It does not itself condense or expand. The font face must already exist as condensed or expanded within the font family.
object.style.fontStretch = "value";
<'font-stretch'> = normal | <percentage> | ultra-condensed | extra-condensed | condensed | semi-condensed | semi-expanded | expanded | extra-expanded | ultra-expanded
Specifies the percentage using a number followed by a percent sign (%).