Was reading through Douglas Crockford's code here and saw a line
var value = +node.getValue();
but I don't see anything at http://www.w3schools.com/js/js_operators.asp which corresponds to an = + or a way that + can be used as a unary operator. So what does this mean?
The - and + operators are both unary in JS and, before forcing the value's sign, must convert the value to a number.
Obviously - will convert to a number and invert the sign, but + only does the first part. Running +"100" will return the number 100.
This behavior is explicitly stated in the spec at 11.4.6, where the unary + operator is defined:
The unary + operator converts its operand to Number type.
It's just a quick way to make sure the variable is an INT, (vs. a STR or BOOL, e.g.).
Just to add on to what's been said do the following:
var a = +'4';
var b = '4';
console.log(typeof(a));//Number
console.log(typeof(b));//String
Related
An interesting thing I've never seen before was posted in another question. They had something like:
var i = + +1;
They thought the extra + converted it to a string, but they were simply adding to a string which is what caused it to convert.
This, however, lead me to the question: what is going on here?
I would have actually expected that to be a compiler error, but JavaScript (at least in Chrome) is just fine with it... it just basically does nothing.
I created a little JSFiddle to demonstrate: Demo
var i = 5;
var j = + +i;
document.body.innerHTML = i === j ? 'Same' : 'Different';
Anyone know what's actually occurring and what JavaScript is doing with this process?
I thought maybe it would treat it like ++i, but i doesn't increment, and you can even do it with a value (e.g., + +5), which you can't do with ++ (e.g., ++5 is a reference error).
Spacing also doesn't affect it (e.g., + + 1 and + +1 are the same).
My best guess is it's essentially treating them as positive/negative signs and putting them together. It looks like 1 == - -1 and -1 == + -1, but that is just so weird.
Is this just a quirky behavior, or is it documented in a standard somewhere?
Putting your the statement through the AST Explorer, we can see that what we get here is two nested Unary Expressions, with the unary + operator.
It's a unary expression consisting of + and +i, and +i is itself a unary expression consisting of + and i.
The unary expression with the unary + operator, will convert the expression portion into a number. So you're essentially converting i to a number, then converting the result of that to a number, again (which is a no-op).
For the sake of completion, it works on as many levels as you add:
var i = 5;
console.log(+ + + + + +i); // 5
console.log(i); // still 5
It's in the specification.
Digging through, we can see from §14.6.2.2 that the increment and decrement operators are listed before (and should be preferred) over the unary operators. So precedence alone won't explain this.
Looking up the the punctuation table in §11.7, we can see that every single instance of ++ (the operator) in the spec shows the two together, without whitespace. That's not conclusive, until you check...
The whitespace rules in §11.2, specifically:
White space code points may occur within a StringLiteral, a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are considered significant code points forming part of a literal value. They may also occur within a Comment, but cannot appear within any other kind of token.
JS does not allow arbitrary whitespace mid-operator.
The JS syntax in both PegJS and Esprima corroborate this, matching on the literal two-character string ++.
For me it's very clear;
var a = +3;
var b = +a; // same as a, could be -a, for instance
var c = + +a; // same as above, same as +(+a)
If you do ++variable the javascript interpreter sees it as the increment operator.
If you do + +variable the javascript interpreter sees it as Unary plus, coercing the value to a number, twice.
So
var a = 1;
var b = +a;
var c = +b;
console.log(c);// still 1
is the same as
var c = + +1;
So the simple answer is that two plus signs can not be separated by a space to be interpreted as incrementation, the space makes it so the interpreter sees two seperate spaces, which is what it really is
The + operators converts into a number, two + operators with a space in between does nothing additional.
Even though it might look very similar, + + and ++ are not at all the same thing for an AST interpreter. The same applies to token separation: varfoo is not the same as var foo.
In the expression + + +i, each + is considered as distinct unary operator, which simply convert your variable to a number. For the incrementation operation, which is ++, no spaces are allowed, neither between the + and the variable token. In the example below, the last line is not valid:
var x = "4";
console.log(+ + +x);
console.log(+ + ++x);
console.log(+ ++ +x);
This question already has answers here:
Why does JavaScript handle the plus and minus operators between strings and numbers differently?
(7 answers)
Closed 5 years ago.
Why does Javascript give an output of 0 when I use the odd operator?
What is the difference between subtraction and addition with a string?
var x = 1;
console.log(x+'1') // Outputs 11
console.log(x-'1') // Outputs 0 -- but why?
So how can I do mathematical calculations?
The + operator has one of two three meanings in javascript. The first is to add numbers, the second is to concatenate strings. When you do 1 + '1' or '1' + 1 the operator will convert one operand that is not a string to a string first, because one other operand is already evaluated to be a string. The - operator on the other hand has just one purpose, which is to subtract the right operand from the left operand. This is a math operation, and so the JS engine will try to convert both operands to numbers, if they are of any other datatype.
