Style fontWeight Property

Set the font weight for an element to "900":

document.getElementById("demo").style.fontWeight = "900";

Description

The fontWeight property sets or returns how thick or thin characters in a text should be displayed.

Property
fontWeight	Yes	Yes	Yes	Yes	Yes

Return the fontWeight property:

object.style.fontWeight

Set the fontWeight property:

Value	Description
normal	Font is normal. This is default
lighter	Font is lighter
bold	Font is bold
bolder	Font is bolder
100 200 300 400 500 600 700 800 900	Defines from light to bold characters. 400 is the same as normal, and 700 is the same as bold
initial	Sets this property to its default value. Read about initial
inherit	Inherits this property from its parent element. Read about inherit

Default Value:	normal
Return Value:	A String, representing the boldness of the font
CSS Version	CSS1

A demonstration of possible values:

var listValue = selectTag.options[selectTag.selectedIndex].text;
document.getElementById("demo").style.fontWeight = listValue;

Return the font weight of an element:

alert(document.getElementById("demo").style.fontWeight);

CSS tutorial: CSS Font

HTML DOM reference: font property