ANSI C-like sprintf() helper for Javascript, with support for alignment and field widths

sprintf.js

function sprintf( )
{
	let regex = /%([-0])?([0-9]+)?(\.[0-9]+)?([sdf%])/g;
	let args = arguments;
	let tmpl = args[ 0 ];
	let idx = 1;

	return tmpl.replace( regex, function ( exp, p0, p1, p2, p3 )
	{
		if( exp == '%%' )
			return '%';
		else if( idx >= args.length )
			return 'undefined';

		let type = p3;
		let prec = p2 != undefined ? parseInt( p2.substr( 1 ) ) : 6;
		let size = p1 != undefined ? parseInt( p1 ) : 0;
		let fill = p0 == '0' && type != 's' ? '0' : ' ';
		let is_sign = type != 's' && args[ idx ] < 0;
		let is_left = p0 == '-';

		let str;
		switch( type ) {
			case 's':
				str = args[ idx ];
				break;
			case 'd':
				str = parseFloat( args[ idx ] ).toFixed( 0 );
				break;
			case 'f':
				str = parseFloat( args[ idx ] ).toFixed( prec );
				break;
		}
		while( str.length < size ) {
			str = is_left ? str + ' ' : fill + str;
			if( fill == '0' && !is_left && is_sign )
				str = '-0' + str.substr( 2 );  // fun corner case!
		}

		idx++;

		return str;
	} )
}

test.html

<script>
document.writeln( '<pre>' );
document.writeln( sprintf( '%-20s  %5s  %6s', 'Item', 'SKU', 'Cost' ) );
document.writeln( '-----------------------------------' );
document.writeln( sprintf( '%-20s  #%04d  %6.2f', 'Wool Coat', 64, 21.10 ) );
document.writeln( sprintf( '%-20s  #%04d  %6.2f', 'Blue Jeans', 311, 19.50 ) );
document.writeln( sprintf( '%-20s  #%04d  %6.2f', 'Mittens', 204, 8.25 ) );
document.writeln( sprintf( '%-20s  #%04d  %6.2f', 'Leather Jacket', 78, 112.50 ) );
document.writeln( sprintf( '%-20s  #%04d  %6.2f', 'Work Boots', 125, 35.00 ) );
document.writeln( '-----------------------------------' );
document.writeln( '</pre>' );
</script>