I'm not sure though why typecasting to strings appears to have precedence over typecasting to numbers, but it obviously does.
(It seems to me the most likely that this is a pure specification decision rather than the result of other language mechanics.)
If you want to make sure that the + operator acts as an addition operator, you can explicitly cast values to a number first. Although javascript does not technically distinguish between integers and floats, two functions exist to convert other datatypes to their number equivalents: parseInt() and parseFloat() respectively:
const x = 10;
const result = x + parseInt('1'); // 11
const y = 5;
const result2 = y + parseFloat('1.5'); // 6.5
const result3 = y + parseInt('1.5'); // 6
Edit
As jcaron states in the comment below, the + operator has a third meaning in the form of an unary + operator. If + only has a right operand, it will try to convert its value to a number almost equivalent as how parseFloat does it:
+ '1'; // returns 1
+ '1.5'; // returns 1.5
// In the context of the previous example:
const y = 5;
const result2 = y + +'1.5'; // 6.5
Dhe difference with parseFloat is that parseFloat will create a substring of the source string to the point where that substring would become an invalid numeric, whereas unary + will always take the entire string as its input:
parseFloat('1.5no-longer-valid'); // 1.5
+ '1.5no-longer-valid'; // NaN
That is because + is a concatenation operator. So javascript considers it to be a concatenation operator rather than a mathematical operator.But it is not the case with / ,* ,/ etc.
This happens because + its also used to concatenate strings. Then, JS always will find the better way to make the correct typecasts basing on types. In this case, the x+'1' operation, will be identified as string type + string type.
Otherwise, x-'1', will become int type - int type.
If you want to work with specific types, try to use type cast conversions, link here.
I am puzzled by this snippet:
var n1 = 5-"4";
var n2 = 5+"4";
alert(n1);
alert(n2);
I understand that n1 is 1. That is because a minus operator would convert the string "4" into number and subtract it from 5.
But why do we get 54 in case of + operator?
Can someone explain this difference between + and = operators to me?
By type conversion any + expression, that contains a strings, will result in a string. Thus all operands (in your case 5) will be converted to a string, before executing the concatenation.
- on the other hand is just an arithmetic operand, thus "4" is converted to an integer and the calculation is performed as you expect.
It's because in n2, + is being treated as concatenation, not addition. So 5 is converted to the string "5" and "4" is concatenated, giving "54".
When there's a string in either side of +, the + will be considered as a string concatenating operator, the other side will be converted to string and then do the concatenating.
And be careful of something like 1+2+'3', the result is '33' rather than '123'.
- operator has only one meaning - numbers subtraction (or negation and in that case, also conversion to number). In case of + operator, however, there are two: number addition and strings concatenation. When one of the operands of + operator is a string it does string concatenation instead of numbers addition.
The entire process is a bit more complicated than that though and involves an algorithm that you can learn a bit more here, for example.
The + operator is also a string operator. Quite every basic type variable in javascript can be interpreted also in its string representation. You are just attaching 5 to 4 getting 54.
The - operator is not a string operator so the compiler tries to interpret "4" as a number, thus getting 1
Javascript takes 5 as a number and "4" as string.
The javascript + operator use to concat two things.
If you want to addition please use parseInt.
var n1 = 5-"4";
var n2 = parseInt(5)+parseInt("4");
alert(n1);
alert(n2);
By this I mean NOT the default javascript behavior of string + number = string, or string + string = string. From http://www.javascriptkit.com/jsref/arithmetic_operators.shtml, the Unary plus will convert a string to a number.
So how would I specify that I want numeric addition with two strings? This seems to work, but is somewhat ugly:
var a = "5";
var b = "2";
var c = +a + +b;
Does there exist a (numeric only +) operator, that always returns a number? Can one be defined like in other languages? For examples, perhaps '%%' or '+^' or '+++'? Or is this just not possible?
No. If you want to perform numeric addition than you have to ensure that both sides of the expression are Numbers before you start.
I have seen a syntax such as the following before:
var mynum = new Number();
var temp = (+mynum); //this line is what i am curious about
var text = temp.toPrecision(3);
Can anyone tell me what this + syntax means?
What I have found is that in some JS implementations, it is somehow necessary as it ensures that the number defined in mynum is valid.
Thanks,
jml
+ is a unary operator which is used to coerce data types into numbers. Unary meaning it only needs one operand.
new Date returns an object, applying + coerces it into a timestamp eg 1277504628812
new Number returns an object, applying + coerces it into the numeric literal 0.
See: http://bclary.com/2004/11/07/#a-11.4.6
This is the ECMAScript documentation, which is the subset of Javascript, in HTML format.