This question already has answers here:
What is the scope of variables in JavaScript?
(27 answers)
Closed 6 years ago.
In the following code:
function outer() {
var x = 'foo';
function inner() {
var y = x; // y == 'foo'
var x = 'bar'; // x == 'bar', y == undefined
}
}
Why does the variable y become undefined within inner()? shouldn't it refer to x in outer()?
If the line var x = 'bar'; is removed then y does indeed have the value 'foo'.
The inner function is interpreted as if it were written like this:
function inner() {
var y;
var x;
y = x; // y == undefined
x = 'bar'; // x == 'bar'
}
The declarations are hoisted, but the initializations are processed top to bottom. Thus, throughout the entire inner function, the symbols x and y both refer to the variables declared locally in that function; in particular x is the local x, not the one in the enclosing context. When the initializer expression for y is evaluated, therefore, x is the local x, which has not yet been initialized; its initializer expression comes after the initializer for y.
Related
This question already has answers here:
What does var mean in Javascript? [closed]
(2 answers)
Closed 3 years ago.
I can't understand why the x after test won't become 30 but still remains 10
<script>
function test(){
var x = 30;
y = 40;
}
</script>
<script>
var x = 10;
var y = 20;
document.write("before test, x: " + x + ", y: " + y + "<br/><br/>");
test();
document.write("after test, x: " + x + ", y: " + y + "<br/><br/>");
</script>
This is because by declaring var x = 30;, you create a variable named x that exists only in the function's scope.
The variable y, however, is only defined at top-level. So when you run the function test, you edit the local x variable, and the global (top-level) y variable.
When you define variables they are hoisted at top of its scope. Let me show you how your current code:
function test(){
var x = 30;
y = 40;
}
var x = 10;
var y = 20;
test();
Will run like this:
// global scope
var x; // x is undefined
var y; // y is undefined
function test() {
// function scope
var x; // x is undefined inside this scope
x = 30; // x is assigned with value 30
y = 40; // y is assigned with value 40
// the global scope y value is assigned
}
x = 10; // x is assigned with value 10
// the global scope x value is assigned
y = 20; // y is assigned with value 20
// the global scope y value is assigned
test();
// when calling test,
// you see x is assigned with its function scope
// and y is assigned with its global scope
// so, at this point
// x becomes 10
// y becomes 40
You can read more about var here in the docs. Also, look in the scope, global scope and local scope.
Also, note that let and const works differently. They are scoped in the block. You can read them in the corresponding links which are linked here in.
This is a thought exercise. I'm not doing anything with this code and the purpose is to better understand how closures work.
Thought Process:
x === 10 in global scope.
outer() function is called.
x === 20 in the global scope and local scope.
inner() function is called.
right side of 'var x' is expressed.
In x + 20, because x is not defined in local scope, it searches outer scope and finds x === 20.
var x = 20 + 20.
var x === 40.
return x.
result === 40.
However, the answer is 20. Why is this?
var x = 10;
function outer () {
x = 20;
function inner () {
var x = x + 20;
return x;
}
inner();
}
outer();
var result = x;
When the inner() function is called, the first thing that happens is var x.
This means the JavaScript interpreter first creates a variable named x to which it assigns undefined.
Then it runs the assignment expression x + 20, which is equivalent to undefined + 20 which is NaN.
Your variable result has nothing to do with your inner() function as you have a local variable (because of that var x) and you ignore the returned result.
In other words, your code is equivalent to just this:
var x = 10;
function outer () {
x = 20;
}
outer();
var result = x;
Because your inner function defined a local var x which will hide the global variable outside. And the outer function uses the global variable x and assign it to 20. Obviously, the global x is 20. Javascript will define every local variable before you call the function in the prototype chain.
var x = 10;
function outer () {
x = 20;
function inner () {
alert(x); // alert undefined
var x = x + 20;
return x;
}
inner();
}
outer();
var result = x;
This question already has answers here:
How do JavaScript closures work?
(86 answers)
Closed 5 years ago.
