I find myself needing to convert a string var (in JavaScript) to a variable name that is called when getting an element. My spontaneous solution to this was writing:
this.name = name;
[...]
this.context.drawImage(imageRepository.(this.name), this.x, this.y);
This does not work however, returning "Unexpected token (". Any suggestions?
You're looking for a property name, not a variable name. You can use bracketed notation for that:
imageRepository[this.name]
In JavaScript, you can refer to properties in two ways: Using dot notation and a property name literal (obj.foo), or using bracketed notation and a property name string (obj["foo"]). In the latter case, the string can be the result of any expression, including looking up a property on another object (this.name).
You need bracket notation in this case.
imageRepository[this.name]
The bracket notation evaluates the variable and selects the appropriate property.
Related
Consider below example:
a={
'firstProperty': "first",
'secondProperty':"second"
};
console.log(a[[[["firstProperty"]]]]);
by using multiple bracket notation i am able to access the firstProperty. How bracket notation is accessing this property??
You are using a nested array and by using a non string or symbol as value, the value is converted to string.
console.log([[["firstProperty"]]].toString());
Because what you provide as the key in a property accessor expression is converted to string if it isn't a Symbol or a string. console.log(a[[[["firstProperty"]]]]); uses an array of arrays as the property name in the accessor expression. Since that isn't a Symbol, it's converted to string. When you convert your array to string, you get the string "firstProperty" because that's how Array.prototype.toString works:
console.log(String([[["firstProperty"]]]));
...and "firstProperty" correctly identifies one of the properties in the object, so the property accessor expression gives you the value of that property.
Using an array like that is unnecessary. Just use
console.log(a["firstProperty"]);
or
console.log(a.firstProperty);
I ran into a problem using the Pipedrive API. I tried to get some data using the below but it returned an error:
$.each(data.data, function(key,value) {
console.log(value.0d1df598a5539ab5b6b410b339dc9218e0acb091);
});
However this works:
$.each(data.data, function(key,value) {
console.log(value.person_name);
});
Why can't I get values of the keys that are complex strings generated by the Pipedrive system?
To retrieve what you require you would need to use bracket notation as the first character of the property identifier is an integer. Try this:
var value = {
'0d1df598a5539ab5b6b410b339dc9218e0acb091': 'foo bar'
}
console.log(value['0d1df598a5539ab5b6b410b339dc9218e0acb091']);
A possible explanation can be summarized in two part
Valid javascript variable(identifier names)
An identifier must start with $, _, or any character in the Unicode categories Uppercase letter (Lu), Lowercase letter, Titlecase letter (Lt), Modifier letter (Lm), Other letter (Lo), or Letter number (Nl).
In your case the identifier name start with an integer(0)
Property accessors
An key of an object in js can be retrieved either by using dot (.) notation or by using Bracket[]` notation
Square brackets notation allows use of characters that cannot be used with dot notation and also to retrieve an identifier which is not valid according to the first point.Beside it also allows to access properties containing special characters.
This is because js interpreter automatically converts the expression within square brackets to a string & retrieves the corresponding value.Actually
js evaluates the first complete expression with square brackets in a statement, runs toString() on it to convert it into a string and then uses that value for the next bracket expression, on down the line till it runs out of bracket expressions.
So dot notation has marginal upper-hand since it wont go throught he above process.
But it cannot be use it with a variable(or number).
It only allow to access explicit key name of a property
Since the identifier in your object identifiers's name starts with an 0 , bracket notation like value['0d1df598a5539ab5b6b410b339dc9218e0acb091'] will give it's value.
In the examples I've seen so far using Symbols in ES6, you have to access symbol properties in Object literals using bracket notation:
let mySymbol = Symbol("mySymbol");
let someObject = {
[mySymbol]: "someValue"
};
console.log(someObject[mySymbol]); // "someValue"
Is there a way to define and access symbol properties using dot notation?
