Get the number of columns in a Jupyter notebook output cell, from within the notebook itself.

get_jupyter_columns.html

%%html
<canvas id="canvas"></canvas>
<script>
/* A function that reports the width of a Jupyter notebook cells */
function reportWidth() {
    /* Get an output area element */ 
    let el = document.querySelector("div.output_subarea.output_text.output_stream.output_stdout > pre");
    
    /* Determine the width of the output area, in pixels */
    let output_width = el.clientWidth;
    
    /* Identify the font size and family */
    let font_size = window.getComputedStyle(el, null).getPropertyValue('font-size');
    let font_family = window.getComputedStyle(el, null).getPropertyValue('font-family');
    
    /* Create canvas+context and set font */
    let can = document.getElementById('canvas');
    let ctx = can.getContext('2d');
    ctx.font = font_size + ' ' + font_family;

    /* Measure the width of a character */
    let char_width = ctx.measureText('M').width;
    
    console.log("Font:", ctx.font);
    console.log("Character width:", char_width);
    console.log("Output width:", output_width);
    console.log("Number of columns:", output_width/char_width);
    
    /* Execute a statement to save the number of columns in an output cell */
    IPython.notebook.kernel.execute("__output_cols=" + Math.floor(output_width/char_width));
}

reportWidth();
</script>