We Keep Coding

What makes function expressions return a value?

I know that function expressions resolve to a value (not to be confused with what they return [as others cleared up for me in another SO question)—all functions return undefined by default), hence the word "expression," whereas function declarations don't. But I'm not sure what exactly is causing this discrepancy. My initial guess was that it is the variable assignment (given what's to the right of the assignment operator is always an expression [I think]), but function expressions don't require variable assignment. To my knowledge the only other difference (besides resolving to a value) between them and function declarations is that you can omit the function name in a function expression. I'd appreciate any insight.

Refer :https://javascript.info/function-expressions-arrows#function-expression-vs-function-declaration
A Function Expression is created when the execution reaches it and is usable from then on.
Once the execution flow passes to the right side of the assignment let sum = function… – here we go, the function is created and can be used (assigned, called etc) from now on.
Function Declarations are different.
A Function Declaration is usable in the whole script/code block.
In other words, when JavaScript prepares to run the script or a code block, it first looks for Function Declarations in it and creates the functions. We can think of it as an “initialization stage”.
And after all of the Function Declarations are processed, the execution goes on.
As a result, a function declared as a Function Declaration can be called earlier than it is defined.
sayHi("John"); // Hello, John
function sayHi(name) {
alert( `Hello, ${name}` );
}
The Function Declaration sayHi is created when JavaScript is preparing to start the script and is visible everywhere in it.
…If it was a Function Expression, then it wouldn’t work:
sayHi("John"); // error!
let sayHi = function(name) { // (*) no magic any more
alert( `Hello, ${name}` );
};
Function Expressions are created when the execution reaches them. That would happen only in the line (*). Too late.
When a Function Declaration is made within a code block, it is visible everywhere inside that block. But not outside of it.
Sometimes that’s handy to declare a local function only needed in that block alone. But that feature may also cause problems.
For instance, let’s imagine that we need to declare a function welcome() depending on the age variable that we get during runtime. And then we plan to use it some time later.
let age = prompt("What is your age?", 18);
// conditionally declare a function
if (age < 18) {
function welcome() {
alert("Hello!");
}
} else {
function welcome() {
alert("Greetings!");
}
}
// ...use it later
welcome(); // Error: welcome is not defined
That’s because a Function Declaration is only visible inside the code block in which it resides.
Here’s another example:
let age = 16; // take 16 as an example
if (age < 18) {
welcome(); // \ (runs)
// |
function welcome() { // |
alert("Hello!"); // | Function Declaration is available
} // | everywhere in the block where it's declared
// |
welcome(); // / (runs)
} else {
function welcome() { // for age = 16, this "welcome" is never created
alert("Greetings!");
}
}
// Here we're out of curly braces,
// so we can not see Function Declarations made inside of them.
welcome(); // Error: welcome is not defined
What can we do to make welcome visible outside of if?
The correct approach would be to use a Function Expression and assign welcome to the variable that is declared outside of if and has the proper visibility.
Now it works as intended:
let age = prompt("What is your age?", 18);
let welcome;
if (age < 18) {
welcome = function() {
alert("Hello!");
};
} else {
welcome = function() {
alert("Greetings!");
};
}
welcome(); // ok now
Or we could simplify it even further using a question mark operator ?:
let age = prompt("What is your age?", 18);
let welcome = (age < 18) ?
function() { alert("Hello!"); } :
function() { alert("Greetings!"); };
welcome(); // ok now

Determine function has defined as a expression or declaried as function

I want to know how the function has been initialized, with the expression or declaried as fuction. _ Amazon interview question
expression : var a = function (){ }
declaration: function a (){ }

You could just do a.toString() and parse out the name. Or do the same with regular expressions
a.toString().test(/^\s*function\s*\(/);
function a(){ }; // gives false
var a = function (){ }; // gives true
Of course as Grundy pointed out this fails with named functions. Something like
var a = function b() {};
or
function b() {};
var a = b;
And ES6 has .name (see the Browser table at the bottom for the current state of affairs) - https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Function/name