No. Symbols must be accessed using bracket notation.
Dot notation is only used for string keys that follow certain rule patterns, mostly about being a valid identifier.
Symbols are not strings, they are a whole something else entirely.
Short rational: One of the design goals of symbols is that they can not clash with property names, that makes them safe to use.
So, if you had an object like this
var = {
prop1: "Value"
};
And you created a symbol called prop1, how could you tell the two apart, and access them differently, using just object notation?
No. Dot notation is reserved for identifiers which resolve to property names (strings). And this won't change for backward compatibility reasons.
This question already has answers here:
JavaScript property access: dot notation vs. brackets?
(17 answers)
Closed 5 years ago.
What is the real difference in using [] and . for accessing array or object properties? Which one to use?
Also why doesn't . operator allow the index property?
Accessing members with . is called dot notation. Accessing them with [] is called bracket notation.
The dot notation only works with property names which are valid identifier names [spec], so basically any name that would also be a valid variable name (a valid identifier, see also What characters are valid for JavaScript variable names?) and any reserved keyword [spec].
Bracket notation expects an expression which evaluates to a string (or can be coerced to a string), so you can use any character sequence as property name. There are no limits to what a string can contain.
Examples:
obj.foo; // valid
obj.else // valid, reserved keywords are valid identifier names
obj.42 // invalid, identifier names cannot start with numbers
obj.3foo // invalid, ""
obj.foo-bar // invalid, `-` is not allowed in identifier names
obj[42] // valid, 42 will be coerced to "42"
obj["--"] // valid, any character sequence is allowed
obj[bar] // valid, will evaluate the variable `bar` and
// use its value as property name
Use bracket notation:
When the property name is contained in a variable, e.g. obj[foo].
The property name contains characters not permitted in identifiers, e.g. starts with a digit†, or contains a space or dash (-), e.g. obj["my property"].
Use dot notation: In all other situations.
There is a caveat though regarding reserved keywords. While the specification permits to use them as property names and with the dot notation, not all browsers or tools respect this (notably older IE versions). So the best solution in my opinion is to avoid using reserved keywords for property names or use bracket notation if you cannot.
†: That's also the reason why you can only use bracket notation to access array elements. Identifiers cannot start with digits, and hence cannot consist only of digits.
You should use . when you know the name of the property
var object = {};
object.property = 'whatever';
, use [] when the name of the property is contained in a variable
var object = {};
var property = 'another-property';
object[property] = 'whatever';
As #DCoder added certain object properties cannot be accessed without using the [] notation because their names break the syntax. E.g. properties named class, default, or data-prop-value
Also why doesn't . operator allow the index property? I really want
full reason. Thank you.
Well if that was possible, consider:
var a = 0.5;
Did you mean the number 0.5 or access the 5 element of the number?
See:
Number.prototype[5] = 3;
0[5] //3
0.5 // 0.5
If you allowed the syntax 0.5 to be equal to 0[5], then how do you know what you mean?
It is however possible to use numbers directly with object literal:
var a = {
0: 3,
1: 5
};
Both dot operator and index(bracket notation) operator are used to access the property of an Object. Generally accessing with dot operator is quite faster because accessing variables by window is significantly slower though. But in case of special character
in the variables, you cannot use dot operator as it will give error. For such cases we need to use index operator and pass the variable name as a string format means underdouble quote otherwise it will give undefined error.
e.g-
var abc = {
font-size : "12px"
}
Using dot operator - abc.font-size; //it will give error (Incorrect)
Using index operator - abc["font-size"]; //12px (Correct)
What's the simplest way to call a string method with jquery?
If I understand your question correctly, you want to call a method whose name is stored in a string.
If that's the case, you should use square bracket notation instead of (not in addition to) dot notation, and delimit the literal string with quotes:
$["STRINGVALUE"]();
You can also use the variable you defined initially, without quotes:
$[myFunction]();