LESS 有“扩展”特性吗？

Yes, Less.js introduced extend in v1.4.0.

:extend()

Rather than implementing the at-rule (@extend) syntax used by SASS and Stylus, LESS implemented the pseudo-class syntax, which gives LESS's implementation the flexibility to be applied either directly to a selector itself, or inside a statement. So both of these will work:

.sidenav:extend(.nav) {...}

or

.sidenav {
&:extend(.nav);
...
}

Additionally, you can use the all directive to extend "nested" classes as well:

.sidenav:extend(.nav all){};

And you can add a comma-separated list of classes you wish to extend:

.global-nav {
&:extend(.navbar, .nav all, .navbar-fixed-top all, .navbar-inverse);
height: 70px;
}

When extending nested selectors you should notice the differences:

nested selectors .selector1 and selector2:

.selector1 {
property1: a;
.selector2 {
property2: b;
}
}

Now .selector3:extend(.selector1 .selector2){}; outputs:

.selector1 {
property1: a;
}
.selector1 .selector2,
.selector3 {
property2: b;
}

, .selector3:extend(.selector1 all){}; outputs:

.selector1,
.selector3 {
property1: a;
}
.selector1 .selector2,
.selector3 .selector2 {
property2: b;
}

,.selector3:extend(.selector2){}; outputs

.selector1 {
property1: a;
}
.selector1 .selector2 {
property2: b;
}

and finally .selector3:extend(.selector2 all){};:

.selector1 {
property1: a;
}
.selector1 .selector2,
.selector1 .selector3 {
property2: b;
}

Easy way to extend a function in Less framework

.sibling-1 {
color: red
}
.sibling-2 {
background-color: #fff;
.sibling-1();
}

Output

.sibling-1 {
color: red
}
.sibling-2 {
background-color: #fff;
color: red
}

Refer https://codepen.io/sprushika/pen/MVZoGB/

Less allows us to do :extend(.class) or :extend(#id)