This question already has answers here:
Why can't I access a property of an integer with a single dot?
(5 answers)
Closed 6 years ago.
56.toString not works throws exception . 56..toString works fine .
Can anyone explain what is the difference between these two statements ?
56.toString(); //throws exception
56..toString(); //Works Fine.
How it works?
Numeric literals are somewhat special, because the property access . can be conflated with a decimal point .
When the parser encounters 56. it expects a digit (or nothing) to follow this. When you write 56..toString() you are getting the toString() of 56.0
that's not problem for toString() method, just the 56. the toString will think it's 56.0, but it have not the '0', so it will fail.
work fine or like this:
(56.).toString()
or
56..toString()
The problem here is when you type
56.toString()
JavaScript thinks 56 is a variable name, and you are trying to access it's toString() method. But this doesn't work because 56 is not a variable, and it's not even a valid variable name, therefore results in a compile error. (Variable name must start with a letter)
Check here for JS variable naming rules
But when you add a dot behind the 56 (56.) it becomes a number literal, and language creates an instance of Number class for that, then tries to resolve toString() method on the Number instance. Since the Number class defines a method called toString() and then it works.
56..toString();
Is equivalent to
(new Number(56.)).toString();
Key to understand here is difference between Number literal (56.) and method access operator (variable.methodName() )
Related
This question already has answers here:
In Typescript, what is the ! (exclamation mark / bang) operator when dereferencing a member?
(5 answers)
Two sets of parentheses after function call
(4 answers)
Closed last month.
In Codemirror 6's documentation, and in the code line 41, ...
while (pending.length) pending.pop()!(data.updates)
What does this syntax mean?
Seems to be typescript specific.
What would be the javascript equivalent?
In TypeScript, the ! operator can also be used to assert that a value is non-nullable. This is called a non-null assertion operator, and it is used to tell the TypeScript compiler that a value is definitely not null or undefined. Check the documentation
The pop() method removes the last element from an array and returns that element. See the documentation.
Since the return value is a function, well you can call it.
This question already has answers here:
Unable to access JSON property with "-" dash [duplicate]
(5 answers)
How can I access object properties containing special characters?
(2 answers)
How can I access and process nested objects, arrays, or JSON?
(31 answers)
Closed 5 years ago.
I have a value in this format:
var state = [{"industry-type":"football","your-role":"coach"}]
I would like to output "football". How can I do this?
I've tried state[0].industry-type but it returns an error:
Uncaught ReferenceError: type is not defined
Any help appreciated.
It does not like the '-' in your property name, try:
state[0]['industry-type']
This happens because you can't access a property with - directly.
var state = [{"industry-type":"football","your-role":"coach"}];
console.log(state[0]['industry-type']);
The - symbol is reserved in Javascript, you cannot use it to refer to an object's property because Javascript thinks you're trying to do subtraction: state[0].industry - type; Hence the error "Uncaught ReferenceError: type is not defined" - it is looking for a variable named type to subtract with, which it can't find.
Instead, refer to it by:
state[0]['industry-type']
Because in Javascript, object.property and object['property'] are equal.
For what it's worth, if you have control over those names, it is best practice in Javascript to name things with Camel Case, so your variable would be defined as:
var state = [{"industryType":"football","yourRole":"coach"}]
Then, you could access it like:
state[0].industryType
In order to be able to use dot notation then your:
...property must be a valid JavaScript identifier, i.e. a sequence of
alphanumerical characters, also including the underscore ("_") and
dollar sign ("$"), that cannot start with a number.
From MDN
Like the other answers pointed out, you have to use square bracket notation to access property names of an object that are not valid JavaScript identifiers.
e.g.
state[0]["industry-type"]
Related SO question:
What characters are valid for JavaScript variable names?
You'll need to use bracket notation for the attribute -
state[0]['industry-type']
This question already has answers here:
Can I get a javascript object property name that starts with a number?
(2 answers)
Closed 8 years ago.
