How can a JavaScript object refer to values in itself?
JavascriptObjectJavascript Problem Overview
Lets say I have the following JavaScript:
var obj = {
key1 : "it ",
key2 : key1 + " works!"
};
alert(obj.key2);
This errors with "key1 is not defined". I have tried
this.key1
this[key1]
obj.key1
obj[key1]
this["key1"]
obj["key1"]
and they never seem to be defined.
How can I get key2 to refer to key1's value?
Javascript Solutions
Solution 1 - Javascript
Maybe you can think about removing the attribute to a function. I mean something like this:
var obj = {
key1: "it ",
key2: function() {
return this.key1 + " works!";
}
};
alert(obj.key2());
Solution 2 - Javascript
This can be achieved by using constructor function instead of literal
var o = new function() {
this.foo = "it";
this.bar = this.foo + " works"
}
alert(o.bar)
Solution 3 - Javascript
You can't refer to a property of an object before you have initialized that object; use an external variable.
var key1 = "it";
var obj = {
key1 : key1,
key2 : key1 + " works!"
};
Also, this is not a "JSON object"; it is a Javascript object. JSON is a method of representing an object with a string (which happens to be valid Javascript code).
Solution 4 - Javascript
One alternative would be to use a getter/setter methods.
For instance, if you only care about reading the calculated value:
var book = {}
Object.defineProperties(book,{
key1: { value: "it", enumerable: true },
key2: {
enumerable: true,
get: function(){
return this.key1 + " works!";
}
}
});
console.log(book.key2); //prints "it works!"
The above code, though, won't let you define another value for key2.
So, the things become a bit more complicated if you would like to also redefine the value of key2. It will always be a calculated value. Most likely that's what you want.
However, if you would like to be able to redefine the value of key2, then you will need a place to cache its value independently of the calculation.
Somewhat like this:
var book = { _key2: " works!" }
Object.defineProperties(book,{
key1: { value: "it", enumerable: true},
_key2: { enumerable: false},
key2: {
enumerable: true,
get: function(){
return this.key1 + this._key2;
},
set: function(newValue){
this._key2 = newValue;
}
}
});
console.log(book.key2); //it works!
book.key2 = " doesn't work!";
console.log(book.key2); //it doesn't work!
for(var key in book){
//prints both key1 and key2, but not _key2
console.log(key + ":" + book[key]);
}
Another interesting alternative is to use a self-initializing object:
var obj = ({
x: "it",
init: function(){
this.y = this.x + " works!";
return this;
}
}).init();
console.log(obj.y); //it works!
Solution 5 - Javascript
Because the statement defining obj
hasn't finished, key1
doesn't exist yet. Consider this solution:
var obj = { key1: "it" };
obj.key2 = obj.key1 + ' ' + 'works!';
// obj.key2 is now 'it works!'
Solution 6 - Javascript
That's not a JSON object, that's a Javascript object created via object literal notation. (JSON is a textual notation for data exchange (more). If you're dealing with JavaScript source code, and not dealing with a string, you're not dealing with JSON.)
There's no way within the object initializer to refer to another key of the object being initialized, because there's no way to get a reference to the object being created until the initializer is finished. (There's no keyword akin to this
or something for this situation.)
Solution 7 - Javascript
You can also reference the obj
once you are inside the function instead of this
.
var obj = {
key1: "it",
key2: function(){return obj.key1 + " works!"}
};
alert(obj.key2());
Solution 8 - Javascript
This is not JSON. JSON was designed to be simple; allowing arbitrary expressions is not simple.
In full JavaScript, I don't think you can do this directly. You cannot refer to this
until the object called obj
is fully constructed. So you need a workaround, that someone with more JavaScript-fu than I will provide.