I don't think it's possible to do so. The only difference between:
var func = function(){ };
and:
function func() { };
Is that the first one gets assigned on runtime. The way I see it, is that both function statements return a reference to their respective function objects. In that sense they are both the same. The only thing you could argue is that one is not named and the other one is, but you could have assigned a named function to a variable too.
However, there seems to be a difference on how they get assigned. The second one seems to get assigned to a variable that its named after, right at the start of the execution context. The first one has to wait for the explicit assignment within the execution context.
So you'd be testing for when they get assigned. You might think that's is possible to do so within the global object like:
//some protected vars that can't work without same-origin
var protected = ['caches', 'localStorage', 'sessionStorage', 'frameElement'];
var definedAtInit = [];
for(prop in window){
if(!isSandboxed(prop) && typeof window[prop] === 'function'){
definedAtInit.push(prop);
}
};
function isSandboxed(prop){
return protected.indexOf(prop) !== -1;
}
function isItDefinedAtInit(funcName){
return definedAtInit.indexOf(funcName) !== -1;
}
var func = function() {
console.log('test');
}
var results = { isItDefinedAtInit : isItDefinedAtInit('isItDefinedAtInit'),
func : isItDefinedAtInit('func')
};
document.getElementById('results').innerHTML = JSON.stringify(results, '/t');
<pre id="results"></pre>
However, you could still do something like:
var isItDefinedAtInit = function() { };
//After this, isItDefinedAtInit('isItDefinedAtInit') would be wrong.
And you still have the problems with other execution contexts, I don't think functions declared within a function execution context get attached to any object.
I think these kind of checks are a bad idea to be honest.

There is only way, we can determine function has defined with function declarations not as expression.
as Grundy mentioned name property of the respective function gives require information, if it has been defined with expression name property holds undefined value, else it holds function name.
Here is the code :
var isDefinedAsFunction = function(fn){
return fn.name !== undefined
}

JAVASCRIPT functions knowing about other functions

Is javascript code read from the top down?
In other words if I have:
function Color(){
}
function Weather() {
}
If I wanted to incorporate some type of weather object inside the Color function would it be possible since Weather is defined afterwards?

The function Color won't know about Weather as it's being parsed, but it will once it is called. For example:
// Since variable declarations are hoisted to the top,
// myVar will be undefined (same as simply saying "var myVar;")
console.log(myVar); // <-- undefined
var myVar = 5;
console.log(myVar); // <-- 5
// test2 hasn't been defined yet, but it doesn't matter
// because test isn't being executed
var test = function() {
test2();
}
// This will throw an error: undefined is not a function
// because test2 isn't defined
test();
var test2 = function() {
alert("test2");
}
// This won't throw an error
test();
Essentially, you should execute after all functions have been defined. However, if you use the function functName() {} syntax, then it is hoisted just like a var statement.
function test() {
test2();
}
// Works!
test();
function test2() { alert("test2"); }

JavaScript is parsed "top-down" and Function Declarations are hoisted constructs - see var functionName = function() {} vs function functionName() {}. Parsing happens before the JavaScript execution itself starts; the following thus covers the evaluation semantics1.
Given the effect of function hoisting, the following code is valid, although it seems like it might not be:
function a(){
return b();
}
alert(a()); // alerts "b", even though "b comes later"
// This declaration IS hoisted
function b() {
return "b";
}
However, considering that even when using "var fn = .." (when the assignment is not hoisted) the ordering usually doesn't matter because the evaluation of the assignments happens before the usage:
var a = function () {
return b();
}
// alert(a()); <- this would fail, because b is not assigned yet
// This assignment is NOT hoisted
var b = function () {
return "b";
}
// But b is assigned before here, meaning that the order of the constructor
// functions still Just Doesn't Matter.
alert(a());
As such, in the case where there are two constructor functions (e.g. Color and Weather) which are [mutually] dependent, it doesn't matter where they are located in the same scope relative to each other.
1The thing that matters is the expression representing the dependency is resolvable when evaluated (which is relative to the evaluation code, but not related to the parsing/placement order of the functions).