I've got an object where one of the properties starts with a number. I've read that this was allowed, but I'm getting this error:
SyntaxError: identifier starts immediately after numeric literal
var stats = {"LOAD_AVG":{"1_MIN":0.08,"5_MIN":0.08,"15_MIN":0.08},"COUNTS":{"TOTAL":888,"RUNNING":1,"SLEEPING":887,"STOPPED":0,"ZOMBIE":0},"CPU":{"USER_CPU_TIME":8.9,"SYSTEM_CPU_TIME":2.4,"NICE_CPU_TIME":0,"IO_WAIT_TIME":1,"HARD_TIME":0,"SOFT_TIME":0.1,"STEAL_TIME":0},"MEMORY":{"PHYSICAL":{"TOTAL":"3921.98mb","IN_USE":"3682.652mb (93.9%)","AVAILABLE":"239.328mb (6.1%)","BUFFERS":"266.492mb (6.8%)"},"SWAP":{"TOTAL":"4194.296mb","IN_USE":"64.264mb (1.5%)","AVAILABLE":"4130.032mb (98.5%)","CACHE":"1191.328mb (28.4%)"}}};
//works fine
window.alert(stats.COUNTS.TOTAL);
//doesn't work
window.alert(stats.LOAD_AVG.1_MIN);
Here's a fiddle.
How can I access the properties that begin with a number without rewriting the PHP that generated it?
You can use bracket access for properties that aren't valid JavaScript identifiers, that goes for property names with spaces or other language symbols like +, *
window.alert(stats.LOAD_AVG["1_MIN"]);
You can use bracket access anywhere really
window.alert(stats["COUNTS"]["TOTAL"]);
This question already has answers here:
How to convert string equation to number in javascript?
(5 answers)
Closed 8 years ago.
Is there any built-in function in JavaScript like eval built-in function in Python?
notice: eval function take an equation as string and returns result. for example assume variable x is 2, then eval("2x+5") returns 9.
Yes, there is eval function in JavaScript too.
Besides, the statement should be valid for evaluation, i.e. eval("2*x+5").
You should also note, that using eval in JavaScript is not recommended. You can read about at MDN.
REF: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/eval
Yes, it's called eval, oddly enough.
http://www.w3schools.com/jsref/jsref_eval.asp
Edit: the mozilla link from VisioN (https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/eval) is better, since it explains why you shouldn't use it.
You can use eval() however you will need to convert any "human" math operators to ones understood by javascript.
var sum = "4x5";
eval(sum.replace("x", "*"));
Update didn't read the part about x being a variable! that does make it slightly more complicated depending on how complex the equations are.
This question already has answers here:
Calling member function of number literal
(3 answers)
Closed 9 years ago.
I'm reading through Douglas Crockford's JavaScript: The Good Parts, and I'm at the point where he defines a fade function. Part of this code boils down to this:
var level = 1;
var hex = level.toString(16);
So I run this in my browser's console to see what I get....
var level = 1;
level.toString(16);
Hey, it returns "1"... Fabuloso! Wunderbar!
Then to be cheeky, I try this to see what I get...
1.toString(16);
And I get
SyntaxError: Unexpected token ILLEGAL
What the what? If level is a variable equal to 1, and running this method on level works fine, then why doesn't running this method on the actual number 1 work? I tried a similar experiment with the toPrecision() method and that worked fine in both cases. What's the issue here? Is this another one of those inherent flaws in the JavaScript implementation, or am I missing something? I am testing in Google Chrome.
Related: Stack Overflow question Why don't number literals have access to Number methods?.
It's just a language grammar limitation.
Since 1. is a legal literal number (and 1.t is not) the tokeniser will split this into the following tokens:
1.
toString
(
)
And that's an illegal sequence of tokens. It's object method, instead of object . method.
In the working versions in #Joey's answer, the braces prevent the tokenizer from treating the dot as part of the number literal instead of as a separate token, as does writing:
1.0.toString()
or
1..toString()
since the tokenizer knows that the second dot must be a token on its own, and not part of the number literal.
You need 1..toString or (1).toString to get the number literal
level is a variable (and thus an object).
1 is a literal. They are not objects and the interpreter thinks about them completely differently.
http://www.cs.brown.edu/courses/bridge/1998/res/javascript/javascript-tutorial.html#4