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JavaScript and Consts [duplicate]

This question already has answers here:
How to use a variable for a key in a JavaScript object literal?
(16 answers)
Closed 4 months ago.
const A = 0;
const LOOKUP = { A : "A"};
console.log(LOOKUP[A]);
console.log(LOOKUP[0]);
Result:
undefined
undefined
Second try:
var A = 0;
const LOOKUP = { A : "A"};
console.log(LOOKUP[A]);
console.log(LOOKUP[0]);
Result:
undefined
undefined
How am I supposed to do this then? And can somebody explain why this doesn't work in JavaScript the way one would expect it to work coming from other languages?

The correct way is:
const A = 0;
const LOOKUP = {};
LOOKUP[A] = 'A';
console.log(LOOKUP[A]);
console.log(LOOKUP[0]);

const LOOKUP = { A : "A"};
The left side of the colon means that the key is the string "A". The string part is implicit, since all keys are strings (or symbols). So to access the property, you need to do console.log(LOOKUP.A) or console.log(LOOKUP["A"])
If you want the key to be a computed value, then you need to use square brackets:
const LOOKUP = { [A]: "A" };
That means that we should resolve the variable A, and use its value as the key. That key is the number 0, which then gets coerced into the string "0". You can then look it up by any of console.log(LOOKUP["0"]), console.log(LOOKUP[0]), or console.log(LOOKUP[A])

Looks like you are searching for some enums (typescript):
enum ETest {
A = 1
};
console.log(ETest['A']); // 1
console.log(ETest[1]); // A

Doing LOOKUP[A] is like doing LOOKUP[0] which is undefined.
You should try it as
console.log(LOOKUP["A"])

This has nothing to do with const or var keyword. The way you are trying to access an object property is incorrect.
const A = 0;
const LOOKUP = { A : "A"};
console.log(LOOKUP["A"]); // Correct Approach: Property access through bracket notation should be done using a string (or a variable assigned to a string).
console.log(LOOKUP[0]); // Property `0` doesn't exist on object `LOOKUP`, so it'll return `undefined`.

Jquery object assign to variable return undefined [duplicate]

