Can you explain the diffrence between two javascript code snippets below? - javascript

i was asked to write out a piece of code that inverts all key value pairs from an object passed in. this is what i wrote out.
invert(object){
let newObj = {};
for(let key in object){
const original = object[key];
newObj = {original : key}
}
return newObj;
}
aparently that dosent work ( and i cant figure out why). the answer to the question was
invert(object){
let invertedObject = {};
for(let key in object){
const originalValue = object[key];
invertedObject = {originalValue : key}
}
return invertedObject;
}
when ran with with a test file, my code fails while the answer code passes. reasoning was mine is returning undefined.
error code from test :
Failed: _.invert({originalKey: "originalValue"})["originalValue"])
returned undefined instead of originalKey

Difference is in the following statement:
newObj = {original : key}
In your code, when you use original as a key in the newObj, instead of using the value of the original variable as a key, 'original' is used literally as a key.
The original variable is unused in your code.
You can fix the problem by using computed property name as:
newObj = { [original]: key };
Without using the computed property, newObj will be:
{
original:"originalKey"
}
but with computed property name, newObj will be:
{
originalValue:"originalKey"
}
The second code example works because in the following statement
invertedObject = {originalValue : key}
they have used the value of the originalKey as the name of the key. So the key of the returned object will be 'originalValue'.
Also note that the following statement in the second code example
const originalValue = object[key];
is unnecessary because originalValue variable is not being used. Second code example will also work without the above statement.
If second code example uses computed property name as:
invertedObject = { [originalValue] : key }
then you need the statement that declares the originalValue variable.
Personally, I think second code example is error prone and is really easy to break; its also not practical for making a reusable function because:
objects passed to this function won't always have a single key
you won't always know the value of each key in the object and even if you do know the values, if the object has multiple keys, this approach won't work
You should use computed property name which doesn't depends on a variable name being the same as the value of a key in the object.

that inverts all key value pairs from an object passed in
You have to add each property to the same object and not create new object for each property.
function invert(object) {
let newObj = {};
for(let key in object){
const original = object[key];
newObj[original] = key; // add property to same 'newObj' object
}
return newObj;
}
console.log(invert({originalKey: "originalValue", anotherKey: "anotherValue"}))

Related

findKey() - recreating lodash library method

I've got a problem with a CodeCademy task. I am to re-create the findKey lodash library method. Here there are the steps of how to do it, but I got stuck, especially at point 5.
Add a method to our _ object called findKey.
Add two parameters to this method: object and predicate. We will
name our predicate function parameter predicate since this is the
name used in the Lodash documentation.
Within the method, use a for ... in loop to iterate through each key
in object.
Within the loop, create a variable called value and set it equal to
the value at the current key in object.
Still within the loop, create another variable called
predicateReturnValue and set it equal to the result of calling
predicate with value.
Finally, still within the loop, use an if statement to check
if predicateReturnValue is truthy. If it is, return the current key
from the method.
Outside of the loop, return undefined to address all cases where no
truthy values were returned from predicate.
This is my code that doesn't work:
findKey(object, predicate) {
for (let key in object) {
let value = object[key];
let predicateReturnValue = predicate(value);
if (predicateReturnValue === 'true') {
return value;
};
};
return undefined;
}
I appreciate your help!
You need to return the key after the truty check of the call of predicate.
function findKey(object, predicate) {
for (let key in object) {
let value = object[key];
let predicateReturnValue = predicate(value);
if (predicateReturnValue) { // just take the value
return key; // return key
}
}
}
const
isStrictEqual = a => b => a === b,
object = { a: 'foo', b: 'bar', c: 'baz' }
console.log(findKey(object, isStrictEqual('bar')));
console.log(findKey(object, isStrictEqual('cat')));

How to add a value to an Object in javascript?

Let's take an example in javascript
var b = function(){
var key = {};
var result = [];
var a =
[{people: "people1"},
{people: "people2"},
{people: "people2"},
{people: "people3"}]
for(i=0;i<a.length;i++)
{
var val = a[i][people];
if(angular.isUndefined(key[val]))
{
Key[val] = "abc"; /////This line is foreign to my knowledge.
result.push(val);
}
}
return result;
}
Now in this Example i am creating an object Key and a array result.
The for loop will loop through the a variable and store the value of people property in the var val.
The angular.Isundefined function check whether the key[val] contains any duplicate data if not then it will add using the
Key[val] = "abc".
1) Now i have no idea how this line is creating the value and key pair in the key object.
2) Please tell me other ways to add value to the object.
O/P is as follows
key = Object {people1: abc, people2: abc, people3: abc}
hence it is adding value to key object without duplicating the value.
P.S. it is just an example not the real code.
From the link of Andreas in comments
I think this solves my problem.
the other way to add the key and value to the JSON object is like this.
obj = {};
obj[people1] = "data";
obj[people2] = "data";
obj[people3] = "data";
console.log(obj);
The key cannot be same but the value can be same.
So this is what my question was
Key[val] = "abc";
this line is getting the val variable dynamically and adding the val variable as the key and the value is abc.
Twist:
Do you think that
key.val = "abc";
work?
No:
This is what provided by the site
Any property name that is not a valid JavaScript identifier (for example, a property name that has a space or a hyphen, or that starts with a number)
can only be accessed using the square bracket notation. This notation is also very useful when property names are to be dynamically determined (when the property name is not determined until runtime).
in the following example i add count in my json my json come from ajax but i want to add count so add count below code that's work fine for me here .count is my custom value added by me, it is not part of my response
$scope.data = response.items;
for (var i = 0; i < response.items.length; i++) {
response.items[i].count = i;
}

