为什么在我的JavaScript中无法为参数设置默认值？

Question

I want to set a default of 'small' if no options object is passed into my constructor:

var Plan = function(options){
  this.name = options.name || 'small';
}

but when I do this:

var smallPlan = new Plan();

console.log(smallPlan.name);

I get Uncaught TypeError: Cannot read property 'name' of undefined

What am I doing wrong? Isn't this the idiomatic way to set default parameter values in javascript?

Answer 1

Instead of over complicating the code to check if options and name is there, check to see if the object is defined, if not, set it to an empty object.

var Plan = function(options){
  options = options || {};
  this.name = options.name || 'small';
}

Answer 2

options is undefined. You can't access options.name if options doesn't exist.

If you want to check for more than just one property, I'd suggest something like this:

var Plan = function(options){
    // Set defaults
    this.name = 'foo';
    this.title = 'bar';
    this.something = 'even more stuff';
    if(options){ // If options exists, override defaults
       this.name = options.name || this.name;
       this.title = options.title || this.title;
       this.something = options.something || this.something;
    }
}

Otherwise, I'd try this:

var Plan = function(options){
    this.name = options ? options.name || 'small' : `small`;
}

It's a little ugly, but you'll have to check if options exists, and if options has a name property.

What this does is:

if(options){
    if(options.name){
        this.name = options.name;
    } else {
        this.name = 'small';
    }
} else {
    this.name = 'small';
}