This question already has answers here:
Convert a JavaScript string in dot notation into an object reference
(34 answers)
Closed 7 years ago.
Working SO code below:
var property_object_parse = jQuery.parseJSON('{"p1":{"TextElement":[{"yscale":"100","xscale":"100","shadow":"false","rotation":"","id":"#1_company name","text":"Company name","font":"Plantagenet Cherokee","size":"9","h":"0.16","w":"1.15","y":"0.3","x":"0.53","lock":"false","customize":"false","Alpha":"100","underline":"false","italic":"","blockName":"company name","caption":"Company name","textType":"Company name","bold":"false","colorspace":"DeviceRGB","colorcode":"rgb(1.00,1.00,1.00)","color":"16777215","hex_color":"","valign":"center","align":"left","sortorder":"0","inputformat":false,"format":false,"clippath":false,"inputrequired":false},{"yscale":"100","xscale":"100","shadow":"false","rotation":"","id":"#1_first name","text":"First name","font":"Plantagenet Cherokee","size":"7","h":"0.1","w":"1.14","y":"0.44","x":"2.3","lock":"false","customize":"false","Alpha":"100","underline":"false","italic":"","blockName":"first name","caption":"First name","textType":"First name","bold":"false","colorspace":"DeviceRGB","colorcode":"rgb(1.00,1.00,1.00)","color":"16777215","hex_color":"","valign":"center","align":"right","sortorder":"1","inputformat":false,"format":false,"clippath":false,"inputrequired":false},{"yscale":"100","xscale":"100","shadow":"false","rotation":"","id":"#1_last name","text":"Last Name","font":"Plantagenet Cherokee","size":"6","h":"0.11","w":"1.16","y":"0.56","x":"2.3","lock":"false","customize":"false","Alpha":"100","underline":"false","italic":"","blockName":"last name","caption":"Last Name","textType":"Last Name","bold":"false","colorspace":"DeviceRGB","colorcode":"rgb(1.00,1.00,1.00)","color":"16777215","hex_color":"","valign":"center","align":"right","sortorder":"2","inputformat":false,"format":false,"clippath":false,"inputrequired":false},{"yscale":"100","xscale":"100","shadow":"false","rotation":"","id":"#1_email","text":"Email","font":"Plantagenet Cherokee","size":"6","h":"0.1","w":"1.14","y":"0.69","x":"2.3","lock":"false","customize":"false","Alpha":"100","underline":"false","italic":"","blockName":"email","caption":"Email","textType":"Email","bold":"false","colorspace":"DeviceRGB","colorcode":"rgb(1.00,1.00,1.00)","color":"16777215","hex_color":"","valign":"center","align":"right","sortorder":"3","inputformat":false,"format":false,"clippath":false,"inputrequired":false},{"yscale":"100","xscale":"100","shadow":"false","rotation":"","id":"#1_phone","text":"Phone","font":"Plantagenet Cherokee","size":"6","h":"0.11","w":"1.14","y":"0.81","x":"2.3","lock":"false","customize":"false","Alpha":"100","underline":"false","italic":"","blockName":"phone","caption":"Phone","textType":"Phone","bold":"false","colorspace":"DeviceRGB","colorcode":"rgb(1.00,1.00,1.00)","color":"16777215","hex_color":"","valign":"center","align":"right","sortorder":"4","inputformat":false,"format":false,"clippath":false,"inputrequired":false},{"yscale":"100","xscale":"100","shadow":"false","rotation":"","id":"#1_street","text":"street","font":"Plantagenet Cherokee","size":"9","h":"0.17","w":"1.16","y":"0.95","x":"2.3","lock":"false","customize":"false","Alpha":"100","underline":"false","italic":"","blockName":"street","caption":"street","textType":"street","bold":"false","colorspace":"DeviceRGB","colorcode":"rgb(1.00,1.00,1.00)","color":"16777215","hex_color":"","valign":"center","align":"right","sortorder":"5","inputformat":false,"format":false,"clippath":false,"inputrequired":false}]},"p2":{"ImageElement":[{"type":"image","text":"Sample.png","blockName":"Back Logo","id":"#2_Back Logo","lock":"false","rotation":"0","h":"0.68","w":"0.69","y":"0.59","x":"0.41","valign":"bottom","halign":"left","clippath":false}],"TextElement":[{"yscale":"100","xscale":"100","shadow":"false","rotation":"","id":"#2_Company Name","text":"Company Name","font":"Plantagenet Cherokee","size":"9","h":"0.16","w":"1.16","y":"1.3","x":"0.24","lock":"false","customize":"false","Alpha":"100","underline":"false","italic":"","blockName":"Company Name","caption":"Company Name","textType":"Company Name","bold":"false","colorspace":"DevicaeRGB","colorcode":"rgb(1.00,0.00,0.00)","color":"16711680","hex_color":"","valign":"bottom","align":"center","sortorder":"0","inputformat":false,"format":false,"clippath":false,"inputrequired":false},{"yscale":"100","xscale":"100","shadow":"false","rotation":"","id":"#2_Tagline","text":"Sample Tagline","font":"Helvetica","size":"12","h":"0.18","w":"0.73","y":"1.52","x":"0.25","lock":"false","customize":"false","Alpha":"100","underline":"false","italic":"","blockName":"Tagline","caption":"Sample Tagline","textType":"Sample Tagline","bold":"false","colorspace":"DeviceRGB","colorcode":"rgb(1.00,1.00,1.00)","color":"16777215","hex_color":"","valign":"top","align":"left","sortorder":"1","inputformat":false,"format":false,"clippath":false,"inputrequired":false}]}}');
var selected_element = 'p1.TextElement[0].size';
console.log(property_object_parse.selected_element);
I am getting undefined when I try to get the value using variable. Is this correct way to access it?
See my jsfiddle here

property_object_parse is a real JavaScript object, so you can just use the member access syntax to access the value you are interested in directly:
console.log(property_object_parse.p1.TextElement[0].size);
Note that you cannot use a dynamic property path string like 'p1.TextElement[0].size', you would have to compile that in a way. For example, you could instead have an array of properties you are trying to access:
var selected_element = ['p1', 'TextElement', '0', 'size'];
var obj = property_object_parse;
for (var i = 0; i < selected_element.length; i++) {
obj = obj[selected_element[i]];
}
console.log(obj);
That has the same result as accessing it all directly as above with property_object_parse.p1.TextElement[0].size.

How to add an object in an array? [duplicate]

This question already has answers here:
How to use a variable for a key in a JavaScript object literal?
(16 answers)
Closed 8 years ago.
I am trying to push object into an array
I have something like these
for(var i=0; i<test.length; i++){
console.log(test[i])
console.log(items[test[i]])
productObj.push( { test[i] : items[test[i]]} );
}
both console.log show stuff but I have an error on the push(). The error is "Uncaught SyntaxError: Unexpected token ["
Can anyone help me about this weird issue? Thanks!

