() – alias to removeAllRanges.

There are also convenience methods to manipulate the selection range directly, without intermediate Range calls:

  • collapse(node, offset) – replace selected range with a new one that starts and ends at the given node, at position offset.
  • setPosition(node, offset) – alias to collapse.
  • collapseToStart() – collapse (replace with an empty range) to selection start,
  • collapseToEnd() – collapse to selection end,
  • extend(node, offset) – move focus of the selection to the given node, position offset,
  • setBaseAndExtent(anchorNode, anchorOffset, focusNode, focusOffset) – replace selection range with the given start anchorNode/anchorOffset and end focusNode/focusOffset. All content in-between them is selected.
  • selectAllChildren(node) – select all children of the node.
  • deleteFromDocument() – remove selected content from the document.
  • containsNode(node, allowPartialContainment = false) – checks whether the selection contains node (partially if the second argument is true)

For most tasks these methods are just fine, there’s no need to access the underlying Range object.

For example, selecting the whole contents of the paragraph <p>:

<p id="p">Select me: <i>italic</i> and <b>bold</b></p>

<script>
  // select from 0th child of <p> to the last child
  document.getSelection().setBaseAndExtent(p, 0, p, p.childNodes.length);
</script>

The same thing using ranges:

<p id="p">Select me: <i>italic</i> and <b>bold</b></p>

<script>
  let range = new Range();
  range.selectNodeContents(p); // or selectNode(p) to select the <p> tag too

  document.getSelection().removeAllRanges(); // clear existing selection if any
  document.getSelection().addRange(range);
</script>
To select something, remove the existing selection first

If a document selection already exists, empty it first with removeAllRanges(). And then add ranges. Otherwise, all browsers except Firefox ignore new ranges.

The exception is some selection methods, that replace the existing selection, such as setBaseAndExtent.

Selection in form controls

Form elements, such as input and textarea provide special API for selection, without Selection or Range objects. As an input value is a pure text, not HTML, there’s no need for such objects, everything’s much simpler.

Properties:

  • input.selectionStart – position of selection start (writeable),
  • input.selectionEnd – position of selection end (writeable),
  • input.selectionDirection – selection direction, one of: “forward”, “backward” or “none” (if e.g. selected with a double mouse click),

Events:

  • input.onselect – triggers when something is selected.

Methods:

  • input.select() – selects everything in the text control (can be textarea instead of input),

  • input.setSelectionRange(start, end, [direction]) – change the selection to span from position start till end, in the given direction (optional).

  • input.setRangeText(replacement, [start], [end], [selectionMode]) – replace a range of text with the new text.

    Optional arguments start and end, if provided, set the range start and end, otherwise user selection is used.

    The last argument, selectionMode, determines how the selection will be set after the text has been replaced. The possible values are:

    • "select" – the newly inserted text will be selected.
    • "start" – the selection range collapses just before the inserted text (the cursor will be immediately before it).
    • "end" – the selection range collapses just after the inserted text (the cursor will be right after it).
    • "preserve" – attempts to preserve the selection. This is the default.

Now let’s see these methods in action.

Example: tracking selection

For example, this code uses onselect event to track selection:

<textarea id="area" style="width:80%;height:60px">
Selecting in this text updates values below.
</textarea>
<br>
From <input id="from" disabled> – To <input id="to" disabled>

<script>
  area.onselect = function() {
    from.value = area.selectionStart;
    to.value = area.selectionEnd;
  };
</script>

Please note:

  • onselect triggers when something is selected, but not when the selection is removed.
  • document.onselectionchange event should not trigger for selections inside a form control, according to the spec, as it’s not related to document selection and ranges. Some browsers generate it, but we shouldn’t rely on it.

Example: moving cursor

We can change selectionStart and selectionEnd, that sets the selection.

An important edge case is when selectionStart and selectionEnd equal each other. Then it’s exactly the cursor position. Or, to rephrase, when nothing is selected, the selection is collapsed at the cursor position.

So, by setting selectionStart and selectionEnd to the same value, we move the cursor.

