[JSX] Parm error when setting characterAttributes.size of Story.textRange

@brian_p_dts, I had a bit of a play and wrote a function "fitText" that aims to fit text to a given textFrame and also takes a maxLineCount parameter, so you can specify that you want, say, 3 lines and if there are more than 3 lines it will reduce point size further. (maxLineCount only works in area text—on path text you just get one line no matter what.) It's a quick script so will definitely fail in edge cases. See what you think.

- Mark

/**
 * @file Fit text to text frame.js
 * 
 * Limitation: this will make all the text in the frame the same size!
 *
 * @author m1b
 * @version 2026-05-17
 * @discussion https://community.adobe.com/t5/illustrator-discussions/jsx-parm-error-when-setting-characterattributes-size-of-story-textrange/m-p/13535839
 */
(function () {

    var doc = app.activeDocument;
    var tf = doc.selection[0];

    if (tf == undefined)
        return alert('Please select a text frame and try again.');

    // some example text
    // tf.textRange.contents = 'The quick brown fox jumps over the lazy dog';

    // fit the text, specifying number of lines
    var numberOfLines = 3;
    fitText(tf, numberOfLines);

    /**
     * Fits text to an area text frame or path text.
     * @author m1b
     * @version 2023-01-29
     * @param {TextFrame} textFrame - an Illustrator TextFrame (area text or path text).
     * @param {Number} [maxLineCount] - the maximum number of lines of the final text (default: no limit). This is ignored for path text.
     * @param {Number} [increment] - the increment of size changing; the precision of the fit (default: 0.05).
     */
    function fitText(textFrame, maxLineCount, increment) {

        maxLineCount = maxLineCount || Infinity;
        increment = increment || 0.05;

        if (TextType.POINTTEXT === textFrame.kind)
            // "fitting" makes no sense with point text
            return;

        var size = textFrame.textRange.characterAttributes.size;

        // make an overflow text frame
        var overflowTextFrame = addOverflowTextFrame(textFrame);

        if (!overflowTextFrame)
            return;
        $.writeln('textFrame.lines.length = ' + textFrame.lines.length);

        // first enlarge text
        while (
            overflowTextFrame.words.length === 0
            && textFrame.lines.length !== 0
        ) {
            size += increment;
            sizeBothTextFrames(size);
        }
        $.writeln('textFrame.lines.length = ' + textFrame.lines.length);

        // now reduce the text size to fit
        while (
            overflowTextFrame.words.length > 0
            || textFrame.lines.length > maxLineCount
        ) {
            size -= increment;
            sizeBothTextFrames(size);
        }

        // cleanup
        overflowTextFrame.remove();

        // helper function
        function sizeBothTextFrames(size) {
            textFrame.textRange.characterAttributes.size = size;
            overflowTextFrame.textRange.characterAttributes.size = size;
        }

    };


    /**
     * Adds a linked area text frame to the given text frame.
     *
     * Tries to overcome a complication that destroys overset
     * text when the new linked frame is created.
     *
     * @author m1b
     * @version 2026-05-17
     * @param {TextFrame} textFrame - an Illustrator TextFrame (area text or path text).
     * @returns {TextFrame} - the overflow textFrame
     */
    function addOverflowTextFrame(textFrame) {

        const EXPAND_AMOUNT = 1000;

        var overflowTextFrame;

        var position = textFrame.position;
        var width = textFrame.textPath.width;
        var height = textFrame.textPath.height;

        // temporarily expand the text frame to ensure all text content is fitting
        textFrame.position = [0, 0];
        textFrame.textPath.width += EXPAND_AMOUNT;
        textFrame.textPath.height += EXPAND_AMOUNT;

        try {

            overflowTextFrame = textFrame.layer.textFrames.areaText(textFrame.layer.pathItems.rectangle(0, 150, 1000, 1000), TextOrientation.HORIZONTAL, textFrame);

        } catch (error) { }

        finally {

            //clean up
            textFrame.textPath.width = width;
            textFrame.textPath.height = height;
            textFrame.position = position;

        }

        return overflowTextFrame;

    };

})();

Edit 2026-05-17: improved `addOverflowTextFrame`.