Can anyone explain how this code works?
function makeAdder(x) {
return function(y) {
return x + y;
};
}
var add5 = makeAdder(5);
var add10 = makeAdder(10);
console.log(add5(2)); // 7
console.log(add10(2)); // 12
Reference: https://mozilla.org/en/docs/Web/JavaScript/Closures
When you call your makeAdder(5), it returns the reference to a new function.
function(y) {
return x + y;
};
So add5 keeps the reference. After it when you call add5(2), you pass the 2 to the function. In it when it wants to return x + y, it starts to find the variables. Starting from x, it looks and see's that the x is not defined in it and goes to the scope in which inner function was defined ` here
function makeAdder(x) {
return function(y) {
return x + y;
};
}
It see's than x was defined here, because you call and pass x during var add5 = makeAdder(5);. After it goes to find y and finds in it's scope. And after all returns x + y.
The idea is this, every function has a reference to it's creater (where it was defined) and when it doesn't find a variable, it goes to find in it's greater and so till to global scope.
function makeAdder(x) {
return function(y) {
return x + y;
};
}
When you do var add5 = makeAdder(5); the add5 becomes a function which was returnd my makeAdder,
the add5 will be following
function(y){
return 5 + y;
}
so when you do add5(2) it will do 5 + 2 and will return 7;
makeAdder(5)
x => 5 and y is not having value so x+y => 5+0 => 5
var add5 = makeAdder(5);
add5(2);
we have created variable for the function makeadder with x => 5
so now we pass y => 2 and x => 5 so x+y => 5+2 => 7
This property of outer function variable accessible in inner function is called closure property in javascript. Also adding to that y variable is not accessible to outer function.
This question already has answers here:
Javascript scoping of variables
(3 answers)
Closed 6 years ago.
i have this code in java script
var x = 5;
function f(y) { return (x + y) - 2 };
function g(h) { var x = 7; return h(x) };
{ var x = 10; z = g(f) };
z value is 15. why?
the expression (x+y)-2 is being evaluated as (10+7)-2.
why does x get the value of 10, and not the value of the previous
block, where x = 7?
thanks for the help
You can completely delete the first assignment. It gets overwritten before you call g(f).
Also, you can remove the parentheses of the last block as there is no block scope in JS (actually block scope got introduced with let, so you wanna use that instead).
var x = 5;
function f(y) {
// global variable x is 10 -> 10 + 7 - 2 = 15
return (x + y) - 2;
}
function g(h) {
// x gets declared locally - local value will be used
var x = 7;
return h(x); // f gets called with y = 7
}
x = 10; //global x gets changed
z = g(f);
... and always place your semicolons. Even though they maybe look optional but in some cases they are obligatory.
Value of variable x is 10 at global execution context.
When function f is finally called the value of the argument which is y, this y actually represent value of x at local execution context of function g, here x is 7.
var x = 5;
function f(y) {
return (x + y) - 2 ;
}; // value of global var x is 10, value of parameter passed is 7
// this value comes from the local var x of g function's execution context.
function g(h) {
var x = 7; return h(x); };
{ var x = 10; z = g(f); };
console.log(z);
Why the y is leaked outside the function as scope in JavaScript are bound to functions.
A detailed explanation would be fruitful.
var x = 0;
function f(){
var x = y = 1; // x is declared locally. y is not!
};
f();
console.log(x, y); // 0, 1
It's just syntax error.
The line var x = y = 1; Means:
Declare local variable x,
y = 1,
Declare global variable y (since it's not declared)
x = y;
Replace
var x = y = 1;
with
var x = 1, y = 1;
or
var x = 1;
var y = 1;
and you will get local variable y
http://www.w3schools.com/js/js_variables.asp
The reason the y variable is global is that in Javascript if you omit the var keyword from an assignment statement, the variable the value is assigned to is declared on the global object.
That means that if you wrote the f() function this way, declaring both x and y with the var keyword:
function f(){
var x, y;
x = y = 1;
};
Then both x and y would be local variables (local x shadowing the global one).
It's bad practice to assign a variable without declaring it (with the var keyword).
New versions of JS will throw an error on this. You can use EC5's strict mode to take advantage of this behavior:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/Strict_mode
so you'd write your function this way:
function f(){
'use strict'
var x, y;
x = y = 1;
};
now if you forget to declare y you get an error yelling at you to do so.