This question already has answers here:
Why does [5,6,8,7][1,2] = 8 in JavaScript?
(3 answers)
Closed 7 years ago.
[1,2,4,8][0,1,2,3]
// equals to 8 (the last element of the indexing array (3) becomes the index)
Why is this not a SyntaxError error (a bad legacy or a purposeful feature)? (A possible duplicate, however I wasn't able to find an answer here.)
Update: Why the contents of the square brackets are treated as an expression?
The first part:
[1,2,4,8]
is interpreted as an array literal. The second part:
[0,1,2,3]
is interpreted as square bracket notation to access a member of the array. The contents of the square brackets are treated as an expression, which is seen as a sequence of comma separated values:
0,1,2,3 // or (0,1,2,3) as an independent expression
That expression returns the last value, so is effectively:
[1,2,4,8][3] // 8
Related
This question already has answers here:
What does the comma operator do in JavaScript?
(5 answers)
Closed 2 years ago.
I really don't know what to google to find out the name for this syntax:
(1,2) seems to evaluate to 2
(1,2,3,"cake") seems to evaluate to "cake".
etc
This is sometimes useful in anonymous functions in Array.reduce, where you need to perform a sequence of operations (say an increment) and also return the element on the right.
But where can I read about it and what's it called?
It's just the comma operator: it evaluates each of its operands (from left to right) and returns the value of the last operand.
This question already has answers here:
Removing everything except numbers in a string
(7 answers)
Closed 3 years ago.
I want to replace the dot in a number in Javascript with regular expression; if country_temp is, for example, 1.234, how can I put the value 1234 in country_temp2?
I have tried the following:
const country_temp2 = country_temp.replace(/[a-zA-Z]*\**\s*\./g, "")
but it's not working.
Edit: the original string is composed by characters, asterisk, a number and a dot
Sorry, but I have written in a very fast way.
Try this:
country_temp.replace(/\./g,'')
This question already has answers here:
Calling member function of number literal
(3 answers)
Closed 3 years ago.
I'm new to Javascript, and I saw code like this:
var myData1 = (5).toString() + String(5);
and the author says he placed the numeric value in parentheses and then called the toString method. This is because you have to allow JavaScript to convert the literal value into a number before you can call the methods that the number type defines.
I'm confused, isn't that 5 is already a number, why 5 needs be converted as (5) to be a number?
The author is partly right. This has nothing todo with turning the literal into a number, this is just about a syntactical distinction: The. can either be used to express fractional numbers (1.1) or it can be used for property access (obj.prop). Now if you'd do:
1.toString()
that would be a syntax error, as the dot is treated as a number seperator. You could do one of the following to use the property access dot instead:
1.0.toString() // as the first dot is the number seperator already, the second dot must be property access
1..toString() // same here
(1).toString() // the dot is clearly not part of the number literal
This question already has answers here:
Calling member function of number literal
(3 answers)
Closed 6 years ago.
Why do I get an error on this, in Javascript:
10000000.toString();
You can see an example in here:
http://jsbin.com/kagijayecu/1/edit?js,console
It's because the JS parser is expecting more digits after the "." rather than a method name, e.g. 1000000.0, and in fact 1000000.0.toString() will work as expected.
wrap it inside () like this (10000000).toString()
When JS parse meets dot after digit it expects floating-point literal, e.g. 1000000.0
This question already has answers here:
What does the comma operator do in JavaScript?
(5 answers)
Closed 6 years ago.
I read a line from doT.js:
var global = (function(){ return this || (0||eval)('this'); }());
After it was minified:
l=function(){return this||(0,eval)("this")}();
So what is the (0,eval), I mean what does the comma do?
I played in Chrome's console, (0,1), (2,1), (2,{}), 2,1, etc, it always returns the last one.
The comma operator evaluates both and always returns the last. Much like you said.
You can read up on the comma operator: http://javascriptweblog.wordpress.com/2011/04/04/the-javascript-comma-operator/
Even though I have no idea the purpose of (0||eval)... (0,eval) is the equivalent and one less character.