what does “if (<name of a function>)” mean?

Question

function addPrivateProperty(o,name,predicate){
    var value;
    o["get"+name]=function(){return value;};
    o["set"+name]=function(v){
        if (predicate && !predicate(v))
            throw Error("set"+name+": invalid value "+v);
        else
            value=v;
    };
}
var o={};
addPrivateProperty(o,"Name",function(x){return typeof x=="string";});

here,what does "if (predicate && !predicate(v))" return? when would it return true/false?

Answer 1

如果未定义则为False，否则为true

Answer 2

I haven't done javascript in a while so you'll have to bear with me but if I remember correctly, all functions are just variables, so saying if(predicate) just means if predicate was defined. In this scope it means if a value (function) was supplied to the parameter.

So essentially if (predicate && !predicate(v)) first checks to see if the function exists before calling it, because they do not want to call a function that doesnt exist

Answer 3

The statement if (predicate && !predicate(v)) evaluates if predicate and the opposite of predicate(v) are both true .

This would be true for predicate or !predicate(v) being equal to anything other than null , false , undefined , 0 , NaN , or an empty string.