How to access properties of object by dot operator in Javascript

var changeJsonKeyName, newObj, obj;
changeJsonKeyName = function(json, oldName, newName) {
json[newName] = json[oldName];
delete json[oldName];
// json.newName = json.oldName;
// delete json.oldName;
// if i use point in this ,i can not get my result that i want
return json;
};
obj = {
'aaa': '1111',
'bb': {
'cc': 333
}
};
newObj = {};
newObj = changeJsonKeyName(obj, 'aaa', 'nnn');
console.log(newObj);
If I use point here ,I can not get my result that's what I want ,what is the wrong,please help me,thank you very much.
I'm not sure if I understood you correctly, but :
json[newName]
access property named with the value of newName variable
json.newName
access a property named 'newName', which does not exist
First, as a comment points out, this is a Javascript question, not a JSON question.
But you seem to be asking why this works:
json[newName] = json[oldName];
delete json[oldName];
but this doesn't:
json.newName. = json.oldName.;
delete json.oldName;
does not.
And the answer is the second form is actually equivalent to
json["newName"] = json["oldName"];
delete json["oldName"];
In other words, you are dealing with attributes whose names are the constants "oldName" and "newName" rather than attributes whose names are passed as parameters to that method.

How to set a Javascript object values dynamically?

It's difficult to explain the case by words, let me give an example:
var myObj = {
'name': 'Umut',
'age' : 34
};
var prop = 'name';
var value = 'Onur';
myObj[name] = value; // This does not work
eval('myObj.' + name) = value; //Bad coding ;)
How can I set a variable property with variable value in a JavaScript object?
myObj[prop] = value;
That should work. You mixed up the name of the variable and its value. But indexing an object with strings to get at its properties works fine in JavaScript.
myObj.name=value
or
myObj['name']=value (Quotes are required)
Both of these are interchangeable.
Edit: I'm guessing you meant myObj[prop] = value, instead of myObj[name] = value. Second syntax works fine: http://jsfiddle.net/waitinforatrain/dNjvb/1/
You can get the property the same way as you set it.
foo = {
bar: "value"
}
You set the value
foo["bar"] = "baz";
To get the value
foo["bar"]
will return "baz".
You could also create something that would be similar to a value object (vo);
SomeModelClassNameVO.js;
function SomeModelClassNameVO(name,id) {
this.name = name;
this.id = id;
}
Than you can just do;
var someModelClassNameVO = new someModelClassNameVO('name',1);
console.log(someModelClassNameVO.name);
simple as this
myObj.name = value;
When you create an object myObj as you have, think of it more like a dictionary. In this case, it has two keys, name, and age.
You can access these dictionaries in two ways:
Like an array (e.g. myObj[name]); or
Like a property (e.g. myObj.name); do note that some properties are reserved, so the first method is preferred.
You should be able to access it as a property without any problems. However, to access it as an array, you'll need to treat the key like a string.
myObj["name"]
Otherwise, javascript will assume that name is a variable, and since you haven't created a variable called name, it won't be able to access the key you're expecting.
You could do the following:
var currentObj = {
name: 'Umut',
age : 34
};
var newValues = {
name: 'Onur',
}
Option 1:
currentObj = Object.assign(currentObj, newValues);
Option 2:
currentObj = {...currentObj, ...newValues};
Option 3:
Object.keys(newValues).forEach(key => {
currentObj[key] = newValues[key];
});

How can I store reference to a variable within an array?

I'm trying to create an array that maps strings to variables. It seems that the array stores the current value of the variable instead of storing a reference to the variable.
var name = "foo";
var array = [];
array["reference"] = name;
name = "bar";
// Still returns "foo" when I'd like it to return "bar."
array["reference"];
Is there a way to make the array refer to the variable?
Put an object into the array instead:
var name = {};
name.title = "foo";
var array = [];
array["reference"] = name;
name.title = "bar";
// now returns "bar"
array["reference"].title;
You can't.
JavaScript always pass by value. And everything is an object; var stores the pointer, hence it's pass by pointer's value.
If your name = "bar" is supposed to be inside a function, you'll need to pass in the whole array instead. The function will then need to change it using array["reference"] = "bar".
Btw, [] is an array literal. {} is an object literal.
That array["reference"] works because an Array is also an object, but array is meant to be accessed by 0-based index. You probably want to use {} instead.
And foo["bar"] is equivalent to foo.bar. The longer syntax is more useful if the key can be dynamic, e.g., foo[bar], not at all the same with foo.bar (or if you want to use a minimizer like Google's Closure Compiler).
Try pushing an object to the array instead and altering values within it.
var ar = [];
var obj = {value: 10};
ar[ar.length] = obj;
obj.value = 12;
alert(ar[0].value);
My solution to saving a reference is to pass a function instead:
If the variable you want to reference is called myTarget, then use:
myRef = function (newVal) {
if (newVal != undefined) myTarget = newVal;
return myTarget;
}
To read the value, use myRef();. To set the value, use myRef(<the value you want to set>);.
Helpfully, you can also assign this to an array element as well:
var myArray = [myRef];
Then use myArray[0]() to read and myArray[0](<new value>) to write.
Disclaimer: I've only tested this with a numerical target as that is my use case.
My solution to saving a reference is to pass a function instead:
If the variable you want to reference is called 'myTarget', then use:
myRef = function (newVal) {
if (newVal != undefined)
myTarget = newVal;
return myTarget;
}
To read the value, use myRef();. To set the value, use myRef(value_to_set);.
Helpfully, you can also assign this to an array element as well:
var myArray = [myRef];
Then use myArray0 to read and myArray[0](value_to_set) to write.
Disclaimer: I've only tested this with a numerical target as that is my use case.

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