How to use a variable to select a document property? - javascript

I want to have
document.body.innerHTML
or
document.head.innerHTML
I tried the obvious:
document.variable.innerHTML
but no go. Also want it to pass jshint.com

Try
document[variable].innerHTML
The . notation requires the property name to be a valid identifier, and a constant. Object property names, however, can be any string. Thus, whenever you have a property name that's an "ugly" string, or a string held in a variable (or computed by some expression), you use the [ ] operator to access it.

Related

node js not accepting hyphen in json [duplicate]

I am using this script to make a style object of all the inherited, etc. styles.
var style = css($(this));
alert (style.width);
alert (style.text-align);
With the following, the first alert will work fine, but the second one doesn't... it's interpreting the - as a minus I assume. The debugger says 'uncaught reference error'. I can't put quotes around it, though, because it isn't a string. So how do I use this object property?
Look at the comments. You will see that for CSS properties, the key notation is not compatible with a number of properties. Using the camel case key notation therefore is the current way:
obj.style-attr // would become
obj["styleAttr"]
Use key notation rather than dot
style["text-align"]
All arrays in JavaScript are objects and all objects are just associative arrays. This means you can refer to a place in an object just as you would refer to a key in an array.
arr[0]
or the object
obj["method"] == obj.method
A couple things to remember when accessing properties this way:
they are evaluated so use strings unless you are doing something with a counter or using dynamic method names.
This means obj[method] would give you an undefined error while obj["method"] would not
You must use this notation if you are using characters that are not allowed in JavaScript variables.
This regex pretty much sums it up:
[a-zA-Z_$][0-9a-zA-Z_$]*
The answer to the original question is: place the property name in quotes and use array style indexing:
obj['property-with-hyphens'];
Several have pointed out that the property you are interested in is a CSS property. CSS properties that have hyphens are automatically converted to camel casing. In that case you must use the camel cased name like:
style.textAlign;
However this solution only works for CSS properties. For example,
obj['a-b'] = 2;
alert(obj.aB); // undefined
alert(obj['a-b']); // 2
CSS properties with a - are represented in camelCase in JavaScript objects. That would be:
alert( style.textAlign );
You could also use a bracket notation to use the string:
alert( style['text-align'] );
Property names may only contain characters, numbers, the well known $ sign and the _ (thanks to pimvdb).
Use brackets:
var notTheFlippingStyleObject = {
'a-b': 1
};
console.log(notTheFlippingStyleObject["a-b"] === 1); // true
More information on objects: MDN
NOTE: If you are accessing the style object, CSSStyleDeclaration, you must use camelCase to access it from JavaScript. More information is here.
alert(style.textAlign)
or
alert(style["textAlign"]);
To directly answer the question: style['text-align'] is how you would reference a property with a hyphen in it. But style.textAlign (or style['textAlign']) is what should be used in this case.
Hyphenated style properties are referenced via camelCase in JavaScript, so use style.textAlign.
To solve your problem: The CSS properties with hyphens in them are represented by JavaScript properties in camelCase to avoid this problem. You want: style.textAlign.
To answer the question: Use square bracket notation: obj.prop is the same as obj["prop"] so you can access property names using strings and use characters that are forbidden in identifiers.
I think in the case of CSS styles they get changed to camelCase in JavaScript, so test-align becomes textAlign.
In the general case, where you want to access a property that contains non-standard characters, you use array-style: ['text-align']
The object property names are not one-to-one matches for the CSS names.
At first, I wondered why the solution didn't work on my end:
api['data-sitekey'] // Returns undefined
...later on I figured out that accessing data attributes was different:
It should be like this:
var api = document.getElementById("some-api");
api.dataset.sitekey

why we need to use [ ] for access variable name include hyphan?

