Tabla de contenido horizontal en indesign

I can get you half way there

But this method automatically inserts a semi-colon and no option to change it - bizarre you can't pick the run-in character... 

When generating your TOC in your TOC options - select your H2 or heading 2 that you want and select Run In at the bottom and they will list horizontally

You might be totally happy with the no-option semi colon approach - and doesn't require any additional changes on your part. 

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I can kinda get you all the way there - RUN IN OPTION change to dassh

Generate your TOC with the Run In as above

Then run the script

Then when you need to update your TOC run the script and it updates the TOC and removes the ; and adds the dash, you can change the character to anything you need.

But there's a flaw in this that I cannot fix - if your text has a ; in the heading listed horizontally it will probably be removed.

// ReplaceRunInCharacter_UpdateTOC.jsx
// This script updates the TOC (if the TOC text frame is selected)
// then replaces the semicolon followed by a space with your chosen character.

if (app.selection.length === 0 || app.selection[0].constructor.name !== "TextFrame") {
    alert("Please select the TOC text frame before running this script.");
} else {
    // First, update the TOC using the menu action
    try {
        var updateAction = app.menuActions.itemByName("Update Table of Contents");
        updateAction.invoke();
        // Wait a bit for the update to process (adjust delay as needed)
        $.sleep(1000);
    } catch (e) {
        alert("Error updating TOC: " + e);
    }
    
    // Now, proceed with the GREP replacement
    var tocStory = app.selection[0].parentStory;
    
    // Clear GREP preferences
    app.findGrepPreferences = app.changeGrepPreferences = NothingEnum.NOTHING;
    
    // Set GREP to find the semicolon and a space (the default run-in separator)
    app.findGrepPreferences.findWhat = ";\\s";
    // Set the replacement to an en dash with spaces (change as desired)
    app.changeGrepPreferences.changeTo = " – ";
    
    // Apply the change to the entire TOC story
    var changedItems = tocStory.changeGrep();
    
    // Reset GREP preferences
    app.findGrepPreferences = app.changeGrepPreferences = NothingEnum.NOTHING;
    
    alert(changedItems.length + " instance(s) replaced.");
}

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I guess the other method mentioned by others is better, DON'T use the RUN IN and replace the \r with a dash 

This just means you don't have to do two steps - update the TOC and then run a GREP - it basically does both things, updates the TOC then applies the changes for the look you want. 

Run the script to update the TOC and add the dashes.

/*
Replace TOC Paragraph Breaks with Dash (excluding last).jsx
- Updates the TOC.
- Replaces paragraph returns (\r) in "TOC Body Text" with a dash, but leaves the last one untouched.
*/

if (app.selection.length === 0 || app.selection[0].constructor.name !== "TextFrame") {
    alert("Please select the TOC text frame before running this script.");
} else {
    try {
        // Update TOC
        var updateAction = app.menuActions.itemByName("Update Table of Contents");
        updateAction.invoke();
        $.sleep(1000);
    } catch (e) {
        alert("Error updating TOC: " + e);
    }

    var tocStory = app.selection[0].parentStory;

    // Clear previous GREP settings
    app.findGrepPreferences = NothingEnum.NOTHING;
    app.changeGrepPreferences = NothingEnum.NOTHING;

    // **Find all instances of \r inside "TOC Body Text"**
    app.findGrepPreferences.appliedParagraphStyle = app.activeDocument.paragraphStyles.itemByName("TOC Body Text");  
    app.findGrepPreferences.findWhat = "\\r"; // Paragraph return

    var foundReturns = tocStory.findGrep();
    
    if (foundReturns.length > 1) {
        for (var i = 0; i < foundReturns.length - 1; i++) { // Skip last return
            foundReturns[i].contents = " – ";
        }
        alert((foundReturns.length - 1) + " instances replaced with a dash.");
    } else {
        alert("No replacements made. Not enough paragraph returns found.");
    }

    // Reset GREP preferences
    app.findGrepPreferences = NothingEnum.NOTHING;
    app.changeGrepPreferences = NothingEnum.NOTHING;
}

=======

These methods may need to be tweaked - or maybe there's a cleverer way to do it that I don't know. 

I'm just learning scripting so this was a fun task to try something new and maybe learn something myself. 

If there's a better script method I'd love to see it.

As others have mentioned, a relatively simple Find/Change should take care of it. Just make sure you have something you can find that won't be confused with other text. (Paragraph styles looks like it should work here.)

If you need to do multiple Find/Changes, you can use the script provided with InDesign or the (now) free Multi-Find/Change: https://www.automatication.com/product/mfc/

Keep in mind that you will have to run the Find/Change each time you update the TOC. Of course, the TOC only needs to be updated when creating a new proof.

in the future, to find the best place to post your message, use the list here, https://community.adobe.com/

p.s. i don't think the adobe website, and forums in particular, are easy to navigate, so don't spend a lot of time searching that forum list. do your best and we'll move the post (like this one has already been moved) if it helps you get responses.

<"moved from using the community">

After generating and regenerating run a find and replace with a GREP to get the toc in its final form. After setting it up, it is a one command task in the future.