Using a functions argument to find and objects key value pair? - javascript

I'm trying to find a key value pair value but i want to pass the key in as an argument but it isn't seeming to work.
function drawPieChart(){
var findData = function(variable){
return dailyLog.find({createdBy: Meteor.user().username}, {fields: {variable: 1}}).fetch()[0].variable;
};
var data = [
{
value: findData(adherence),
color: "#CBDDE7"
}...
I want variable to be passed in twice, once to sort and other to find the value but it is actually looking for the key value pair "variable" which obviously doesn't exist.
How do i make it be seen as the argument?

There are two aspects to this:
Creating the fields object with a property whose name is the value of variable rather than the literal name variable, and
Accessing the resulting field based on the value of the variable (rather than the literal name variable)
Dealing with #2 first because it's easier: In JavaScript, you can access a property on an object either using dot notation and a property name literal (obj.foo), or using brackets notation and a property name string* (obj["foo"]). In the latter case, the string can be the result of any expression, including a variable lookup. So if variable contains "foo", then obj[variable] will get or set the foo property on obj.
Back to #1: For now, you have to create the object you're going to pass as fields first and then assign the property value via brackets notation rather than in an object initializer:
var fields = {};
fields[variable] = 1;
If variable contains "foo", then fields[variable] = 1 sets the foo property on fields to 1.
So putting that all together:
var findData = function(variable){
var fields = {};
fields[variable] = 1;
return dailyLog.find({createdBy: Meteor.user().username}, {fields: fields}).fetch()[0][variable];
// Note ------------------------------------------------------------------------------^--------^
};
In the next version of JavaScript, ECMAScript6 (aka ES6), you'll be able to do #1 with a "computed property name" in the object initializer (and still retrieve it with brackets notation). Perhaps unsurprisingly, computed property names use...brackets!
// ES6 only!!
var findData = function(variable){
return dailyLog.find({createdBy: Meteor.user().username}, {fields: {[variable]: 1}}).fetch()[0].[variable];
// Note ------------------------------------------------------------^--------^ --- and ---------^--------^
};
* Side note: In ES6, brackets notation can be used with things called Symbols as well as strings. It's not relevant to your question, but I said "string" above, and soon that won't be true, so...

Related

Complex use of function parameter in javascript

function me(a, b) {
function he(c, d) {
if(!a[c]) {
}
}
}
Please someone explain 'if(!a[c])' for me.
why this square bracket is used here in [c] though it is a parameter. it is not an array obviously.
what does if(!a[c]) make sense? how two parameters are used like this?
There is nothing special about that code.
It is saying, in English, If the property c of a is falsey, then the condition is true.
In JavaScript, bracket notation can be used to access properties of an object or members of an array.
For example, someArray[5] will access the 6th member of the array, while someObject['someProp'] will access someProp of the object someObject.
The argument a is likely an object. The syntax:
if (!a[c])
Checks to see if the property in the variable c on the a object does not have a truthy value. This could be if it was null, false, undefined or any other falsey value.
The bracket notation can be used with property names. So, if you have an object like this:
var x = { name: "John"};
Then, you can access that property like in any of these ways:
x.name
x["name"];
// Or, if the property name is in a variable
var prop = "name";
x[prop]
Your example is using a version of the last of the above three options when the property name is in another Javascript variable.
In javascript, properties can be accessed dynamically using the square-bracket syntax.
Consider the following:
var person = {name:'Sarah'};
console.log(person.name); // logs 'Sarah';
Sometimes, you might want to dynamically access properties of an object, using a variable that holds the name of the property you want to access. The above example could also be written like this:
var person = {name:'Sarah'};
var prop = 'name';
console.log(person[prop]); // logs 'Sarah';

Get data from array of object and put it to key value

I want to take that._mCon.usa which have array of object's and put the data like key value
in _mUsa object,In the object instance I've name and path ,I try like following and its not working the _mUsa is not filled with data...any idea what im doing wrong here ?
_mUsa{
},
for(var i = 0; i <= that._mCon.usa.length; i++) {
that._mUsa[that._mCon.usa[i][name]] = that._mUsa[that._mCon.usa[i][path]];
}
this is that._mCon.usa with the name and path properties
Object properties are accessed using .propertyname, so it should be:
that._mUsa[that._mCon.usa[i].name] = that._mUsa[that._mCon.usa[i].path];
You use [name] when the property name is dynamic, and name is a variable containing the property name.
You can use [] with a literal string, e.g. ['name'] and ['path'], but there's little point to that; if the property is known, just use the normal dot notation.

Use key's value as key in key-value pair in Javascript

How can you use the value of a key-value pair as the key in a different key-value pair in javascript?
I'd like to do the following:
var gizmos = {gizmo1: "G1"};
var things = {gizmos.gizmo1: "T1"};
So that things essentially equals:
var things = {"G1": "T1"};
Like this:
var gizmos = {gizmo1: "G1"};
var things = {};
things[gizmos.gizmo1] = "T1";
There's no way to do it as part of the object initializer (aka object "literal"), you have to do it after.
The reason it works is that in JavaScript, you can access (get or set) a property on an object in two ways: Either using dotted notation and a literal, e.g. foo.bar, or using bracketed notation and a string, e.g. foo["bar"]. In the latter case, the string doesn't have to be a string literal, it can be the result of any expression (including, in this case, a property lookup on another object).
Side Note: If you change gizmos.gizmo1 after you do the things[gizmos.gizmo1] = "T1"; line, it does not change the name of the property on things. There's no enduring link, because the value of gizmos.gizmo1 was used to determine the property name during the things[gizmos.gizmo1] = "T1"; line (just like it is in any other expression).
var gizmos = {gizmo1: "G1"};
var things = {};
things[gizmos.gizmo1]="T1";
To get the value for a given key on an object use object["key"] or object.key

Can an Object object be coerced into an Array object?