Either if you saying '-' takes as a subtract expression.
eg. Json = {'Me-m':123}
But have you idea why we need to use [] for access this hyphen variable.
Such like that Json['Me-m'].
Writting
Json.Me-m
will evaluate to
Json.Me - m
It will try to access Me property from Json object and subtract m variable, which probably will be undefined.
Because Me-m is not a valid property name to be used as property hence you are forced to use as Maps read this or check here for a valid propert name
That's because '-' has a meaning in javaScript, so if you write Json.Me-m JavaScript interprets: 'Substract the value of the variable m to the value of the attribute Me of the object ``Json`'.
In javascript, you can access any member of an object by name, using [], and inside a string, javascript is not going to try to evaluate anything, so when you write Json[Me-m], it's interpreted as: 'Retrieve the value of the member named Me-m of the object Json'
The reason is simple: in Javascript each object it's an indexed key/value dictionary, accessing to the variable with the "." it's a shortcut, and obviously the fact of the "dictionary object" it's a performance issue, but despite of this matter, javascript it's really fast in comparison with other scripting language.
Answering your question, by desing an index can be a string containing the "-" char, but not a property... so if you have a property with that char, it won't be accessible with the "." notation.
Because you are passing a special character in key part of obj and for accessing that value of obj we need to use [].
eg. json['Me-m']

Accessing response data using Angular.js with numerical named field

My response object has a field called "50", so right now I'm trying to access and save that data doing something like this:
var thing = $scope.data.array[0].50;
However I'm getting an error on the console when I simply reload the page with the function not even running. When I get rid of the 50, everything is fine. There is indeed a field called "50" inside the $scope.data.array[0] and I do not have access to change the response. Is there something wrong with this because the field is called "50" and maybe JS is interrupting that as a number instead??
Also when I changed "50" to something random like "af", then I get no errors on refresh.
this doesn't work
var thing = $scope.data.array[0].50;
this works
var thing = $scope.data.array[0].af;
The following should work if your first element of the array has a property called "50".
var thing = $scope.data.array[0]["50"];
Property accessors provide access to an object's properties by using the dot notation or the bracket notation.
Syntax
object.property
object["property"]
JavaScript objects are also associative arrays (hashes). Using these you can associate a key string with a value string as shown in the example above.
The reason as to why you don't get an error when accessing $scope.data.array[0].af; is because "af" is valid identifier for a property. Dot notation only works with property names that are valid identifiers. An identifier must start with a letter, $, _ or unicode escape sequence.
For all other property names, you must use bracket notation.

variable in javascript statement

How does one add a variable string in this javascript statement?
where name may correspond to any valid string , say WebkitTransform or Moztransform,etc
document.getElementById('test').style.VARIABLE_NAME = 'rotate(15deg)';
My code doesn't seem to work when i set the VARIABLE_NAME to WebkitTransform, but it works fine if I use WebkitTransform directly, as in without naming it via a variable.
Thanks in advance :)
There are two ways to access members of a Javascript object.
Dot notation, which uses an identifier to access the member:
obj.member;
Bracket notation, which uses a string to access the member:
obj['member']
The latter uses a string to locate the member and you can just as easily use any expression. The value of the expression will be converted to a string so these are equivalent:
obj[{}]
obj['[object Object]']
If your expression is already a string it will be used as is, and in your case your variable holds a string so you can just do:
document.getElementById('test').style[VARIABLE_NAME] = 'rotate(15deg)';
There are 2 ways of accessing values in javascript objects. The first one is by using the dot operator(e.g. object.memberName). The second one is by using the square bracket notation(e.g. object['memberName']).

Javascript Object Access

I am using a Comet Push Engine called APE (Ajax Push Engine) and whenever I receive a realtime event I receive it in an javascript object called 'raw'.
So if for example if the raw object contains a 'location' value, I can print 'raw.location' and it will give me the value,
alert(raw.location);
So I have another object called currentSensor, which contains a value like this (in my example it would contain the string 'location'):
currentSensor.value
How do I programmatically use the currentSensor.value variable to access the 'raw' object? I have tried this:
var subsensor = currentSensor.sensorKey;
and then
alert(raw.subsensor);
But I keep getting undefined because the raw object doesn't contain a key called "subsensor" its actually "location". I hope this makes sense!
Thanks!
When using dot-notation, you use a literal property name. If you want to use a string, use square bracket notation.
foo.bar === foo['bar'];
Strings can be variables.
baz = 'bar';
foo.bar === foo[baz];
like this:
console.log(raw[currentSensor.value]);
Here you go:
alert(raw[subsensor]);
The dot syntax cannot help you when you need to access variable indexes. You need to use the array access method.
Note: The dot access method is just syntactic sugar and is not really needed in any place, but it is useful for code readability.
For your entertainment:
"1,2,3"["split"](",")["join"]("|")

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