在javascript中，评估的参数顺序和延迟评估

Question

1. if function( arg1, arg2 ) , am I correct in believing arg1 is guaranteed to evaluated before arg2 (unlike classic C for example)?

2. is there any way to have a function where the arguments are not evaluated, but rather evaluated on demand? for example, if( cond1 || cond2 ) evaluates cond2 if and only if cond1 is false. is it possible to write our own if -like function?

for example, could I write a function like oracle's nvl( arg1, arg2, ... ) , which returns the first non-null argument, evaluating them lazily. in a regular function call, all arguments are evaluated before the function body is executed.

Answer 1

Function arguments are evaluated before they're passed to the function, so what you're asking for isn't technically possible. However, you could do this:

function nvl() {
    for (var i = 0; i < arguments.length; i++) {
        var res = arguments[i]();
        if (res)
            return res;
    }
}

nvl(function() { return false; }, 
    function() { return 7; }, 
    function() { return true; });

The call to nvl returns 7 .

All of the function wrappers are created, but only the bodies of the first two are evaluated inside nvl .

This isn't exactly pretty, but it would allow you to prevent an expensive operation in the body of any of the functions after the first one returning a truthy value.

Answer 2

in your first example, yes, if you're calling a function they will be evaluated in order.

In the 2nd example, JS does not evaluate every param (just like C), eg:

if(true || x++){

x++ will never run.

What exactly are you trying to do?

Answer 3

1. Yes
2. Yes, with if(cond1 || cond2) , cond2 will be evaluated iff cond1 is false.

Here is a function that return the first truthy argument:

function firstDefinedArgument(){
    for(var i in arguments){
        if(arguments[i]){
           return arguments[i];
        }
     }
 }

So, for example, firstDefinedArgument(0, null, undefined, "", "hello", "goodbye") would return "hello" .

If you wanted to return the first non-null argument instead of the first truthy one, you would replace if(arguments[i]) with if(arguments[i] !== null) . This function would return 0 in the above example.