Crockford writes in http://javascript.crockford.com/survey.html:
"There are two ways to make a new array:
var myArray = [];
var myArray = new Array();"
So I'm confused by these two lines in some AJAX code I am reading:
var obj={}; // obj is an Object object (i.e. a hash table)
obj[4] = 'x'; // now obj is suddenly an Array object via an integer key?
In JavaScript are an object and an array really just the same thing, but with a variant on the key type?
In other words, is this the same as in php where we can use either a name (string) or an integer for a hash key?
I've Googled for an answer on this but can't seem to nail down an article which discusses this issue.
One possibility that comes to mind is that perhaps the first line is syntactic lint because the 2nd line overwrites the previous definition of obj as it creates a new Array object.
it does not become an array, it is simply an Object with a '4' property, like this:
var obj = {
'4': 'x'
};
it is just converted to a string when used as a property like obj['4'] = 'x';
Everything but primitive datatypes is an object in JavaScript. Objects can have a properties and there are two ways to access object properties:
Dot notation, foo.bar, which you can use as long as the property name is a valid identifier.
Bracket notation, foo['bar'] which you have to use if the key is not a valid identifier [spec]. For example, if it is a number, or contains a space or you have a variable with the name.
Hence, bracket notation is not a characteristic of arrays and if you see it, it does not mean the value is an array. It is simple one of two ways of accessing properties.
The elements of an array are just properties with numeric keys. Arrays are built on top of objects and implement some additional methods which treat these numeric properties in a special way. For example the .length property is automatically updated when you add new elements. But ultimately they are just normal properties.
In your example you have a simple object. You have to access the property with obj[4] or obj['4'] because obj.4 is invalid since 4 is not a valid identifier (basically everything that you can use as variable name is a valid identifier. var 4 = 'foo'; is invalid).
And since arrays are just objects, if you could use numbers as identifiers, you were also able to access an element with arr.4.
As far as I know, no, an object can't be coerced into an array. But, it can look and act like an array, and that's what's happening here. Numbers, and anything else that can be coerced to a string, are perfectly valid property names for Javascript objects, so
obj[4] = 1;
obj['spam'] = 2;
are both valid ways of setting a property on the object. That doesn't make the object an array. An Array is a special class of object with specific methods (.slice(), .concat(), etc) and a length property that's kept up to date with the number of items in the array.
Yes
Javascript Array is very different from tradition array, you can think of it as object.
var array = [1,2,3] is equivalent to var object = {'0' : 1, '1' : 2, '2' : 3}
except array inherited from Array.prototype and object inherited from Object.prototype, where Array.prototype will contain method such as length.
Javascript is a loosely-typed, prototype-based language. Even primitive types like a boolean can be treated like an object (though you aren't going to get far). Almost everything in javascript is, at root, an object.
Understanding this, an array IS an object. You can arbitrarily add properties to any object:
var xml = new XMLHttpRequest();
xml[4] = 'x';
console.log(xml);
That object is still an instance of XMLHttpRequest. It now has a property labeled 4 with a value of x. You can treat anything like this -- even a function:
var test_func = function () {
alert('woah!');
}
test_func[4] = 'x';
console.log(test_func[4]);
The take-away here is that the obj[key] = value notation is NOT indicative of an "array" type, like it is in languages such as PHP. Rather, it is an alternate way to access properties of any object, and is equivalent to obj.key = value (you can't use obj.4 = 'x', though, that's invalid syntax). The other take-away is that any object in javascript can be modified or used in pretty much any way. You shouldn't misuse objects, but you can
Check it out here: http://jsfiddle.net/w2AqJ/
Documentation
Array on MDN - https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Array
Javascript "associative arrays" considered harmful by Andrew Dupont - http://andrewdupont.net/2006/05/18/javascript-associative-arrays-considered-harmful/

how to get the value 'dynamically' from an associative array in javascript?

i am trying to get a value from a key stored on a string variable proyNombre, but whenever i call it via the common method "myAssociativeArray.MyKey", it gets the variable 'proyNombre' as the key, instead of getting its value and passing it as a key.
proyectos.each(function(index){
var proyNombre = this.value;
if(!(proyNombre in myArray)){ // whenever the variable is undefined, define it
myArray[proyNombre] = horas[index].value-0 + minutos[index].value/60;
}
else{
console.log(myArray.proyNombre); //This doesnt work, it tries to give me the value for the key 'proyNombre' instead of looking for the proyNombre variable
console.log(myArray.this.value); //doesnt work either
}
});
Try:
console.log(myArray[proyNombre]);
myArray is actually an object in javascript. You can access object properties with object.propertyName or, object['propertyName']. If your variable proyNombre contained the name of a property (which it does) you can use the second form, like I did above. object.proyNombre is invalid - proyNombre is a variable. You can't do for example:
var myObject = {};
myObject.test = 'test string';
var s = 'test';
console.log(myObject.s); // wrong!!
but you could then do:
console.log(myObject.test);
console.log(myObject['test']);
console.log(myObject[s]);
You need to use the same syntax you used to set the value:
console.log(myArray[proyNombre]);
Simply access the value with myArray[proyNombre].
You're doing it right in the assignment: myArray[proyNombre]. You can use the same method to retrieve the variable.
If you change:
console.log(myArray.proyNombre);
console.log(myArray.this.value);
to
console.log(myArray[proyNombre]);
console.log(myArray[this.value]);
You should get the same value (the value for the key represented by the variable proyNombre) logged twice.
It's true that Javascript doesn't have associative arrays but objects in Javascript can be treated like associative arrays when accessing their members.

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