so basically yeah you can implement something to a function that appears in a previous code.
hole this helps you understand it better.
var weather = function() {
if (weather === red) {
console.log("weather is represented by the color red");
}
else {
console.log("Not the right color");
}
};
weather(red);

Adding a function to global windows object

I was playing around with javascript objects to understand "this" and function context better. I stumbled across this problem. I get the error "obj2 is not defined" unless I run window.obj2() after assigning it, but I don't know why. Shouldn't it be enough to assign the function to window.obj2 without also executing it immediately afterwards? I know that you're not supposed to pollute the window object, this is just a test.
Thanks!
window.obj2 = function(){
console.log('obj2 in window.object',this);
}
window.obj2(); // problem when this line is commented out
(function () {
var parent = {
obj : function(){
//console.log(this);
obj2();
this.obj2();
window.obj2();
},
obj2 :function(){
console.log('obj2 in parent',this);
}
}
parent.obj();
}());
EXPLANATION
OP asks why does he have to execute the function after defining it in order for it to become defined later in the code... See what happens when you comment out the problem line.

Solution to the mystery:
You forgot a semicolon:
window.obj2 = function(){
console.log('obj2 in window.object',this);
}; // <--
Without it, the code will be interpreted as
// I'm naming the functions to refer to them later
window.obj2 = function a(){
...
}(function b() { ... }());
I.e. the parenthesis around b are interpreted as call operation of a (just like you did with b itself: (function b() {...}()).
The engine first executes b in order to pass the return value as argument to a, and only after that the return value is assigned to window.obj2.
So, at the moment b is called, window.obj2 does indeed not exist yet.
So, the reason why adding window.obj2() makes it work is not because you are accessing window.obj2, but because it makes the code un-ambigious. The following parenthesis cannot be interpreted as call operation anymore. You could use any statement there, e.g.
window.obj2 = function(){
console.log('obj2 in window.object',this);
}
"foo";
(function () {
obj2();
}());

If you defined function window.obj2 inside the anonymous function which does call itself then it works fine, have a look code.
<script>
//window.obj2(); // problem
(function (window) {
window.obj2 = function(){
console.log('obj2 in window.object',this);
}
var parent = {
obj : function(){
//console.log(this);
obj2();
this.obj2();
window.obj2();
},
obj2 :function(){
console.log('obj2 in parent',this);
}
}
parent.obj();
}(window));
</script>
<body>
</body>

What are the usage scenarios or advantages of defining functions after the return expression

En example can be found in Twitter'a typeahead.js here:
function () {
// ...
return this.each(initialize);
function initialize() {
// ...
}
}
Questions:
What are the scopes and what function sees what?
What is the reason for using such a construct (usage scenarios and advantages)?

Javascript has function based scope, which means that every thing defined inside a function is available from the first line, since the definition is "hoisted" by the complier.
That goes for both variable and function definitions - variable values however, are not available until after assignment.
You can read all about javascript scoping and hoisting here
This means that the function initialize is available from the first line of the wrapping anonymous function.
There is no real reason, and no advantages, for doing it that way, unless you count the code structure as an advantage.

Personally I don't see any reason to do this. For me even it looks a little bit weird. Martin is right. You should be careful, because the defined variables are not accessible like functions. For example this doesn't work:
var getValue = function(func) {
return func();
}
var f = function() {
return getValue(now);
var now = function() {
return 10;
}
}
alert(f());
However, this works:
var getValue = function(func) {
return func();
}
var f = function() {
return getValue(now);
function now() {
return 10;
}
}
alert(f());