Why is semicolon required for this statement? - javascript

The following code returns an error that says
"console.log(...) is not a function"
if (4<5) console.log('hi')
(4<5) ? console.log('hi') : console.log('bye')
The following code does not return any error
if (4<5) console.log('hi')
if (4<5) console.log('hi')
Why is this so?

Without a semicolon at the end of the first line, the code tries to use return value of the first console.log as a function and call it with the argument 4<5; this is clearer if you remove the line break:
if (4<5) console.log('hi')(4<5) ? console.log('hi') : console.log('bye')
// ^^^^^^^^^^^^^^^^^^^^^^---- looks like calling `console.log` and then
// using the result as a function
There's the potential for this any time you combine leaving off semicolons (which means you're relying on an error-correction mechanism1) with expression statements. Since expression statements are, by their nature, expressions, if the parser can use them in the previous expression or statement, it will.
FWIW, astexplorer.net is a cool tool I've recently found (thanks to the Babel project). It's an interactive syntax tree explorer which can use any of several parsers to parse your code and tell you exactly how it was parsed. And from the github account, it was started by our own Felix Kling.
1 Quoting Brendan Eich:
ASI is (formally speaking) a syntactic error correction procedure. If you start to code as if it were a universal significant-newline rule, you will get into trouble.

Related

eval is not a function [duplicate]

This question already has answers here:
TypeError: console.log(...) is not a function [duplicate]
(6 answers)
Closed 5 years ago.
It might interest somebody and save few hours, a nasty bundling problem, eval is from one file and function is from second file.
eval('console.log(0)')
(function(x){console.log(x)})(1)
will result with: Uncaught TypeError: eval(...) is not a function
and this is the fix
eval('console.log(0)');
(function(x){console.log(x)})(1)
missing semicolon, I've read few times that semicolon in JS optional most of the time.
any comments why eval is not a function in this context?
The JS runtime is not perfect at guessing where semi-colons should go.
When it sees this:
eval('console.log(0)')
(function(x){console.log(x)})(1)
It incorrectly assumes that the result of the immediately invoked function expression is part of the eval line (a potential parameter to be passed to the potential function that the eval evaluates to).
Here's an example of when that would work:
eval("(function(message){ alert(message); })")
("Hi there!");
But, in your case, eval is not evaluating to a function, so trying to pass an argument to it fails, thus your error message.
But, it actually has less to do with eval(), in particular. This can pop up anytime a semi-colon is omitted just prior to an IIFE or any expression starting with a (.
Adding the semi-colon, allows the runtime to know that the IIFE is separate.
The rule is that you must insert semi colons on any line that is followed by a ( (as this is how an expression can begin) in order to keep them separate . But, the better rule is to not rely on automatic semi-colon insertion at all and always put them in yourself.
This is one of the few situations in which Javascript's semicolon insertion will trip you up.
This code:
eval('console.log(0)')
(function(x){console.log(x)})(1)
Is equivalent to this code:
eval('console.log(0)')(function(x){console.log(x)})(1)
In other words, Javascript thinks that the expression eval('console.log(0)') evaluations to a function which you are trying to call with the parameter (function(x){console.log(x)}). That is obviously not what you intended, so you need semicolons at the end of your lines.

The logic behind Ractive.DEBUG minify assertion

I saw in Ractive docs this line of code:
Ractive.DEBUG = /unminified/.test(function() {/*unminified*/});
Can you explain the logic behind this ?
The function should have the same value, despite whether it has inline comments or not.
Minification will remove the comment from the code so the function becomes function() {} when regex test calls .toString() on the function, thus yielding false.
Without minification, the regex test will find the occurrence of unminified and therefore DEBUG will be true

Using semi-colons to close functions in JavaScript. Necessary? [duplicate]

