如何检查object中的object是否存在

Question

It seems that the following technique for checking the existence of an object member produces an error because the 'bar' parent object hasn't been declared before the check, which means I either have to declare it before the check or use two 'typeof' expressions, either of which would be excess code:

var foo = {},
    newVal = (typeof foo.bar.myVal !== 'undefined' ? foo.bar.myVal : null );

Error: foo.bar is undefined

So, how do you check if a member within an undeclared object exists without producing an error?

I love javascript, but sometimes...

Answer 1

It can be done simply using the code below:

var newVal = (foo && foo.bar && typeof foo.bar.myVal !== 'undefined') ? foo.bar.myVal : foo.bar.myVal

A property is null or undefined, it will be evaluated as false so the above code will only process up to the first 'false' statement.

Answer 2

var newVal = ('foo' in window && // could be typeof foo !== 'undefined' if you want all scopes
             'bar' in foo &&
             'myVal' in foo.bar) ? foo.bar.myVal : null;

To be fair to javascript, that reads almost like natural language.

Answer 3

The simplest test is:

if (foo && foo.bar) {
  // play with foo.bar.myVal  ... 
}

Answer 4

As Matthew Abbott gave in his response, you can write a method for it to use it later if you do a lot of diving into child properties where the whole object or somewhere along the chain can be null. Below is my implementation using lodash .

checkGraph(obj, graphPath) {
    if (obj) {
      let root = obj;
      _.each(graphPath.split('.'), part => {
        if (root[part]) {
          root = root[part];
        } else {
          return false; // bad property
        }
      });
      return true;
    }
    return false; // bad object
  }

You can call the method like this:

if (this.checkGraph(object, 'property.subproperty') {
    // do thing
}

Answer 5

You could wrap it up into a method:

function checkGraph(obj, graphPath)
{
  var parts = graphPath.split(".");
  var root = obj;

  for (var i = 0; i < parts.length; i++)
  {
    var part = parts[i];
    if (root[part] && root.hasOwnProperty(part))
      root = root[part];
    else
      return false;
  }

  return true;
}

Where you could call it as:

var foo = {},
    newVal = checkGraph(foo, "bar.myVal") ? foo.bar.myVal : null;

Answer 6

There is another way is you use a JS transpiler like Babel :

if (foo?.bar) {
  // play with foo.bar.myVal  ... 
}