setting a default value to an undefined variable in javascript

问题

Ideally I want to be able to write something like:

function a( b ) {
    b.defaultVal( 1 );
    return b;
}

The intention of this is that if b is any defined value, b will remain as that value; but if b is undefined, then b will be set to the value specified in the parameter of defaultVal(), in this case 1.

Is this even possible?

Ive been toying about with something like this:

String.prototype.defaultVal=function(valOnUndefined){
    if(typeof this==='undefined'){
        return valOnUndefined;
    }else{
        return this;
    }
};

But I'm not having any success applying this kind of logic to any variable, particularly undefined variables.

Does anyone have any thoughts on this? Can it be done? Or am I barking up the wrong tree?

回答1:

Well the most important thing to learn here is if a variable is undefined you, by default, do not have any methods available on it. So you need to use a method that can operate on the variable, but is not a member/method of that variable.

Try something like:

function defaultValue(myVar, defaultVal){
    if(typeof myVar === "undefined") myVar = defaultVal;
    return myVar;
}

What you're trying to do is the equivalent of this:

undefined.prototype.defaultValue = function(val) { }

...Which for obvious reasons, doesn't work.

回答2:

Why not use the default operator:

function someF(b)
{
    b = b || 1;
    return b;
}

Job done! Here's more info about how it works.

Just as a side-note: your prototype won't work, because you're augmenting the String prototype, whereas if your variable is undefined, the String prototype doesn't exactly apply.

To be absolutely, super-duper-sure that you're only switching to the default value, when b really is undefined, you could do this:

var someF = (function(undefined)//get the real undefined value
{
    return function(b)
    {
        b = b === undefined ? 1 : b;
        return b;
    }
})(); // by not passing any arguments

回答3:

While the two answers already provided are correct, the language has progressed since then. If your target browsers support it, you can use default function parameters like this:

function greetMe(greeting = "Hello", name = "World") {
  console.log(greeting + ", " + name + "!");
}

greetMe(); // prints "Hello, World!"
greetMe("Howdy", "Tiger"); // prints "Howdy, Tiger!"

Here are the MDN docs on default function parameters.

来源：https://stackoverflow.com/questions/12409813/setting-a-default-value-to-an-undefined-variable-in-javascript

标签

javascript

undefined

default-value