This question already has answers here:
Do you recommend using semicolons after every statement in JavaScript?
(11 answers)
Closed 6 years ago.
When executing a function in JavaScript, I've always ended my code block with a semi-colon by default, because that's what I've been taught to do. Coming from Java it felt a bit unorthodox at first, but syntax is syntax.
function semiColon(args) {
// code block here
};
or
function sloppyFunction(args) {
// code block here
}
Lately I've been seeing more and more code where the developer left the semi-colon out after functions, but the intended code still executed normally. So are they actually required? If not, why is it common practice to include them? Do they serve another purpose?
Function declarations do not need a semi-colon, though if you put a semi-colon there, it won't be harmful, it is just a redundant empty statement.
function semiColon(args) {
// code block here
};;;; // 4 empty statements
Most statements require a semi-colon, but if you leave the semi-colon out it will be inserted automatically in most cases by Automatic Semi-Colon Insertion, with caveats. In general it is easier to just always add a semi-colon after your statements, so that you, and other developers working with your code, don't have to worry about those caveats.
This code is correct:
function semiColon(args) {
// code block here
} // No need for semi-colon
var semiColon = function (args) {
// code block here
}; // Semi-colon required here
Whereas this code is wrong, but will still usually work:
function semiColon(args) {
// code block here
}; // Redundant unnecessary Empty Statement
var semiColon = function (args) {
// code block here
} // Semi-colon required here,
// but ASI will sometimes insert it for you, depending on
// the subsequent token
NO - using semicolons to end function declarations are NOT necessary in JavaScript. While they will not throw an error, they are the equivalent of using more than one semicolon to end a line of code - harmless, but unnecessary. Being superfluous, they are considered poor stylistic and programming practice.
The one exception is a function expression, e.g.
var my_function = function(a, b){ };
where you DO need the semicolon to terminate the line.
You shall not add a semicolon after a function declaration.
Because, after checking the Javascript grammar:
StatementList:
StatementListItem
StatementList
StatementListItem:
Statement
Declaration
Declaration:
HoistableDeclaration
ClassDeclaration
LexicalDeclaration
HoistableDeclaration:
FunctionDeclaration
GeneratorDeclaration
Here's the grammar production for a function:
FunctionDeclaration → HoistableDeclaration → Declaration → StatementListItem → StatementList
which proves that my former response is wrong (no need to look at the former edits, as it's wrong! ;) ).
A function xx() {} construct alone is a special case, neither — strictly speaking — a statement or an expression, and thus is NOT to be ended with a semicolon.
You only need to add a semicolon (or leave the ASI take care of it) if you're using the expression function() construct, which exists when it is part of a statement. To make it a statement you need to either have it part of a statement:
var foo = function() {};
or embedded within another expression:
(function() {})();
!function x() { ... }();
And in either cases, you need to add the semicolon at the end of the full statement, obviously.
Generally speaking, I like the python mantra "explicit is better than implicit" so when you hesitate to add a semicolon that the ASI would add otherwise, just add it.
sorry for being wrong in the first version of my answer, I'll debug some PHP code as a penitence. ☺
HTH

jshint complains about shorthand AND operator with boolean

I've got a javascript function that needs to be executed based on a boolean value. I really like to use the && operator for this (which only executes the second part if the first results in true).
someBoolean && executeFunction();
However, when JSHint checks my code, I get the following message:
Expected an assignment or function call and instead saw an expression.
I'm wondering why JSHint throws this message. I know I can easily avoid it by using a simple if statement, but I really like the simplicity of this one. Maybe I need to fiddle a bit with some JSHint configuration? Is there some hidden danger in this line of code?

Are semicolons needed after an object literal assignment in JavaScript?

