Simple function to cast ASCII-string-text to barcode128-text for font. That text can be used with Barcode 128 Font and it's recognized by Barcode readers.

text2Barcode128B.js

/* BARCODE 128 FONT (with or without text):
 *   https://fonts.google.com/specimen/Libre+Barcode+128+Text
 *   https://fonts.google.com/specimen/Libre+Barcode+128
 *
 * EXAMPLES (with checksum < 126 or checksum > 126 ):
 *   - textToBarcode128B( 'Hello World!' ); // "ÌHello World!WÎ"
 *   - textToBarcode128B( 'Hello W4rld!' ); // "ÌHello W4rld!ÆÎ"
 *
 * NOTE: 
 *   The font takes miliseconds to refresh the barcode-text without checksum char.
 */

function textToBarcode128B( str ) {
    /* Type B start */
    var sumValue = 104;  

    /* Multiply char position with decimal value of character */
    for( var i = 0, len = str.length; i < len; i++ ) {
        sumValue += ( ( i + 1 ) * ( str.charCodeAt( i ) - 32 ) );
    }

    /* Use Modulus to find checksum */
    var modValue = ( sumValue % 103 ) + 32;

    /* Checksum with shift control */
    var checksum = String.fromCharCode( modValue > 126 ? modValue + 68 : modValue );

    return String.fromCharCode( 204 ) + str + checksum + String.fromCharCode( 206 );
}