What is the scope of variables in javascript? Do they have the same scope inside as opposed to outside a function? Or does it even matter? Also, where are the variables stored if they are defined globally?
转载于:https://stackoverflow.com/questions/500431/what-is-the-scope-of-variables-in-javascript
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I think about the best I can do is give you a bunch of examples to study. Javascript programmers are practically ranked by how well they understand scope. It can at times be quite counter-intuitive.
A globally-scoped variable
// global scope
var a = 1;
function one() {
alert(a); // alerts '1'
}
Local scope
// global scope
var a = 1;
function two(a) { // passing (a) makes it local scope
alert(a); // alerts the given argument, not the global value of '1'
}
// local scope again
function three() {
var a = 3;
alert(a); // alerts '3'
}
Intermediate: No such thing as block scope in JavaScript (ES5; ES6 introduces let)
a.
var a = 1;
function four() {
if (true) {
var a = 4;
}
alert(a); // alerts '4', not the global value of '1'
}
b.
var a = 1;
function one() {
if (true) {
let a = 4;
}
alert(a); // alerts '1' because the 'let' keyword uses block scoping
}
Intermediate: Object properties
var a = 1;
function Five() {
this.a = 5;
}
alert(new Five().a); // alerts '5'
Advanced: Closure
var a = 1;
var six = (function() {
var a = 6;
return function() {
// JavaScript "closure" means I have access to 'a' in here,
// because it is defined in the function in which I was defined.
alert(a); // alerts '6'
};
})();
Advanced: Prototype-based scope resolution
var a = 1;
function seven() {
this.a = 7;
}
// [object].prototype.property loses to
// [object].property in the lookup chain. For example...
// Won't get reached, because 'a' is set in the constructor above.
seven.prototype.a = -1;
// Will get reached, even though 'b' is NOT set in the constructor.
seven.prototype.b = 8;
alert(new seven().a); // alerts '7'
alert(new seven().b); // alerts '8'
Global+Local: An extra complex Case
var x = 5;
(function () {
console.log(x);
var x = 10;
console.log(x);
})();
This will print out undefined and 10 rather than 5 and 10 since JavaScript always moves variable declarations (not initializations) to the top of the scope, making the code equivalent to:
var x = 5;
(function () {
var x;
console.log(x);
x = 10;
console.log(x);
})();
Catch clause-scoped variable
var e = 5;
console.log(e);
try {
throw 6;
} catch (e) {
console.log(e);
}
console.log(e);
This will print out 5, 6, 5. Inside the catch clause e shadows global and local variables. But this special scope is only for the caught variable. If you write var f; inside the catch clause, then it's exactly the same as if you had defined it before or after the try-catch block.
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