The following code illustrates an object literal being assigned, but with no semicolon afterwards:
var literal = {
say: function(msg) { alert(msg); }
}
literal.say("hello world!");
This appears to be legal, and doesn't issue a warning (at least in Firefox 3). Is this completely legal, or is there a strict version of JavaScript where this is not allowed?
I'm wondering in particular for future compatibility issues... I would like to be writing "correct" JavaScript, so if technically I need to use the semicolon, I would like to be using it.
Not technically, JavaScript has semicolons as optional in many situations.
But, as a general rule, use them at the end of any statement. Why? Because if you ever want to compress the script, it will save you from countless hours of frustration.
Automatic semicolon insertion is performed by the interpreter, so you can leave them out if you so choose. In the comments, someone claimed that
Semicolons are not optional with statements like break/continue/throw
but this is incorrect. They are optional; what is really happening is that line terminators affect the automatic semicolon insertion; it is a subtle difference.
Here is the rest of the standard on semicolon insertion:
For convenience, however, such semicolons may be omitted from the source text in certain situations. These situations are described by saying that semicolons are automatically inserted into the source code token stream in those situations.
The YUI Compressor and dojo shrinksafe should work perfectly fine without semicolons since they're based on a full JavaScript parser. But Packer and JSMin won't.
The other reason to always use semi-colons at the end of statements is that occasionally you can accidentally combine two statements to create something very different. For example, if you follow the statement with the common technique to create a scope using a closure:
var literal = {
say: function(msg) { alert(msg); }
}
(function() {
// ....
})();
The parser might interpret the brackets as a function call, here causing a type error, but in other circumstances it could cause a subtle bug that's tricky to trace. Another interesting mishap is if the next statement starts with a regular expression, the parser might think the first forward slash is a division symbol.
JavaScript interpreters do something called "semicolon insertion", so if a line without a semicolon is valid, a semicolon will quietly be added to the end of the statement and no error will occur.
var foo = 'bar'
// Valid, foo now contains 'bar'
var bas =
{ prop: 'yay!' }
// Valid, bas now contains object with property 'prop' containing 'yay!'
var zeb =
switch (zeb) {
...
// Invalid, because the lines following 'var zeb =' aren't an assignable value
Not too complicated and at least an error gets thrown when something is clearly not right. But there are cases where an error is not thrown, but the statements are not executed as intended due to semicolon insertion. Consider a function that is supposed to return an object:
return {
prop: 'yay!'
}
// The object literal gets returned as expected and all is well
return
{
prop: 'nay!'
}
// Oops! return by itself is a perfectly valid statement, so a semicolon
// is inserted and undefined is unexpectedly returned, rather than the object
// literal. Note that no error occurred.
Bugs like this can be maddeningly difficult to hunt down and while you can't ensure this never happens (since there's no way I know of to turn off semicolon insertion), these sorts of bugs are easier to identify when you make your intentions clear by consistently using semicolons. That and explicitly adding semicolons is generally considered good style.
I was first made aware of this insidious little possibility when reading Douglas Crockford's superb and succinct book "JavaScript: The Good Parts". I highly recommend it.
In this case there is no need for a semicolon at the end of the statement. The conclusion is the same but the reasoning is way off.
JavaScript does not have semicolons as "optional". Rather, it has strict rules around automatic semicolon insertion. Semicolons are not optional with statements like break, continue, or throw. Refer to the ECMA Language Specification for more details; specifically 11.9.1, rules of automatic semicolon insertion.
Use JSLint to keep your JavaScript clean and tidy
JSLint says:
Error:
Implied global: alert 2
Problem at line 3 character 2: Missing
semicolon.
}
The semi-colon is not necessary. Some people choose to follow the convention of always terminating with a semi-colon instead of allowing JavaScript to do so automatically at linebreaks, but I'm sure you'll find groups advocating either direction.
If you are looking at writing "correct" JavaScript, I would suggest testing things in Firefox with javascript.options.strict (accessed via about:config) set to true. It might not catch everything, but it should help you ensure your JavaScript code is more compliant.
This is not valid (see clarification below) JavaScript code, since the assignment is just a regular statement, no different from
var foo = "bar";
The semicolon can be left out since JavaScript interpreters attempt to add a semicolon to fix syntax errors, but this is an extra and unnecessary step. I don't know of any strict mode, but I do know that automated parsers or compressors / obfuscators need that semicolon.
If you want to be writing correct JavaScript code, write the semicolon :-)
According to the ECMAscript spec, http://www.ecma-international.org/publications/standards/Ecma-262.htm, the semicolons are automatically inserted if missing. This makes them not required for the script author, but it implies they are required for the interpreter. This means the answer to the original question is 'No', they are not required when writing a script, but, as is pointed out by others, it is recommended for various reasons.

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