less.js/lib/less/functions/boolean.js

var functionRegistry = require("./function-registry"),
    Anonymous = require("../tree/anonymous"),
    Keyword = require("../tree/keyword");

functionRegistry.addMultiple({
    boolean: function(condition) {
        return condition ? Keyword.True : Keyword.False;
    },

    'if': function(condition, trueValue, falseValue) {
        return condition ? trueValue
            : (falseValue || new Anonymous);
    }
});