Object literal keys need to be constant. test[i] isn't going to work. You'll need to create an empty object and add the key/value pair:
var o = {};
o[ test[i] ] = items[test[i]];
productObj.push( o );

The keys in an object literal are not evaluated and must be valid identifiers.
You would get the same error just from the object literal: { test[i]: 1 }.
You can use the new keyword in conjunction with an anonymous function to create the object instead:
productObj.push( new function () { this[test[i]] = items[test[i]]; } );
If you find that the above is less readable, then you should create the object in advance:
var temp = {};
temp[test[i]] = items[test[i]];
productObj.push( temp );

The key of an entry in the {} syntax for creating objects is not looked up as a variable, it must be a valid identifier. test[i] is not a valid identifier due to the brackets, and even if it was it would not get evaluated.
To create a key from test[i], you have to manually add it using the [] syntax.
var v = {};
v[test[i]] = ...

jQuery expression as object key [duplicate]

This question already has answers here:
How to create an object property from a variable value in JavaScript? [duplicate]
(9 answers)
Accessing an object property with a dynamically-computed name
(19 answers)
Closed 8 years ago.
I need to figure out how to create a dynamic key string for an object. This expression makes JavaScript complain.
return {$(this).val(): true}; // returns an object e.g. {2: true}
What am I doing wrong?

You have to create the object, then use bracket notation for the dynamic key
var obj = {};
var val = $(this).val();
obj[val] = true;
return obj;
or a completely unnecessary one-liner
return (function(o,e) {o[e.value]=true; return o;})({}, this);

The JavaScript object literal syntax {x: y} specifies that x will be a (possibly) quoteless string, and y any value. You can't use this syntax for dynamic keys.
Use this instead:
var foo = {};
foo[$(this).val()] = true;
return foo;

create object using variables for property name [duplicate]

This question already has answers here:
How to use a variable for a key in a JavaScript object literal?
(16 answers)
Closed 7 years ago.
Is it at all possible to use variable names in object literal properties for object creation?
Example
function createJSON (propertyName){
return { propertyName : "Value"};
}
var myObject = createJSON("myProperty");
console.log(myObject.propertyName); // Prints "value"
console.log(myObject.myProperty); // This property does not exist

If you want to use a variable for a property name, you can use Computed Property Names. Place the variable name between square brackets:
var foo = "bar";
var ob = { [foo]: "something" }; // ob.bar === "something"
If you want Internet Explorer support you will need to use the ES5 approach (which you could get by writing modern syntax (as above) and then applying Babel):
Create the object first, and then add the property using square bracket notation.
var foo = "bar";
var ob = {};
ob[foo] = "something"; // === ob.bar = "something"
If you wanted to programatically create JSON, you would have to serialize the object to a string conforming to the JSON format. e.g. with the JSON.stringify method.

ES6 introduces computed property names, which allow you to do
function CreateJSON (propertyName){
var myObject = { [propertyName] : "Value"};
}
Note browser support is currently negligible.

You can sort of do this:
var myObject = {};
CreateProp("myProperty","MyValue");
function CreateProp(propertyName, propertyValue)
{
myObject[propertyName] = propertyValue;
alert(myObject[propertyName]); // prints "MyValue"
};
I much perfer this syntax myself though:
function jsonObject()
{
};
var myNoteObject = new jsonObject();
function SaveJsonObject()
{
myNoteObject.Control = new jsonObject();
myNoteObject.Control.Field1= "Fred";
myNoteObject.Control.Field2= "Wilma";
myNoteObject.Control.Field3= "Flintstone";
myNoteObject.Control.Id= "1234";
myNoteObject.Other= new jsonObject();
myNoteObject.Other.One="myone";
};
Then you can use the following:
SaveJsonObject();
var myNoteJSON = JSON.stringify(myNoteObject);
NOTE: This makes use of the json2.js from here:http://www.json.org/js.html

One thing that may be suitable (now that JSON functionality is common to newer browsers, and json2.js is a perfectly valid fallback), is to construct a JSON string and then parse it.
function func(prop, val) {
var jsonStr = '{"'+prop+'":'+val+'}';
return JSON.parse(jsonStr);
}
var testa = func("init", 1);
console.log(testa.init);//1
Just keep in mind, JSON property names need to be enclosed in double quotes.