For example:

<textarea id="area" style="width:80%;height:60px">
Focus on me, the cursor will be at position 10.
</textarea>

<script>
  area.onfocus = () => {
    // zero delay setTimeout to run after browser "focus" action finishes
    setTimeout(() => {
      // we can set any selection
      // if start=end, the cursor is exactly at that place
      area.selectionStart = area.selectionEnd = 10;
    });
  };
</script>

Example: modifying selection

To modify the content of the selection, we can use input.setRangeText() method. Of course, we can read selectionStart/End and, with the knowledge of the selection, change the corresponding substring of value, but setRangeText is more powerful and often more convenient.

That’s a somewhat complex method. In its simplest one-argument form it replaces the user selected range and removes the selection.

For example, here the user selection will be wrapped by *...*:

<input id="input" style="width:200px" value="Select here and click the button">
<button id="button">Wrap selection in stars *...*</button>

<script>
button.onclick = () => {
  if (input.selectionStart == input.selectionEnd) {
    return; // nothing is selected
  }

  let selected = input.value.slice(input.selectionStart, input.selectionEnd);
  input.setRangeText(`*${selected}*`);
};
</script>

With more arguments, we can set range start and end.

In this example we find "THIS" in the input text, replace it and keep the replacement selected:

<input id="input" style="width:200px" value="Replace THIS in text">
<button id="button">Replace THIS</button>

<script>
button.onclick = () => {
  let pos = input.value.indexOf("THIS");
  if (pos >= 0) {
    input.setRangeText("*THIS*", pos, pos + 4, "select");
    input.focus(); // focus to make selection visible
  }
};
</script>

Example: insert at cursor

If nothing is selected, or we use equal start and end in setRangeText, then the new text is just inserted, nothing is removed.

We can also insert something “at the cursor” using setRangeText.

Here’s a button that inserts "HELLO" at the cursor position and puts the cursor immediately after it. If the selection is not empty, then it gets replaced (we can detect it by comparing selectionStart!=selectionEnd and do something else instead):

<input id="input" style="width:200px" value="Text Text Text Text Text">
<button id="button">Insert "HELLO" at cursor</button>

<script>
  button.onclick = () => {
    input.setRangeText("HELLO", input.selectionStart, input.selectionEnd, "end");
    input.focus();
  };
</script>

Making unselectable

To make something unselectable, there are three ways:

  1. Use CSS property user-select: none.

    <style>
    #elem {
      user-select: none;
    }
    </style>
    <div>Selectable <div id="elem">Unselectable</div> Selectable</div>

    This doesn’t allow the selection to start at elem. But the user may start the selection elsewhere and include elem into it.

    Then elem will become a part of document.getSelection(), so the selection actually happens, but its content is usually ignored in copy-paste.

  2. Prevent default action in onselectstart or mousedown events.

    <div>Selectable <div id="elem">Unselectable</div> Selectable</div>
    
    <script>
      elem.onselectstart = () => false;
    </script>

    This prevents starting the selection on elem, but the visitor may start it at another element, then extend to elem.

    That’s convenient when there’s another event handler on the same action that triggers the select (e.g. mousedown). So we disable the selection to avoid conflict, still allowing elem contents to be copied.

  3. We can also clear the selection post-factum after it happens with document.getSelection().empty(). That’s rarely used, as this causes unwanted blinking as the selection appears-disappears.

References

Summary

We covered two different APIs for selections:

  1. For document: Selection and Range objects.
  2. For input, textarea: additional methods and properties.

The second API is very simple, as it works with text.

The most used recipes are probably:

  1. Getting the selection:
    let selection = document.getSelection();
    
    let cloned = /* element to clone the selected nodes to */;
    
    // then apply Range methods to selection.getRangeAt(0)
    // or, like here, to all ranges to support multi-select
    for (let i = 0; i < selection.rangeCount; i++) {
      cloned.append(selection.getRangeAt(i).cloneContents());
    }
  2. Setting the selection:
    let selection = document.getSelection();
    
    // directly:
    selection.setBaseAndExtent(...from...to...);
    
    // or we can create a range and:
    selection.removeAllRanges();
    selection.addRange(range);

And finally, about the cursor. The cursor position in editable elements, like <textarea> is always at the start or the end of the selection. We can use it to get cursor position or to move the cursor by setting elem.selectionStart and elem.selectionEnd.

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